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13.7 Cosmos & Culture

The truth about the left brain / right brain relationship.

Tania Lombrozo

It's time to rethink whatever you thought you knew about how the right and left hemispheres of the brain work together. iStockphoto hide caption

It's time to rethink whatever you thought you knew about how the right and left hemispheres of the brain work together.

Sometimes ideas that originate in science seep out into the broader culture and take on a life of their own. It's still common to hear people referred to as "anal," a Freudian idea that no longer has much currency in contemporary psychology. Ideas like black holes and quantum leaps play a metaphorical role that's only loosely tethered to their original scientific meanings.

What about the idea that some people are more right-brained and others more left-brained? Or that there's a distinctive analytic and verbal style of thinking associated with the left hemisphere of the brain, and a more holistic, creative style associated with the right? Are these scientific facts or cultural fictions?

An infographic reproduced just last month at Lifehack.org , for example, promises to explain "why you act the way you do" by revealing "which side of your brain you tend to use more." An article at Oprah.com explains " how to tap into right-brain thinking ." And decades of research using behavioral and neuro-scientific techniques do reveal fascinating and systematic differences across brain regions.

On the other hand, some recent headlines challenge the left brain / right brain dichotomy. One highly publicized paper , summarized at The Guardian , failed to find evidence that individuals tend to have stronger left- or right-sided brain networks. A new book by Stephen M. Kosslyn and G. Wayne Miller argues that the left / right brain divide is largely bogus , and should instead be replaced by a top brain / bottom brain distinction.

So while there's something deeply compelling about the clear-cut, right-brain versus left-brain classification (or is that just my left hemisphere speaking?), we have good reasons for skepticism. The real story, as you might expect, is a bit more complicated — but arguably more interesting — than the infographics and popular headlines seem to suggest.

To get a clearer picture of what we do and don't know about hemispheric brain differences in humans, I was fortunate to have an opportunity to interview a leading cognitive neuroscientist, Kara D. Federmeier , whose research focuses on language, memory and hemispheric asymmetries throughout the lifespan. Dr. Federmeier is a professor of psychology at the University of Illinois at Urbana-Champaign, where she's also affiliated with the Neurosciences Program and The Beckman Institute for Advanced Science and Technology. (And, full disclosure, she was also one of my first scientific mentors and co-authors.)

One idea that's often heard in popular discussions of psychology is that the left brain is the seat of language and more "logical," while the right brain is more creative. Is there any truth to this idea?

One problem with answering this question is that we would first have to agree on what "logical" and "creative" even mean. So let's consider a (relatively) more well-defined case: math skills, which are often taken to be part of what the "logical" left hemisphere would be good at. There are different kinds of math skills, ranging from being able to estimate which of two sets of things has a greater number of items, to counting, to various types of calculations. Research shows that, overall, the abilities that make up math skills arise from processing that takes place in BOTH hemispheres (especially the brain area in each hemisphere that is known as the intraparietal sulcus) and that damage to either hemisphere can cause difficulties with math. A left hemisphere advantage for math is mostly seen for tasks like counting and reciting multiplication tables, which rely heavily on memorized verbal information (thus, not exactly what we think of as "logical"!). And there are right hemisphere advantages on some math-related tasks as well, especially estimating the quantity of a set of objects. This kind of pattern, in which both hemispheres of the brain make critical contributions, holds for most types of cognitive skills. It takes two hemispheres to be logical – or to be creative. The claim that the left hemisphere is the seat of language, however, is a little different. That idea comes from observations that damage to the left hemisphere (for example, due to a stroke) is often associated with difficulties producing language, a problem known as aphasia. Similar damage to the right hemisphere is much less likely to cause aphasia. In fact, for most people, the left hemisphere does play a much more important role in the ability to speak than the right hemisphere does. However, this does not mean that the right hemisphere is "nonverbal." My laboratory studies the hemispheres' ability to comprehend (rather than produce) language, and we, like others, have shown that both hemispheres can figure out the meaning of words and sentences – and that they have differing strengths and weaknesses when it comes to comprehending. So, like other complex skills, the ability to understand what we read or what someone is saying to us requires both hemispheres, working together and separately.

Early studies of hemispheric asymmetries often relied on "split-brain" patients who had the corpus callosum — the bundle of neural fibers that connects the two hemispheres — severed as a treatment for severe epilepsy. In such studies, information could be provided to a single hemisphere at a time by presenting people with input to one side of the visual field, since the right visual field is processed by the left hemisphere, and vice versa.

Your lab uses contemporary neuro-scientific techniques, such as measures of brain wave activity (EEG and ERP) to investigate hemispheric asymmetries, and typically does so in individuals with intact brains. How do you do so, and do your findings corroborate or challenge earlier inferences made from the behavior of split-brain patients?

We actually use the same basic technique, known as "visual half field presentation." As an aside, I should point out that many times people misunderstand and think that each EYE is connected to a different hemisphere. That's not true. (It would make our studies so much easier if it were, since we could just ask people to close one eye!) Instead, half of the information coming into each eye goes to each of the hemispheres, with the result, as you point out, that if you are looking forward, things you see to the right of where you are looking are being picked up initially by your left hemisphere and things to the left by your right hemisphere. To look at hemispheric differences, we ask our participants, who are usually either college students or retired adults, to look at the center of the screen. We then display words (or pictures, or other types of stimuli) fairly rapidly – so people can't move their eyes fast enough to fixate them directly – to the left or the right side of a computer screen. By comparing how people respond (for example, whether they can accurately remember a word) when it was processed first by the left hemisphere versus by the right hemisphere, we can test ideas about what each hemisphere is capable of and whether one hemisphere has better, or different, abilities compared to the other. Often, we also measure brain electrical activity in these experiments because that provides rich information about how processing is unfolding over time: we can track what happens as the eyes send information to visual processing areas in the brain, as people pay attention to a word, access its meaning from memory, and add this new information into their unfolding understanding of a sentence, and as people, in some cases, decide how to respond and then prepare to press a button to register their response. With electrophysiological measures we can thus find out not only THAT the two hemispheres do something different but WHEN and HOW. In general, the kinds of hemispheric differences that were uncovered in split-brain patients have been replicated (and then extended) using these techniques in people with intact brains. This sometimes surprises people, including my fellow cognitive neuroscientists. The idea that the two hemispheres perceive things differently, attach different significance to things, obtain different meanings from stimuli, and, sometimes, make different decisions about what to do seems like it should be an exotic side effect of the split-brain condition. When the hemispheres are connected, don't they just share all the information and operate in a unified fashion? The answer is, no, they don't. They don't, in part, because they can't. Processing within each hemisphere relies on a rich, dense network of connections. The corpus callosum that connects the hemispheres is big for a fiber tract, but it is tiny compared to the network of connections within each hemisphere. Physically, then, it doesn't seem feasible for the hemispheres to fully share information or to operate in a fully unified fashion. Moreover, in a lot of cases, keeping things separate is (literally!) the smarter way for the hemispheres to function. Dividing up tasks and allowing the hemispheres to work semi-independently and take different approaches to the same problem seems to be a good strategy for the brain ... just as it often is in a partnerships between people.

It makes sense to have specialized brain regions, just as it makes sense to have divisions of labor in other areas of life. But why have specialized hemispheres? In other words, do you think there's something general that can be said about the sorts of processing that occur in the left hemisphere versus the right hemisphere, or is each simply a constellation of somewhat distinct, specialized regions?

Specifically how and why the hemispheres differ remains a mystery. They are actually remarkably similar physically, and this is one reason I think that studying hemispheric differences is critical for the field. Over the past decade or so, a lot of effort has been put into "mapping" the human brain – that is, linking areas that differ anatomically (have different inputs, outputs, types or arrangements of neurons, and/or neuropharmacology) to different functions. From this, we hope we can learn something about how and why these anatomical differences matter. However, in doing this, the field has also uncovered a lot of hemispheric asymmetries – cases in which, for example, a left hemisphere brain area becomes active and its right hemisphere homologue (with the SAME basic inputs, outputs, etc.) is much less active (or vice versa). This should really surprise us: here are two brain areas that are essentially the same on all the dimensions the field is used to thinking about, yet they behave strikingly differently. There must be physical differences between them, of course – but then, this means that those "subtle" differences are much more critical for function than the field has appreciated. My own view is that studies of hemispheric differences will help to move the field away from thinking in terms of mapping functions onto localized brain areas. I believe that cognitive functions arise from dynamically configured neural networks. On this view, the role played by any given brain area is different depending on the state of the network of which it is currently a part, and how activity unfolds over time often matters more than where it is in the brain. Why do the hemispheres differ? I think it is because even small differences in something like the strength with which areas are connected can lead to very different dynamic patterns of activation over time – and thus different functions. For language comprehension in particular, my work has shown that left hemisphere processing is more influenced by what are sometimes called "top-down" connections, which means that the left hemisphere is more likely to predict what word might be coming up next and to have its processing affected by that prediction. The right hemisphere, instead, shows more "feedforward" processing: it is less influenced by predictions (which can make its processing less efficient) but then more able to later remember details about the words it encountered. Because of what is likely a difference (possibly small) in the efficacy of particular connections within each hemisphere, the same brain areas in the two interact differently, and this leads to measurable and important asymmetries in how words are perceived, linked to meaning, remembered, and responded to. This is unlikely to be the only difference between the hemispheres, of course. But I think the answer to your question is that what we see across the pattern of asymmetries is neither a random collection of unrelated differences nor divisions based on one or even a small set of functional principles (e.g., the left hemisphere is "local" and the right hemisphere is "global" ... another popular one). Rather, some of the underlying biology is skewed, and this has far reaching consequences for the kinds of patterns that can be set up over time in the two hemispheres, leading to sets of functional differences that we can hopefully eventually link systematically to these underlying biological causes, and thereby deepen our understanding of how the brain works.

What's surprised you most about the hemispheric asymmetries you've found (or failed to find!) in your own research?

One of my favorite findings came from an experiment in which we used adjectives to change the meaning of the same noun. For example, the word "book" in "green book" refers to something concrete – that is, something for which it is easy to create a mental image. However, given "interesting book" people now usually think about the content of the book rather than its physical form, so the same word has become more "abstract" in meaning. A lot of research shows that concrete and abstract words are processed differently in the brain. We wanted to see if those differences could be found for exactly the same word depending on what it was referring to, and whether the two hemispheres were similarly affected by concreteness. We found in this experiment, as we had previously in many others, that the left hemisphere is very sensitive to the predictability of word combinations. Fewer nouns can go with "green" than with "interesting," and brain activity elicited in response to "book" reflected this when the words were presented initially to the left hemisphere. However, to our surprise, it was the right hemisphere that elicited imagery-related brain activity to "green book" compared to "interesting book." Thus, although the left hemisphere is clearly important for language processing, the right hemisphere may play a special role in creating the rich sensory experience that often accompanies language comprehension ... and that makes reading such a pleasure.

Another popular idea is that some people are more "left brained" and others more "right brained." Is there any evidence for individual differences in the extent to which people rely on one hemisphere versus another? More generally, what kinds of individual differences do you see in hemispheric specialization?

There are certainly individual differences in hemispheric specialization across people, but they are very difficult to reliably determine. Where this matters most is in medical contexts: when people are going to have brain surgery (e.g., for epilepsy or tumor resection), physicians would like to make sure that in removing certain brain tissue they are not going to disrupt critical functions like language. As I mentioned already, most of the time the left hemisphere is more important for speaking, for example, but that isn't true in absolutely everyone. In order to determine if a person's left or right hemisphere is more important for their language production, physicians use things like the WADA test, in which a barbiturate is injected into one hemisphere to temporarily shut it down, allowing the physician to see what each hemisphere can do on its own. This is obviously a very invasive test (and not perfect at that). If it were possible to instead figure out whether someone relied more on their left or right hemisphere by having them look at a spinning figure or answer a few questions, that would obviously be preferable ... but it doesn't work. There are, of course, differences in how people learn and think, what they like, and what they are like (although, since everyone's brain is different, I think the similarities are actually more surprising than the differences). Some of these differences may arise because of individual differences in how the hemispheres are organized or which hemisphere tends to be used in particular circumstances. Given that the hemispheres do operate somewhat independently, the question of how their independent processing is eventually combined and/or which hemisphere gets to "take control" of processing for a particular task is one that we are only beginning to understand. (In some cases, split-brain patients' hands – one controlled by each hemisphere – literally fought for control of a particular task; it is intriguing to imagine that kind of struggle routinely taking place internally for everyone else!) However, it seems safe to say that for the most part we all use both sides of our brains almost all the time. We do know a few factors that influence how functions are lateralized and how much they are lateralized. For example, having a "reversed" laterality (with, for example, control of speech in the right rather than the left hemisphere) is more likely for left-handed than right-handed people (although it is important not to overgeneralize from this: the vast majority of left-handed people have the typical lateralization pattern). Moreover, differences have been seen among right-handed people depending on whether or not they have left-handed biological relatives; this is something my lab is beginning to explore. Again, small biological shifts, caused in part by (complex) genetic differences, can lead to different functional patterns, including whether a function tends to be very lateralized or accomplished by both hemispheres. I will end with one last fact about hemispheric differences that many people may not be aware of, and that is that lateralization of function changes with normal aging. The kinds of lateralized patterns of brain activity I mentioned earlier when talking about brain mapping studies are more common in young adults. Across many types of tasks and many brain areas, these lateralized patterns tend to switch to bilateral patterns in healthy older adults. Is this because older adults have better learned how to be both logical AND creative? Maybe :-). It is actually difficult to know when this kind of a shift is helpful – for example, bringing extra processing resources to bear on a task to compensate for age-related declines in function – versus when it might be a sign that the brain is simply less good at maintaining a healthy division of labor. Understanding hemispheric specialization is thus also important for discovering ways to help us all maintain better cognitive functioning with age. This is something my laboratory actively investigates, aided by support from the National Institute of Aging as well as the James S. McDonnell Foundation.

Finally, can you recommend any accessible resources for readers who want to learn more about hemispheric asymmetries?

My own interest in hemispheric differences was sparked, in part, by books like Left Brain, Right Brain by Sally Springer and Georg Deutsch and Hemispheric Asymmetry: What's Right and What's Left by Joseph Hellige. These are accessible books written by scientists and well-grounded in the research – although both books are now more than a decade old, so don't reflect current developments in the field. Unfortunately, I don't know of more recent books that are comparably reliable and accessible. Some readers may be interested to read journal articles on the topic. For example, I drew some of my information about math and the hemispheres from the article , "Arithmetic and the brain" by Stanislas Dehaene, Nicolas Molko, Laurent Cohen and Anna J Wilson in the journal Current Opinion in Neurobiology (2004; Volume 14, pages 218-224). For those interested in language, I (with coauthors Edward Wlotko and Aaron Meyer) have written a fairly accessible review called "What's "right" in language comprehension: ERPs reveal right hemisphere language capabilities" published in Language and Linguistics Compass (2008; Volume 2, pages 1-17).

You can keep up with more of what Tania Lombrozo is thinking on Twitter: @TaniaLombrozo

• Kara Federmeier
• right brain
• hemispheric asymmetries
• neuroscience

Left Brain, Right Brain: An Outdated Argument

• By Kevin Boehm
• April 15, 2012

“I am definitely a left-brained person — I am not very artistic.” How many times have we characterized ourselves as either left-brained and logical people or right-brained and creative people? This popular myth, which conjures up an image of one side of our brains crackling with activity while the other lies dormant, has its roots in outdated findings from the 1970s, and it seems to imply that humans strongly favor using one hemisphere over the other. More recent findings have shown that although there are indeed differences between the hemispheres, they may not be as clear-cut as we once thought.

Our personalities and abilities are not determined by favoring one hemisphere over the other — that much is certain. Many other functions, however, such as response to danger and language generation, are lateralized in the brain. Researchers hypothesize that these differences arose from early vertebrates. Originally, it seems that the right hemisphere began to respond more quickly to danger. In fact, when we are suddenly confronted by a dangerous stimulus, we will respond more quickly with our left hand, which is controlled by the right hemisphere. The left hemisphere, on the other hand, has developed to handle more common, routine tasks, such as feeding and hand control. Since this hemisphere controls the right hand, a strong right-handed preference has arisen in most of us, providing one explanation of why most people are right-hand dominant.

Language is another process that is lateralized in the brain, though a study conducted by researchers at Ghent University has shown that the asymmetry differs when generating versus receiving language. When children were shown images and asked to tell a story about them, function was lateralized strongly in the left hemisphere for over 90 percent of participating children. However, when asked to listen to an emotional story, both hemispheres of the brain were activated to a similar degree as planning and articulation require more processing involving more regions on both sides of the brain. The stories the children listened to, unlike the pictures, were emotional, which may indicate that the observed involvement of the right hemisphere is linked to emotional regulation.

Olivia Farr, a neuroscience Ph.D. candidate at the Yale School of Medicine, explains that this language lateralization is the source of many generalizations. “In some of the first studies conducted on hemispheric lateralization, split-brained patients without an intact corpus callosum, or bridge between the two hemispheres, were examined,” says Farr. Because visual information from the right eye goes to the left hemisphere, when split-brained patients saw a word with their right eye, they could speak it but not draw it. When the patients saw a word with the left eye, they could draw but not speak it. These results contributed to the belief that hemispheres operate independently of each other for most tasks, which then developed into the myth of being exclusively left-brained or right-brained. There was so little known about the brain that it was convenient to attribute poorly understood traits, such as personality or thinking habits, to a clear-cut difference in lateralization. However, “we now know that hemispheres are always communicating, and that even these lateralization rules don’t always apply,” Farr affirms.

Hemispheres sometimes do perform tasks nearly independently, but the integration of the two yields some of our most uniquely human characteristics. For example, when we make errors, our realization and ability to correct them is a result of the synergy of the two halves of our brain. In fact, patients with damage to the corpus callosum have difficulties correcting their errors as compared to patients with intact corpora callosa, further suggesting that the two halves of the brain are both involved in processing the error.

Even though some tasks usually occur preferentially in one half of the brain, it is possible for the part directly opposite to take control of the process. Such a process takes time, but after damage in the left inferior frontal gyrus (referred to as Broca’s area) — a region of the brain linked to speech production — researchers have found that activity in the right inferior frontal gyrus begins to increase during language generation. Our brains have enough plasticity to adapt to damage and change conformations, even as adults.

Knowing that language processing usually occurs on the left side of the brain and response to danger generally occurs on the right does not comprehensively summarize our beings. Lateralization of the brain is still not well understood, and there are very few, if any, hard and fast rules of lateralization that actually make an impact on our behavior. We are still every bit as human and unpredictable as before, but we now understand a bit more of what makes us that way.

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Parts of the Brain: Anatomy, Structure & Functions

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On This Page:

The brain controls all functions of the body, interprets information from the outside world, and defines who we are as individuals and how we experience the world.

The brain receives information through our senses: sight, touch, taste, smell, and hearing. This information is processed in the brain, allowing us to give meaning to the input it receives.

The brain is part of the central nervous system ( CNS ) along with the spinal cord. There is also a peripheral nervous system (PNS) comprised of 31 pairs of spinal nerves that branch from the spinal cord and cranial nerves that branch from the brain.

Brain Parts

The brain is composed of the cerebrum, cerebellum, and brainstem (Fig. 1).

Figure 1. The brain has three main parts: the cerebrum, cerebellum, and brainstem.

The cerebrum is the largest and most recognizable part of the brain. It consists of grey matter (the cerebral cortex ) and white matter at the center. The cerebrum is divided into two hemispheres, the left and right, and contains the lobes of the brain (frontal, temporal, parietal, and occipital lobes).

The cerebrum produces higher functioning roles such as thinking, learning, memory, language, emotion, movement, and perception.

The Cerebellum

The cerebellum is located under the cerebrum and monitors and regulates motor behaviors, especially automatic movements.

This structure is also important for regulating posture and balance and has recently been suggested for being involved in learning and attention.

Although the cerebellum only accounts for roughly 10% of the brain’s total weight, this area is thought to contain more neurons (nerve cells) than the rest of the brain combined.

The brainstem is located at the base of the brain. This area connects the cerebrum and the cerebellum to the spinal cord, acting as a relay station for these areas.

The brainstem regulates automatic functions such as sleep cycles, breathing, body temperature, digestion, coughing, and sneezing.

Right Brain vs. Left Brain

The cerebrum is divided into two halves: the right and left hemispheres (Fig. 2). The left hemisphere controls the right half of the body, and the right hemisphere controls the left half.

The two hemispheres are connected by a thick band of neural fibers known as the corpus callosum, consisting of about 200 million axons.

The corpus callosum allows the two hemispheres to communicate and allows information being processed on one side of the brain to be shared with the other.

Figure 2. The cerebrum is divided into left and right hemispheres. The nerve fibers corpus callosum connects the two sides.

Hemispheric lateralization is the idea that each hemisphere is responsible for different functions. Each of these functions is localized to either the right or left side.

The left hemisphere is associated with language functions, such as formulating grammar and vocabulary and containing different language centers (Broca’s and Wernicke’s area).

The right hemisphere is associated with more visuospatial functions such as visualization, depth perception, and spatial navigation. These left and right functions are the case in most people, especially those who are right-handed.

Lobes of the Brain

Each cerebral hemisphere can be subdivided into four lobes, each associated with different functions.

The four lobes of the brain are the frontal, parietal, temporal, and occipital lobes (Figure 3).

Figure 3. The cerebrum is divided into four lobes: frontal, parietal, occipital, and temporal.

Frontal lobes

The frontal lobes are located at the front of the brain, behind the forehead (Figure 4).

Their main functions are associated with higher cognitive functions, including problem-solving, decision-making, attention, intelligence, and voluntary behaviors.

The frontal lobes contain the motor cortex  responsible for planning and coordinating movements.

It also contains the prefrontal cortex, which is responsible for initiating higher-lever cognitive functioning, and Broca’s Area, which is essential for language production.

Figure 4. Frontal lobe structure.

Temporal lobes

The temporal lobes are located on both sides of the brain, near the temples of the head, hence the name temporal lobes (Figure 5).

The main functions of these lobes include understanding, language, memory acquisition, face recognition, object recognition, perception, and auditory information processing.

There is a temporal lobe in both the left and right hemispheres. The left temporal lobe, which is usually the most dominant in people, is associated with language, learning, memorizing, forming words, and remembering verbal information.

The left lobe also contains a vital language center known as Wernicke’s area, which is essential for language development. The right temporal lobe is usually associated with learning and memorizing non-verbal information and determining facial expressions.

Figure 5. Temporal lobe structure.

Parietal lobes

The parietal lobe is located at the top of the brain, between the frontal and occipital lobes, and above the temporal lobes (Figure 6).

The parietal lobe is essential for integrating information from the body’s senses to allow us to build a coherent picture of the world around us.

These lobes allow us to perceive our bodies through somatosensory information (e.g., through touch, pressure, and temperature). It can also help with visuospatial processing, reading, and number representations (mathematics).

The parietal lobes also contain the somatosensory cortex, which receives and processes sensory information, integrating this into a representational map of the body.

This means it can pinpoint the exact area of the body where a sensation is felt, as well as perceive the weight of objects, shape, and texture.

Figure 6. Parietal lobe structure.

Occipital lobes

The occipital lobes are located at the back of the brain behind the temporal and parietal lobes and below the occipital bone of the skull (Figure 7).

The occipital lobes receive sensory information from the eyes’ retinas, which is then encoded into different visual data. Some of the functions of the occipital lobes include being able to assess the size, depth, and distance, determine color information, object and facial recognition, and mapping the visual world.

The occipital lobes also contain the primary visual cortex, which receives sensory information from the retinas, transmitting this information relating to location, spatial data, motion, and the colors of objects in the field of vision.

Figure 7. Occipital lobe structure.

Cerebral Cortex

The surface of the cerebrum is called the cerebral cortex  and has a wrinkled appearance, consisting of bulges, also known as gyri, and deep furrows, known as sulci (Figure 8).

A gyrus (plural: gyri) is the name given to the bumps and ridges on the cerebral cortex (the outermost layer of the brain). A sulcus (plural: sulci) is another name for a groove in the cerebral cortex.

Figure 8. The cortex contains neurons (grey matter) interconnected to other brain areas by axons (white matter). The cortex has a folded appearance. A fold is called a gyrus, and the valley between is a sulcus.

The cerebral cortex is primarily constructed of grey matter (neural tissue made up of neurons), with between 14 and 16 billion neurons found here.

The many folds and wrinkles of the cerebral cortex allow a wider surface area for an increased number of neurons to live there, permitting large amounts of information to be processed.

Deep Structures

The amygdala is a structure deep in the brain that is involved in the processing of emotions and fear learning. The amygdala is a part of the limbic system, a neural network that mediates emotion and memory (Figure 9).

This structure also ties emotional meaning to memories, processes rewards, and helps us make decisions. This structure has also been linked with the fight-or-flight response.

Figure 9. The amygdala in the limbic system plays a key role in how animals assess and respond to environmental threats and challenges by evaluating the emotional importance of sensory information and prompting an appropriate response.

Thalamus and Hypothalamus

The thalamus relays information between the cerebral cortex, brain stem, and other cortical structures (Figure 10).

Because of its interactive role in relaying sensory and motor information, the thalamus contributes to many processes, including attention, perception, timing, and movement. The hypothalamus modulates a range of behavioral and physiological functions.

It controls autonomic functions such as hunger, thirst, body temperature, and sexual activity. To do this, the hypothalamus integrates information from different brain parts and responds to various stimuli such as light, odor, and stress.

Figure 10. The thalamus is often described as the brain’s relay station as a great deal of information that reaches the cerebral cortex first stops in the thalamus before being sent to its destination.

Hippocampus

The hippocampus is a curved-shaped structure in the limbic system associated with learning and memory (Figure 11).

This structure is most strongly associated with the formation of memories, is an early storage system for new long-term memories, and plays a role in the transition of these long-term memories to more permanent memories.

Figure 11. Hippocampus location in the brain

Basal Ganglia

The basal ganglia are a group of structures that regulate the coordination of fine motor movements, balance, and posture alongside the cerebellum.

These structures are connected to other motor areas and link the thalamus with the motor cortex. The basal ganglia are also involved in cognitive and emotional behaviors, as well as playing a role in reward and addiction.

Figure 12. The Basal Ganglia Illustration

Ventricles and Cerebrospinal Fluid

Within the brain, there are fluid-filled interconnected cavities called ventricles , which are extensions of the spinal cord. These are filled with a substance called cerebrospinal fluid, which is a clear and colorless liquid.

The ventricles produce cerebrospinal fluid and transport and remove this fluid. The ventricles do not have a unique function, but they provide cushioning to the brain and are useful for determining the locations of other brain structures.

Cerebrospinal fluid circulates the brain and spinal cord and functions to cushion the brain within the skull. If damage occurs to the skull, the cerebrospinal fluid will act as a shock absorber to help protect the brain from injury.

As well as providing cushioning, the cerebrospinal fluid circulates nutrients and chemicals filtered from the blood and removes waste products from the brain. Cerebrospinal fluid is constantly absorbed and replenished by the ventricles.

If there were a disruption or blockage, this can cause a build-up of cerebrospinal fluid and can cause enlarged ventricles.

Neurons are the nerve cells of the central nervous system that transmit information through electrochemical signals throughout the body. Neurons contain a soma, a cell body from which the axon extends.

Axons are nerve fibers that are the longest part of the neuron, which conduct electrical impulses away from the soma.

There are dendrites at the end of the neuron, which are branch-like structures that send and receive information from other neurons.

A myelin sheath, a fatty insulating layer, forms around the axon, allowing nerve impulses to travel down the axon quickly.

There are different types of neurons. Sensory neurons transmit sensory information, motor neurons transmit motor information, and relay neurons allow sensory and motor neurons to communicate.

The communication between neurons is called synapses. Neurons communicate with each other via synaptic clefts, which are gaps between the endings of neurons.

During synaptic transmission, chemicals, such as neurotransmitters, are released from the endings of the previous neuron (also known as the presynaptic neuron).

These chemicals enter the synaptic cleft to then be transported to receptors on the next neuron (also known as the postsynaptic neuron).

Once transported to the next neuron, the chemical messengers continue traveling down neurons to influence many functions, such as behavior and movement.

Glial Cells

Glial cells are non-neuronal cells in the central nervous system which work to provide the neurons with nourishment, support, and protection.

These are star-shaped cells that function to maintain the environment for neuronal signaling by controlling the levels of neurotransmitters surrounding the synapses.

They also work to clean up what is left behind after synaptic transmission, either recycling any leftover neurotransmitters or cleaning up when a neuron dies.

Oligodendrocytes

These types of glial have the appearance of balls with spikes all around them. They function by wrapping around the axons of neurons to form a protective layer called the myelin sheath.

This is a substance that is rich in fat and provides insulation to the neurons to aid neuronal signaling.

Microglial cells have oval bodies and many branches projecting out of them. The primary function of these cells is to respond to injuries or diseases in the central nervous system.

They respond by clearing away any dead cells or removing any harmful toxins or pathogens that may be present, so they are, therefore, important to the brain’s health.

Ependymal cells

These cells are column-shaped and usually line up together to form a membrane called the ependyma. The ependyma is a thin membrane lining the spinal cord and ventricles of the brain .

In the ventricles, these cells have small hairlike structures called cilia, which help encourage the flow of cerebrospinal fluid.

Cranial Nerves

There are 12 types of cranial nerves which are linked directly to the brain without having to pass through the spinal cord. These allow sensory information to pass from the organs of the face to the brain:

Mnemonic for Order of Cranial Nerves:

S ome S ay M arry M oney B ut M y B rother S ays B ig B rains M atter M ore

• Cranial I: Sensory
• Cranial II: Sensory
• Cranial III: Motor
• Cranial IV: Motor
• Cranial V: Both (sensory & motor)
• Cranial VI: Motor
• Cranial VII: Both (sensory & motor)
• Cranial VIII: Sensory
• Cranial IX: Both (sensory & motor)
• Cranial X: Both (sensory & motor)
• Cranial XI: Motor
• Cranial XII: Motor

Purves, D., Augustine, G., Fitzpatrick, D., Katz, L., LaMantia, A., McNamara, J., & Williams, S. (2001). Neuroscience 2nd edition . sunderland (ma) sinauer associates. Types of Eye Movements and Their Functions.

Mayfield Brain and Spine (n.d.). Anatomy of the Brain. Retrieved July 28, 2021, from: https://mayfieldclinic.com/pe-anatbrain.htm

Robertson, S. (2018, August 23). What is Grey Matter? News Medical Life Sciences. https://www.news-medical.net/health/What-is-Grey-Matter.aspx

Guy-Evans, O. (2021, April 13). Temporal lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/temporal-lobe.html

Guy-Evans, O. (2021, April 15). Parietal lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/parietal-lobe.html

Guy-Evans, O. (2021, April 19). Occipital lobe: definition, functions, and location. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/occipital-lobe.html

Guy-Evans, O. (2021, May 08). Frontal lobe function, location in brain, damage, more. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/frontal-lobe.html

Guy-Evans, O. (2021, June 09). Gyri and sulci of the brain. Simply Psychology. www.www.www.www.www.www.simplypsychology.org/gyri-and-sulci-of-the-brain.html

July 1, 2009

16 min read

Evolutionary Origins of Your Right and Left Brain

The division of labor by the two cerebral hemispheres—once thought to be uniquely human—predates us by half a billion years. Speech, right-handedness, facial recognition and the processing of spatial relations can be traced to brain asymmetries in early vertebrates

By Peter F. MacNeilage , Lesley J. Rogers & Giorgio Vallortigara

The left hemisphere of the human brain controls language, arguably our greatest mental attribute. It also controls the remarkable dexterity of the human right hand. The right hemisphere is dominant in the control of, among other things, our sense of how objects interrelate in space. Forty years ago the broad scientific consensus held that, in addition to language, right-handedness and the specialization of just one side of the brain for processing spatial relations occur in humans alone. Other animals, it was thought, have no hemispheric specializations of any kind.

Those beliefs fit well with the view that people have a special evolutionary status. Biologists and behavioral scientists generally agreed that right-handedness evolved in our hominid ancestors as they learned to build and use tools, about 2.5 million years ago. Right-handedness was also thought to underlie speech. Perhaps, as the story went, the left hemisphere simply added sign language to its repertoire of skilled manual actions and then converted it to speech. Or perhaps the left brain’s capacity for controlling manual action extended to controlling the vocal apparatus for speech. In either case, speech and language evolved from a relatively recent manual talent for toolmaking. The right hemisphere, meanwhile, was thought to have evolved by default into a center for processing spatial relations, after the left hemisphere became specialized for handedness.

In the past few decades, however, studies of many other animals have shown that their two brain hemispheres also have distinctive roles. Despite those findings, prevailing wisdom continues to hold that people are different. Many investigators still think the recently discovered specializations of the two brain hemispheres in nonhumans are unrelated to the human ones; the hemispheric specializations of humans began with humans.

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Here we present evidence for a radically different hypothesis that is gaining support, particularly among biologists. The specialization of each hemisphere in the human brain, we argue, was already present in its basic form when vertebrates emerged about 500 million years ago. We suggest that the more recent specializations of the brain hemispheres, including those of humans, evolved from the original ones by the Darwinian process of descent with modification. (In that process, capabilities relevant to ancient traits are changed or co-opted in the service of other developing traits.) Our hypothesis holds that the left hemisphere of the vertebrate brain was originally specialized for the control of well-established patterns of behavior under ordinary and familiar circumstances. In contrast, the right hemisphere, the primary seat of emotional arousal, was at first specialized for detecting and responding to unexpected stimuli in the environment.

In early vertebrates such a division of labor probably got its start when one or the other hemisphere developed a tendency to take control in particular circumstances. From that simple beginning, we propose, the right hemisphere took primary control in potentially dangerous circumstances that called for a rapid reaction from the animal—detecting a predator nearby, for instance. Otherwise, control passed to the left hemisphere. In other words, the left hemisphere became the seat of self-motivated behavior, sometimes called top-down control. (We stress that self-motivated behavior need not be innate; in fact, it is often learned.) The right hemisphere became the seat of environmentally motivated behavior, or bottom-up control. The processing that directs more specialized behaviors—language, toolmaking, spatial interrelations, facial recognition, and the like—evolved from those two basic controls.

The Left Hemisphere Most of the evidence that supports our hypothesis does not come from direct observation of the brain but rather from observations of behavior that favors one or the other side of the body. In the vertebrate nervous system the connections cross between body and brain—to a large degree, nerves to and from one side of the body are linked to the opposite-side hemisphere of the brain.

Evidence for the first part of our hypothesis—that the vertebrate left hemisphere specializes in controlling routine, internally directed behaviors—has been building for some time. One routine behavior with a rightward bias across many vertebrates is feeding. Fishes, reptiles and toads, for instance, tend to strike at prey on their right side under the guidance of their right eye and left hemisphere. In a variety of bird species—chickens, pigeons, quails and stilts—the right eye is the primary guide for various kinds of food pecking and prey capture. In one instance, such a lateralized feeding preference has apparently led to a lateralized bias in the animal’s external anatomy. The beak of the New Zealand wry-billed plover slopes to the right; that way, the plover’s right eye can guide the beak as the bird seeks food under small river stones.

As for mammals, the feeding behavior of humpback whales is a spectacular example of a lateral feeding preference. Phillip J. Clapham, now at the Alaska Fisheries Science Center in Seattle, and his colleagues discovered that 60 out of 75 whales had abrasions only on the right jaw; the other 15 whales had abrasions only on the left jaw. The findings were clear evidence that whales favor one side of the jaw for food gathering and that “right-jawedness” is by far the norm.

In short, in all vertebrate classes—fishes, reptiles, amphibians, birds and mammals—animals tend to retain what was probably an ancestral bias toward the use of the right side in the routine activity of feeding.

Origins of Right-Handedness What do these findings say about the alleged uniqueness of human right-handedness? Evidence for a right-side bias in birds and whales is intriguing, but it hardly makes a convincing argument against the old belief that right-handedness in humans had no evolutionary precursors. Yet more than a dozen recent studies have now demonstrated a right-handed bias among other primates, our closest evolutionary relatives—clearly suggesting that human right-handedness descended from that of earlier primates. The right-hand preference shows itself in monkeys (baboons, Cebus monkeys and rhesus macaques) as well as in apes, particularly in chimpanzees.

Many of the studies of apes have been done by William D. Hopkins of the Yerkes National Primate Research Center in Atlanta and his colleagues. Hopkins’s group observed right-hand preferences particularly in tasks that involved either coordinating both hands or reaching for food too high to grab without standing upright. For example, experimenters placed honey (a favorite food) inside a short length of plastic pipe and gave the pipe to one of the apes. To get the honey, the ape had to pick up the pipe in one hand and scrape out the honey with one finger of the opposite hand. By a ratio of 2 to 1, the apes preferred to scrape honey out with a finger of the right hand. Similarly, in the reaching experiments, the apes usually grabbed the food they wanted with the right hand.

The Yerkes findings also suggest to us that as early primates evolved to undertake harder and more elaborate tasks for finding food, their handedness preferences became stronger, too. The reason, we suspect, is that performing ever more complex tasks made it increasingly necessary for the control signals from the brain to pass as directly as possible to the more skilled hand. Since the most direct route from the left hemisphere—the hemisphere specialized for routine tasks—to the body follows the body-crossing pathways of the peripheral nerves, the right hand increasingly became the preferred hand among nonhuman primates for performing elaborate, albeit routine, tasks.

Communication and the Left Brain The evolutionary descent of human right-handed dexterity via the modification of ancient feeding behavior in ancestral higher primates now seems very likely. But could feeding behavior also have given rise to the left-brain specialization for language? Actually we do not mean to suggest that this development was direct. Rather we argue that the “language brain” emerged from an intermediate and somewhat less primitive specialization of the left hemisphere—namely, its specialization for routine communication, both vocal and nonvocal. But contrary to long-held beliefs among students of human prehistory, neither of those communicative capabilities first arose with humans; they, too, are descended from hemispheric specializations that first appeared in animals that lived long before our species emerged.

In birds, for instance, studies have shown that the left hemisphere controls singing. In sea lions, dogs and monkeys, the left hemisphere controls the perception of calls by other members of the same species. One of us (Rogers), in collaboration with Michelle A. Hook-Costigan, now at Texas A&M University, observed that common marmosets open the right side of their mouths  wider than the left side when making friendly calls to other marmosets. People also generally open the right side of their mouths to a greater extent than the left when they speak—the result of greater activation of the right side of the face by the left hemisphere.

Little is universal in nature, though, and in some animals a vocal response to highly emotional circumstances has also been linked to the left brain, not—as one might expect—to the right. When a male frog is clasped from behind and held by a rival male, for instance, the left hemisphere seems to control the vocal responses of the first frog. The left hemisphere in mice controls the reception of distress calls from infant mice, and in gerbils it controls the production of calls during copulation. But those animals may be exceptions. In humans and monkeys—and perhaps in most other animals—the right brain takes control in highly emotional vocalizing; the left brain sticks to the routine.

Nonvocal communication in humans has evolutionary antecedents as well. Not only do chimpanzees tend to be right-handed when they manipulate objects, but they also favor the right hand for communicative gestures. Gorillas, too, tend to incorporate the right hand into complex communications that also involve the head and the mouth. Adrien Meguerditchian and Jacques Vauclair, both at the University of Provence in France, have even observed a right-handed bias for one manual communication (patting the ground) in baboons.

The evolutionary significance of all this becomes clear as soon as one notes that humans also tend to make communicative gestures with the right hand. The lateralized behavior we share with baboons suggests that right-handed communications arose with the first appearance of the monkeylike ancestor we share with baboons. That creature emerged perhaps 40 million years ago—well before hominids began to evolve.

Evolution of Speech A fundamental question remains: Just how could any of the behaviors already controlled by the left brain—feeding, vocalizing, communicating with the right hand—have been modified to become speech—one of the most momentous steps in the history of life on earth?

One of us (MacNeilage) has hypothesized that it required the evolution of the syllable, the basic organizational unit underlying a stream of speech in time. The typical syllable is a rhythmic alternation between consonants and vowels. (Consonants are the sounds created when the vocal tract is momentarily closed or almost closed; vowels are the sounds created by resonance with the shape of the vocal tract as air flows relatively freely out through the open mouth.) The syllable may have evolved as a by-product of the alternate raising (consonant) and lowering (vowel) of the mandible, a behavior already well established for chewing, sucking and licking. A series of these mouth cycles, produced as lip smacks, may have begun to serve among early humans as communication signals, just as they do to this day among many other primates.

Somewhat later the vocalizing capabilities of the larynx could have paired with the communicative lip smacks to form spoken syllables. Syllables were perhaps first used to symbolize individual concepts, thus forming words. Subsequently, the ability to form sentences (language) presumably evolved when early humans combined the two kinds of words that carry the main meaning of sentences: those for objects (nouns) and those for actions (verbs).

The Right Hemisphere What about the second half of our hypothesis? How strong is the evidence that, early in vertebrate evolution, the right hemisphere specialized in detecting and responding to unexpected stimuli? In what ways has that underlying specialization evolved and been transformed?

One set of findings that lend strong support to our hypothesis comes from studies of the reactions to predators by various animals. After all, few events in ancient vertebrate environments could have been more unexpected and emotion-laden than the surprise appearance of a deadly predator. Sure enough, fishes, amphibians, birds and mammals all react with greater avoidance to predators seen in the left side of their visual field (right side of the brain) than in their right visual field.

Evidence that the same hemispheric specialization for reactions holds for humans comes from brain-imaging studies. In a summary of those studies, Michael D. Fox and his colleagues at Washington University in St. Louis conclude that humans possess an “attentional system” in the right hemisphere that is particularly sensitive to unexpected and “behaviorally relevant stim­uli”—or in other words, the kind of stimuli that say, in effect, Danger ahead! The existence of such an attentional system helps to make sense of an otherwise inexplicable human propensity: in the laboratory, even right-handed people respond more quickly to unexpected stimuli with their left hand (right hemisphere) than with their right hand.

Even in nonthreatening circumstances, many vertebrates keep a watchful left eye on any visible predators. This early right-hemisphere specialization for wariness in the presence of predators also extends in many animals to aggressive behavior. Toads, chameleons, chicks and baboons are more likely to attack members of their own species to their left than to their right.

In humans the relatively primitive avoidance and wariness behaviors that manifest right-hemisphere attentiveness in nonhuman animals have morphed into a variety of negative emotions. Nineteenth-century physicians noticed that patients complained more often of hysterical limb paralyses on the left side than on the right. There is some evidence for right-hemisphere control of emotional cries and shouts in humans—in striking contrast with the emotionally neutral vocalizations controlled by the left hemisphere. People are more likely to become depressed after damage to the left hemisphere than to the right. And in states of chronic depression the right hemisphere is more active than the left.

Recognizing Others Along with the sudden appearance of a predator, the most salient environmental changes to which early vertebrates had to react quickly were en­counters with others of their own species. In fishes and birds the right hemisphere recognizes social companions and monitors social behavior that might require an immediate reaction. Hence, the role of the right hemisphere in face perception must have descended from abilities of relatively early vertebrates to recognize the visual appearance of other individuals of their species.

For example, only some species of fishes—among the earliest evolving vertebrates—may be able to recognize individual fish, but birds in general do show a right-hemisphere capacity to recognize individual birds. Keith M. Kendrick of the Babraham Institute in Cambridge, England, has shown that sheep can recognize the faces of other sheep (and of people) from memory and that the right hemisphere is preferentially involved. Charles R. Hamilton and Betty A. Vermeire, both at Texas A&M, have observed similar behavior in monkeys.

In humans neuroscientists have recently recognized that the right hemisphere specializes in face recognition. Prosopagnosia, a neurological disorder that impairs that ability, is more often a result of damage to the right hemisphere than to the left. Extending face recognition to what seems another level, both monkeys and humans interpret emotional facial expressions more accurately with the right hemisphere than with the left. We think that this ability is part of an ancient evolutionary capacity of the right hemisphere for determining identity or familiarity—for judging whether a present stimulus, for instance, has been seen or encountered before.

Global and Local We have argued for a basic distinction between the role of the left hemisphere in normal action and the role of the right hemisphere in unusual circumstances. But investigators have highlighted additional dichotomies of hemispheric function as well. In humans the right hemisphere “takes in the whole scene,” attending to the global aspects of its environment rather than focusing on a limited number of features. That capacity gives it substantial advantages in analyzing spatial relations. Memories stored by the right hemisphere tend to be organized and recalled as overall patterns rather than as a series of single items. In contrast, the left hemisphere tends to focus on local aspects of its environment.

Striking evidence for the global-local dichotomy in humans has been brought to light by a task invented by David Navon of the University of Haifa in Israel. Brain-damaged patients are asked to copy a picture in which 20 or so small copies of the uppercase letter A have all been arranged to form the shape of a large capital H. Patients with damage to the left hemisphere often make a simple line drawing of the H with no small A letters included; patients with damage to the right hemisphere scatter small A letters unsystematically all over the page.

A similar dichotomy has been detected in chickens, suggesting its relatively early evolution. Richard J. Andrew of the University of Sussex in England and one of us (Vallortigara) have discovered that, as in humans, the domestic chick pays special attention to broad spatial relations with its right hemisphere. Moreover, chicks with the right eye covered, hence receiving input only to the right hemisphere, show interest in a wide range of stimuli, suggesting they are attending to their global environment. Chicks that can attend only with the left hemisphere (left eye covered) focus only on specific, local landmark features.

Why Do Hemispheres Specialize? Why have vertebrates favored the segregation of certain functions in one or the other half of the brain? To assess an incoming stimulus, an organism must carry out two kinds of analyses simultaneously. It must estimate the overall novelty of the stimulus and take decisive emergency action if needed (right hemisphere). And it must determine whether the stimulus fits some familiar category, so as to make whatever well-established response, if any, is called for (left hemisphere).

To detect novelty, the organism must attend to features that mark an experience as unique. Spatial perception calls for virtually that same kind of “nose for novelty,” because almost any standpoint an animal adopts results in a new configuration of stimuli. That is the function of the right hemisphere. In contrast, to categorize an experience, the organism must recognize which of its features are recurring, while ignoring or discarding its unique or idiosyncratic ones. The result is selective attention, one of the brain’s most important capabilities. That is the function of the left hemisphere.

Perhaps, then, those hemispheric specializations initially evolved because collectively they do a more efficient job of processing both kinds of information at the same time than a brain without such specialized systems. To test this idea, we had to compare the abilities of animals having lateralized brains with animals of the same species having nonlateralized brains. If our idea was correct, those with lateralized brains would be able to perform parallel functions of the left and right hemisphere more efficiently than those with nonlateralized brains.

Fortunately, one of us (Rogers) had already shown that by exposing the embryo of a domestic chick to light or to dark before hatching, she could manipulate the development of hemispheric specialization for certain functions. Just before hatching, the chick embryo’s head is naturally turned so that the left eye is covered by the body and only the right eye can be stimulated by light passing through the egg shell. The light triggers some of the hemispheric specializations for visual processing to develop. By incubating eggs in the dark, Rogers could prevent the specializations from developing. In particular, she found, the dark treatment prevents the left hemisphere from developing its normal superior ability to sort food grains from small pebbles, and it also prevents the right hemisphere from being more responsive than the left to predators.

Rogers and Vallortigara, in collaboration with Paolo Zucca of the University of Teramo in Italy, tested both kinds of chicks on a dual task: the chicks had to find food grains scattered among pebbles while they monitored for the appearance of a model predator overhead. The chicks incubated in light could perform both tasks simultaneously; those incubated in the dark could not—thereby confirming that a lateralized brain is a more efficient processor.

Social “Symmetry Breaking” Enabling separate and parallel processing to take place in the two hemispheres may increase brain efficiency, but it does not explain why, within a species, one or the other specialization tends to predominate. Why, in most animals, is the left eye (and the right hemisphere) better suited than the right eye (and the left hemisphere) for vigilance against predation? What makes the predominance of one kind of handedness more likely than a symmetric, 50–50 mixture of both?

From an evolutionary standpoint a “broken” symmetry, in which populations are made up mainly of left types or mainly of right types, could be disadvantageous because the behavior of individuals would be more predictable to predators. Predators could learn to approach on the prey’s less vigilant side, thereby reducing the chance of being detected. The uneven proportion of left- and right-type individuals in many populations thus indicates that the imbalance must be so valuable that it persists despite the increased vulnerability to predators. Rogers and Vallortigara have suggested that, among social animals, the advantage of conformity may lie in knowing what to expect from others of one’s own species.

Together with Stefano Ghirlanda of the Universities of Stockholm in Sweden and of Bologna in Italy, Vallortigara recently showed mathe­matically that populations dominated by left-type or by right-type individuals can indeed arise spontaneously if such a population has frequency-dependent costs and benefits. The mathematical theory of games often shows that the best course of action for an individual may depend on what most other members of its own group decide to do. Applying game theory, Ghirlanda and Vallortigara demonstrated that left- or right-type behavior can evolve in a population under social selection pressures—that is, when asymmetrical individuals must coordinate with others of their species. For example, one would expect schooling fish to have evolved mostly uniform turning preferences, the better to remain together as a school. Solitary fish, in contrast, would probably vary randomly in their turning preferences, because they have little need to swim together. This is in fact the case.

With the realization that the asymmetrical brain is not specific to humans, new questions about a number of higher human functions arise: What are the relative roles of the left and right hemispheres in having self-awareness, consciousness, empathy or the capacity to have flashes of insight? Little is known about those issues. But the findings we have detailed suggest that these functions—like the other human phenomena discussed here—will be best understood in terms of the descent with modification of prehuman capabilities.

Did the Syllable Evolve from Chewing? According to one of the authors (MacNeilage), the origin of human speech may be traceable to the evolution of the syllable—typically an alternation between consonant and vowel. In the word “mama,” for instance, each syllable begins with the consonant sound [m] and ends with the vowel sound [a]. As the cutaway diagrams show, the [m] sound is made by temporarily raising the jaw, or lower mandible, and stopping the flow of air from the lungs by closing the lips (below left). To make the following vowel sound [a], the jaw drops and air flows freely through the vocal tract (below right). MacNeilage has thus proposed that the making of syllabic utterances is an evolutionary modification of routine chewing behavior, which first evolved in mammals 200 million years ago.

A Lateralized Brain Is More Efficient One of the authors (Rogers) discovered that if she exposed chick embryos to light or to dark before they hatched, she could control whether the two halves of the chick brains developed their specializations for visual processing—that is, whether the chicks hatched with weakly or strongly lateralized brains. Rogers and another one of the authors (Vallortigara), with Paolo Zucca of the University of Teramo in Italy, then compared normal, strongly lateralized chicks with weakly lateralized chicks on two tasks. One task was to sort food grains from small pebbles (usually a job for the left hemisphere); the other task was to respond to a model of a predator (a cutout in the shape of a hawk) that was passed over the chicks (usually a task for the right hemisphere). The weakly lateralized chicks had no trouble learning to tell grains from pebbles when no model hawk was present. But when the hawk “flew” overhead, they frequently failed to detect it, and they were much slower than normal chicks in learning to peck at grains instead of pebbles. In short, without the lateral specializations of their brain, the chicks could not attend to two tasks simultaneously.

Note: This article was originally printed with the title, "Origins of the Left and Right Brain."

Left Brain - Right Brain

Reviewed by Psychology Today Staff

The human brain includes two hemispheres connected by a bundle of nerves. The left hemisphere controls movement for the right side of the body, while the right hemisphere directs the left side. The hemispheres specialize in distinct mental functions—different aspects of visual perception, for example—but most behaviors and abilities require activity in both halves of the brain.

• How Both Sides Work Together
• “Left-Brained” and “Right-Brained” People

Both sides of the brain collaborate to handle major functions such as language processing and vision. But they are also, to a degree, specialized. Some areas of the brain are more active than others during particular tasks, and one hemisphere may be more involved than the other in specific parts of a larger mental operation.

For example, Broca’s area and Wernicke’s area are both linked to language and are most commonly located on the left side of the brain. Yet the right hemisphere is also known to play a role in language processing. Meanwhile, the limbic system—which includes the amygdalae and hippocampi and is involved in functions such as emotion and memory —resides on both sides of the brain.

In language processing, it is usually the left brain that properly orders words during speech, while in visual perception, it registers the locations of objects in space relative to other objects. 

The right brain, like the left brain , supports language, including in processing the correct meaning of a set of words with more than one possible implication (as in the case of figurative speech). And in visual perception, it processes the distance between objects.

Each brain hemisphere controls the movement of the opposite side of the body. In left-handed people, the motor cortex in the right hemisphere is dominant for fine motor behaviors, such as writing with a pencil. The reverse is true for right-handed people—the left hemisphere is stronger when it comes to such movements.

A widespread myth suggests that some people, whose left hemisphere is dominant overall, are more quantitative, logical, and analytical, while right-brained individuals are more emotional, intuitive, and creative. Like many ways of categorizing people, the left brain/right brain dichotomy is appealing , promising to teach individuals about how they think and why. But the reality of hemispheric specialization is much more complex than this popular concept suggests.

No. While the brain’s left and right hemispheres do handle distinct functions , their duties aren’t strictly divided according to broad categories like “logical” or “creative.” People can't be split neatly into groups with special strengths based on one hemisphere being more dominant.

The “left-brained” type has been described as logical, analytical, and detail-oriented, while “right-brained” people have been characterized as creative and intuitive—though other supposed traits have been attributed to each. (In any case, the "left-brained" and "right-brained" types are not scientifically supported .)

The concept of “left-brained” and “right-brained” individuals appears to stem from research in the 1960s on “split-brain” patients, whose corpus callosum (the bridge between hemispheres) had been separated. The research revealed that patients responded in different ways to stimuli such as images, depending on which brain hemisphere perceived them.

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Have you ever wondered why some dogs are left-pawed? A new study found that the handedness of the owner may play an important role in dogs’ pawedness.

New research shows how noninvasive neurostimulation of the right cerebellum improves episodic memory and may offset age-related cognitive decline.

When our usual attempts to cope with the inner turmoil of shame fail, mindfulness can help.

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Left Brain, Right Brain: Facts and Fantasies

* E-mail: [email protected]

Affiliation School of Psychology, University of Auckland, Auckland, New Zealand

• Michael C. Corballis

Published: January 21, 2014

• https://doi.org/10.1371/journal.pbio.1001767
• Reader Comments

Handedness and brain asymmetry are widely regarded as unique to humans, and associated with complementary functions such as a left-brain specialization for language and logic and a right-brain specialization for creativity and intuition. In fact, asymmetries are widespread among animals, and support the gradual evolution of asymmetrical functions such as language and tool use. Handedness and brain asymmetry are inborn and under partial genetic control, although the gene or genes responsible are not well established. Cognitive and emotional difficulties are sometimes associated with departures from the “norm” of right-handedness and left-brain language dominance, more often with the absence of these asymmetries than their reversal.

Citation: Corballis MC (2014) Left Brain, Right Brain: Facts and Fantasies. PLoS Biol 12(1): e1001767. https://doi.org/10.1371/journal.pbio.1001767

Copyright: © 2014 Michael C Corballis. This is an open-access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: Some of the research summarised in this article was funded by Contract UOA from the Marsden Fund of the Royal Society of New Zealand. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The author has declared that no competing interests exist.

“That raven on yon left-hand oak (Curse his ill-betiding croak) Bodes me no good!” —from Fables, by John Gay (1688–1732)

Introduction

The most obvious sign that our brains function asymmetrically is the near-universal preference for the right hand, which goes back at least as far as the historical record takes us, and has long been a powerful source of symbolism, with the dexterous right associated with positive values and the sinister left with negative ones [1] . This has often led to stigmatization of left-handed individuals, sometimes forcing them to switch hand use, occasionally with grievous consequences. Superstitions about left and right were compounded by the discovery, in the 1860s, that speech was based predominantly in the left hemisphere of the brain [2] . Since language itself is uniquely human, this reinforced the idea that brain asymmetry more generally is a distinctive mark of being human [3] . Because the left hemisphere also controls the dominant right hand, it came to be widely regarded as the dominant or major hemisphere, and the right as nondominant or minor. Nevertheless, further evidence that the right hemisphere was the more specialized for perception and emotion also led to speculation, some of it far-fetched, about the complementary roles of the two sides of the brain in maintaining psychological equilibrium [4] .

Interest flagged for a while, but was revived a century later, in the 1960s, with the study of patients who had undergone split-brain surgery, in which the main commissures connecting the two hemispheres were cut as a means of controlling intractable epilepsy. Testing of each disconnected hemisphere again revealed the left to be specialized for language and the right for emotional and nonverbal functions [5] , [6] . This work won Roger W. Sperry the Nobel Prize for Physiology and Medicine in 1981, but again led to speculation, most of it exaggerated or ill-founded, about the complementary functions of the two sides of the brain.

One popular example is Betty Edwards' Drawing on the Right Side of the Brain , first published in 1979 but now in its fourth edition [7] , which epitomizes the popular view that the right hemisphere is responsible for creativity. Brain imaging shows, though, that creative thought activates a widespread network, favoring neither hemisphere [8] . A more recent example is Iain McGilchrist's 2009 book The Master and His Emissary , which draws on cerebral asymmetry in a sweeping account of the forces that shaped Western culture, and provocatively declares the right hemisphere to be the dominant one (“the master”) [9] . Although widely acclaimed, this book goes far beyond the neurological facts. Polarities of left and right brain are broadly invoked in art, business, education, literary theory, and culture, but owe more to the power of myth than to the scientific evidence [10] .

Evolution of Brain Asymmetries, with Implications for Language

One myth that persists even in some scientific circles is that asymmetry is uniquely human [3] . Left–right asymmetries of brain and behavior are now known to be widespread among both vertebrates and invertebrates [11] , and can arise through a number of genetic, epigenetic, or neural mechanisms [12] . Many of these asymmetries parallel those in humans, or can be seen as evolutionary precursors. A strong left-hemispheric bias for action dynamics in marine mammals and in some primates and the left-hemisphere action biases in humans, perhaps including gesture, speech, and tool use, may derive from a common precursor [13] . A right-hemisphere dominance for emotion seems to be present in all primates so far investigated, suggesting an evolutionary continuity going back at least 30 to 40 million years [14] . A left-hemisphere dominance for vocalization has been shown in mice [15] and frogs [16] , and may well relate to the leftward dominance for speech—although language itself is unique to humans and is not necessarily vocal, as sign languages remind us. Around two-thirds of chimpanzees are right-handed, especially in gesturing [17] and throwing [18] , and also show left-sided enlargement in two cortical areas homologous to the main language areas in humans—namely, Broca's area [19] and Wernicke's area [20] (see Figure 1 ). These observations have been taken as evidence that language did not appear de novo in humans, as argued by Chomsky [21] and others, but evolved gradually through our primate lineage [22] . They have also been interpreted as evidence that language evolved not from primate calls, but from manual gestures [23] – [25] .

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Image credit: Todd Preuss, Yerkes Primate Research Center ( http://commons.wikimedia.org/wiki/File:Human_and_chimp_brain.png ).

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Some accounts of language evolution (e.g., [25] ) have focused on mirror neurons, first identified in the monkey brain in area F5 [26] , a region homologous to Broca's area in humans, but now considered part of an extensive network more widely homologous to the language network [27] . Mirror neurons are so called because they respond when the monkey performs an action, and also when they see another individual performing the same action. This “mirroring” of what the monkey sees onto what it does seems to provide a natural platform for the evolution of language, which likewise can be seen to involve a mapping of perception onto production. The motor theory of speech perception, for example, holds that we perceive speech sounds according to how we produce them, rather than through acoustic analysis [28] . Mirror neurons in monkeys also respond to the sounds of such physical actions as ripping paper or dropping a stick onto the floor, but they remain silent to animal calls [29] . This suggests an evolutionary trajectory in which mirror neurons emerged as a system for producing and understanding manual actions, but in the course of evolution became increasingly lateralized to the left brain, incorporating vocalization and gaining grammar-like complexity [30] . The left hemisphere is dominant for sign language as for spoken language [31] .

Mirror neurons themselves have been victims of hyperbole and myth [32] , with the neuroscientist Vilayanur Ramachandran once predicting that “mirror neurons will do for psychology what DNA did for biology” [33] . As the very name suggests, mirror neurons are often taken to be the basis of imitation, yet nonhuman primates are poor imitators. Further, the motor theory of speech perception does not account for the fact that speech can be understood by those deprived of the ability to speak, such as those with damage to Broca's area. Even chimpanzees [34] and dogs [35] can learn to respond to simple spoken instructions, but cannot produce anything resembling human speech. An alternative is that mirror neurons are part of a system for calibrating movements to conform to perception, as a process of learning rather than direct imitation. A monkey repeatedly observes its hand movements to learn to reach accurately, and the babbling infant calibrates the production of sounds to match what she hears. Babies raised in households where sign language is used “babble” by making repetitive movements of the hands [36] . Moreover, it is this productive aspect of language, rather than the mechanisms of understanding, that shows the more pronounced bias to the left hemisphere [37] .

Inborn Asymmetries

Handedness and cerebral asymmetries are detectable in the fetus. Ultrasound recording has shown that by the tenth week of gestation, the majority of fetuses move the right arm more than the left [38] , and from the 15th week most suck the right thumb rather than the left [39] —an asymmetry strongly predictive of later handedness [40] (see Figure 2 ). In the first trimester, a majority of fetuses show a leftward enlargement of the choroid plexus [41] , a structure within the ventricles known to synthesize peptides, growth factors, and cytokines that play a role in neurocortical development [42] . This asymmetry may be related to the leftward enlargement of the temporal planum (part of Wernicke's area), evident at 31 weeks [43] .

Image credit: jenny cu ( http://commons.wikimedia.org/wiki/File:Sucking_his_thumb_and_waving.jpg ).

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In these prenatal brain asymmetries, around two-thirds of cases show the leftward bias. The same ratio applies to the asymmetry of the temporal planum in both infants and adults [44] . The incidence of right-handedness in the chimpanzee is also around 65–70 percent, as is a clockwise torque, in which the right hemisphere protrudes forwards and the left hemisphere rearwards, in both humans and great apes [45] . These and other asymmetries have led to the suggestion that a “default” asymmetry of around 65–70 percent, in great apes as well as humans, is inborn, with the asymmetry of human handedness and cerebral asymmetry for language increased to around 90 percent by “cultural literacy” [46] .

Variations in Asymmetry

Whatever their “true” incidence, variations in handedness and cerebral asymmetry raise doubts as to the significance of the “standard” condition of right-handedness and left-cerebral specialization for language, along with other qualities associated with the left and right brains that so often feature in popular discourse. Handedness and cerebral asymmetry are not only variable, they are also imperfectly related. Some 95–99 percent of right-handed individuals are left-brained for language, but so are about 70 percent of left-handed individuals. Brain asymmetry for language may actually correlate more highly with brain asymmetry for skilled manual action, such as using tools [47] , [48] , which again supports the idea that language itself grew out of manual skill—perhaps initially through pantomime.

Even when the brain is at rest, brain imaging shows that there are asymmetries of activity in a number of regions. A factor analysis of these asymmetries revealed four different dimensions, each mutually uncorrelated. Only one of these dimensions corresponded to the language regions of the brain; the other three had to do with vision, internal thought, and attention [49] —vision and attention were biased toward the right hemisphere, language and internal thought to the left. This multidimensional aspect throws further doubt on the idea that cerebral asymmetry has some unitary and universal import.

Handedness, at least, is partly influenced by parental handedness, suggesting a genetic component [50] , but genes can't tell the whole story. For instance some 23 percent of monozygotic twins, who share the same genes, are of opposite handedness [51] . These so-called “mirror twins” have themselves fallen prey to a Through the Looking Glass myth; according to Martin Gardner [52] , Lewis Carroll intended the twins Tweedledum and Tweedledee in that book to be enantiomers, or perfect three-dimensional mirror images in bodily form as well as in hand and brain function. Although some have argued that mirroring arises in the process of twinning itself [53] , [54] , large-scale studies suggest that handedness [55] , [56] and cerebral asymmetry [57] in mirror twins are not subject to special mirroring effects. In the majority of twins of opposite handedness the left hemisphere is dominant for language in both twins, consistent with the finding that the majority of single-born left-handed individuals are also left-hemisphere dominant for language. In twins, as in the singly born, it is estimated that only about a quarter of the variation in handedness is due to genetic influences [56] .

The manner in which handedness is inherited has been most successfully modeled by supposing that a gene or genes influence not whether the individual is right- or left-handed, but whether a bias to right-handedness will be expressed or not. In those lacking the “right shift” bias, the direction of handedness is a matter of chance; that is, left-handedness arises from the lack of a bias toward the right hand, and not from a “left-hand gene.” Such models can account reasonably well for the parental influence [58] – [60] , and even for the relation between handedness and cerebral asymmetry if it is supposed that the same gene or genes bias the brain toward a left-sided dominance for speech [60] , [61] . It now seems likely that a number of such genes are involved, but the basic insight that genes influence whether or not a given directional bias is expressed, rather than whether or not it can be reversed, remains plausible (see Box 1 ).

Box 1. The Genetics of Handedness and Cerebral Asymmetry

Linkage analyses have often revealed candidate laterality genes, but all too often these fail in follow-up analysis—a common problem in the search for genes related to human behavior. Part of the problem is the sheer immensity of the genome, which means that candidates are likely to surface by chance, and the problem is compounded by the likelihood of a strong chance element in the determination of handedness itself. With appropriate statistical control, several large-scale genome-wide studies have failed to reveal any single locus to be significantly associated with handedness [68] , [69] , including one study [70] based on a large sample of twins, which also failed specifically to support the single-gene model developed by McManus [60] , or weaker versions of that model. The authors of one study estimate that as many as 40 different loci may be involved [71] , but note that it would be difficult to distinguish multilocus models from a single-gene model, such as that of McManus, in terms of handedness pedigrees.

The study of one candidate gene, PCSK6 , has led to some insight as to polygenic control of handedness. Across three independent samples of individuals with dyslexia, a genome-wide assay revealed the minor allele at the rs11855415 locus within this gene to be significantly associated with increased right-handedness [72] . This allele was not significantly associated with handedness in a large sample from the general population. Another targeted search within the PCSK6 gene failed to confirm a role for rs11855415 in a large sample from the general population, but revealed that a tandem repeat polymorphism at another locus, rs10523972, was associated with the degree, but not the direction, of handedness [73] . PCSK6 is involved in regulating NODAL, which plays a role in the development of the left–right axis in vertebrates, and knock-out of PCSK6 in mice results in defects in the placement of normally asymmetrical internal organs. Several other genes in the pathway that leads to anomalies of left–right development in mice proved to be associated as a group with human handedness in the general population, leading to the suggestion that handedness is indeed a polygenic trait partly controlled by the genes that establish body asymmetry early in development [74] .

Another gene of interest is LRRTM1 , which has been associated with handedness and schizophrenia when inherited through the father [75] , where a particular haplotype consisting of minor alleles at three locations within the gene significantly shifted handedness to the left—a finding partially confirmed elsewhere [76] . Again, though, LRRTM1 does not stand out in genome-wide assays in samples from the general population. Nevertheless, schizophrenia has long been associated with increased left-handedness or ambidexterity [77] , [78] , as have schizotypy and tendencies to magical thinking [79] – [81] . Just as the association of PCSK6 with dyslexia led to suggestion of a polygenic pathway, so the association of LRRTM1 with schizophrenia may lead to other pathways influencing handedness and brain asymmetry.

Another suggestion is that cerebral asymmetry, and even a disposition to schizophrenia, was critical to human speciation, involving a rearrangement within the X and Y chromosomes, and that it was this event that constituted the supposed “big bang” that created language de novo in our species [82] . The idea that language emerged in this saltatory fashion, still championed by Chomsky [21] , is now widely questioned [83] , [84] . Linkage analysis gives little support to the involvement of the X and Y chromosomes, although one study has shown that repeats of a CAG sequence in the androgen receptor locus on the X chromosome are linked to handedness. In females the incidence of left-handedness increased with the number of repeats, while in males it was reduced with the number of repeats. This finding supports a role for testosterone in the determination of handedness [85] . In recent formulations of the X–Y theory, it has been proposed that handedness and cerebral asymmetry are facultative traits, universally encoded in the human genome, and that the variations giving rise to schizophrenia or anomalies of handedness and cerebral asymmetry are epigenetic, and therefore not coded in the nucleotide sequence [86] . It appears that epigenetic change through DNA methylation can be transmitted between generations [87] , which might explain pedigree effects that are not detected in linkage analyses.

Another gene that has been linked to language evolution is the FOXP2 gene, following the discovery that about half the members of an extended family possessed a mutation of this gene that caused a severe deficit in articulating speech [88] . Unlike the unaffected family members, they all failed to show activation of Broca's area when asked to silently generate words, and indeed showed no consistent asymmetry at all [89] . A more recent study also shows widespread anatomical differences between the affected and unaffected family members, including bilateral reduction of the caudate nucleus in the affected members, along with a reduction of grey matter in Broca's area on the left [90] . All of the affected individuals are right-handed, though, so the effect of the mutation appears to involve the brain circuits involved in speech, and possibly more generally in language and other motor skills, but not in handedness itself. Although highly conserved in mammalian evolution, the human FOXP2 gene differs in two locations from that in the chimpanzee, leading to the suggestion that it may have played a role in the evolution of language [91] . Evidence that the most recent mutation was also present in Neanderthal DNA [92] again argues against the “big bang” theory that language evolved uniquely in humans.

Genetic considerations aside, departures from right-handedness or left-cerebral dominance have sometimes been linked to disabilities. In the 1920s and 1930s, the American physician Samuel Torrey Orton attributed both reading disability and stuttering to a failure to establish cerebral dominance [62] . Orton's views declined in influence, perhaps in part because he held eccentric ideas about interhemispheric reversals giving rise to left–right confusions [63] , and in part because learning-theory explanations came to be preferred to neurological ones. In a recent article, Dorothy Bishop reverses Orton's argument, suggesting that weak cerebral lateralization may itself result from impaired language learning [64] . Either way, the idea of an association between disability and failure of cerebral dominance may be due for revival, as recent studies have suggested that ambidexterity, or a lack of clear handedness or cerebral asymmetry, is indeed associated with stuttering [65] and deficits in academic skills [66] , as well as mental health difficulties [67] and schizophrenia (see Box 1 ).

Although it may be the absence of asymmetry rather than its reversal that can be linked to problems of social or educational adjustment, left-handed individuals have often been regarded as deficient or contrarian, but this may be based more on prejudice than on the facts. Left-handers have excelled in all walks of life. They include five of the past seven US presidents, sports stars such as Rafael Nadal in tennis and Babe Ruth in baseball, and Renaissance man Leonardo da Vinci, perhaps the greatest genius of all time.

Author Contributions

The author(s) have made the following declarations about their contributions: Conceived and written by: MC.

• 1. Hertz R (1960) Death and the right hand. Aberdeen (United Kingdom): Cohen & West.
• View Article
• Google Scholar
• 4. Harrington A (1987) Medicine, mind, and the double brain. Princeton (New Jersey): Princeton University Press.
• 7. Edwards B (2012) Drawing on the right side of the brain. New York: Penguin Putnam.
• 9. McGilchrist I (2009) The master and his emissary. New Haven (Connecticut): Yale University Press.
• 10. Corballis MC (1999) Are we in our right minds? In: Della Sala S, editor. Mind myths. Chichester (United Kingdom): John Wiley & Sons. pp. 26–42.
• 11. Rogers LJ, Vallortigara G, Andrew RJ (2013) Divided brains: the biology and behaviour of brain asymmetries. Cambridge: Cambridge University Press.
• 21. Chomsky N (2010) Some simple evo devo theses: how true might they be for language? In: Larson RK, Déprez V, Yamakido H, editors. The evolution of human language. Cambridge: Cambridge University Press. pp. 45–62.
• 22. Corballis MC (2012) Lateralization of the human brain. In: Hofman MA, Falk D, editors. Progress in brain research, Vol. 195. Amsterdam: Elsevier. pp. 103–121.
• 24. Corballis MC (2002) From hand to mouth: the origins of language. Princeton (New Jersey): Princeton University Press.
• 34. Savage-Rumbaugh S, Shanker SG, Taylor TJ (1998) Apes, language, and the human mind. Oxford: Oxford University Press.
• 50. McManus IC, Bryden MP (1992) The genetics of handedness, cerebral dominance and lateralization. In: Rapin I, Segalowitz SJ, editors. Handbook of neuropsychology, Vol. 6: developmental neuropsychology, Part 1. Amsterdam: Elsevier. pp. 115–144.
• 52. Gardner M, Carroll L (1960) The annotated Alice. New York: Bramhall House.
• 58. Annett M (2002) Handedness and brain asymmetry: the right shift theory. Hove (United Kingdom): Psychology Press.
• 60. McManus C (2002) Right hand, left hand. London: Weidenfeld & Nicolson.
• 62. Orton ST (1937) Reading, writing and speech problems in children. New York: Norton.
• 63. Corballis MC, Beale IL (1993) Orton revisited: dyslexia, laterality, and left-right confusion. In: Willows DM, Kruk RS, Corcos E, editors. Visual processes in reading and reading disabilities. Hillsdale (New Jersey): Lawrence Erlbaum Associates. pp. 57–73.

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Choosing words: left hemisphere, right hemisphere, or both? Perspective on the lateralization of word retrieval

Stephanie k. ries.

1 Department of Psychology and Helen Wills Neuroscience Institute, University of California, Berkeley, Berkeley, California

2 Center for Aphasia and Related Disorders, Veterans Affairs Northern California Health Care System, Martinez, California

Nina F. Dronkers

3 Department of Neurology, University of California, Davis, Davis, California

4 Neurolinguistics Laboratory, National Research University Higher School of Economics, Moscow, Russian Federation

Robert T. Knight

Language is considered to be one of the most lateralized human brain functions. Left hemisphere dominance for language has been consistently confirmed in clinical and experimental settings and constitutes one of the main axioms of neurology and neuroscience. However, functional neuroimaging studies are finding that the right hemisphere also plays a role in diverse language functions. Critically, the right hemisphere may also compensate for the loss or degradation of language functions following extensive stroke-induced damage to the left hemisphere. Here, we review studies that focus on our ability to choose words as we speak. Although fluidly performed in individuals with intact language, this process is routinely compromised in aphasic patients. We suggest that parceling word retrieval into its sub-processes—lexical activation and lexical selection—and examining which of these can be compensated for after left hemisphere stroke can advance the understanding of the lateralization of word retrieval in speech production. In particular, the domain-general nature of the brain regions associated with each process may be a helpful indicator of the right hemisphere's propensity for compensation.

Introduction

Language is left lateralized in 95–99% of right-handed individuals and about 70% of left-handed individuals. 1 Perhaps an even more striking testament of the left hemisphere dominance for language is that crossed aphasia, a language disorder due to a right hemisphere lesion in right handers, occurs in only 1–13% of individuals. 2

Historically, language was the first human brain function found to contradict Bichat's law of symmetry, which assumed the symmetrical representation of brain function over the left and right cerebral hemispheres. a In the 1860s, independent reports by Paul Broca and Gustave Dax indicated that speech output processes (i.e., referred to as “articulated language”) appeared to be left lateralized. b , 5 , 6 The left lateralization of language functioning was then extended to language comprehension by Wernicke, who showed a lesion in the superior left temporal lobe could be associated with a loss of what was referred to as “speech-specific sound images”. 7 The association of language functioning with the left hemisphere has been prevalent ever since these findings were reported and constitutes one of the axioms of modern neurology and neuroscience.

In this review, we focus on a process that is core to our ability to produce language: conceptually driven word retrieval, which allows us to retrieve words from long-term memory as we speak. In individuals with normal language, this process is remarkably efficient, enabling adult speakers to produce two to four words per second, selected from 50,000 to 100,000 words in the mental lexicon, and erring no more than once or twice every 1,000 words. 8 This is, however, not the case in people with aphasia, who represent approximately 1 million people in the United States, according to the National Institute of Neurological Disorders and Stroke. Word-finding difficulty is the universal complaint in these patients. 9 Thus, understanding its cerebral basis, and whether it can be compensated for after left hemisphere damage, is of primary importance.

Conceptually driven word retrieval is enabled through lexical activation and selection. Lexical activation is the process by which a set of words is quickly activated through spreading activation from a corresponding set of features in semantic memory. Thus, when a speaker wants to say the word dog , semantic features such as mammalian, domestic , and terrestrial will be activated in semantic memory. Activation from these conceptual features will spread onto a set of words such as cat, horse, rabbit , and dog . Lexical selection is the process by which the intended word is then selected from this set (see Box 1 for a short perspective on the neurobiological underpinnings of the mental lexicon and associated notions of lexical activation and selection). Lexical activation and selection are usually thought to be dissociated processes, although lexical selection is possible only if lexical activation has taken place. c , 8 , 12 , 13 , 14 , 15 , 16 It has been proposed that these two sub-processes engage different brain regions: lexical activation has been associated with left temporal regions whereas lexical selection has been associated with left lateral and medial frontal regions. 11 , 17 Although word retrieval is traditionally thought to be supported by predominantly left-lateralized brain regions, 18 an increasing number of neuroimaging studies are also pointing to the presence of right-sided brain activity when engaged in tasks requiring word retrieval. 18 , 19 , 20 A key question concerns the nature of these right-sided brain activities in word retrieval: Are these activations merely epiphenomenal or do right-sided brain regions play a causal role in supporting word retrieval?

Short perspective on the neurobiological underpinnings of the mental lexicon

The concept of a mental lexicon and associated notions of lexical activation and selection stem from the field of psycholinguistics. There, words and how they are retrieved have been modeled in different ways, especially using neural network models (see Refs. 10 or 74 for examples of language production models). However, the neurobiological underpinning of these cognitive representations and how they are accessed remain to be investigated and constitutes a fascinating topic for future investigations. One promising direction is that of recent electrocortigraphic studies investigating the electrical oscillation patterns associated with different speech gestures, phonetic features, and words recorded directly at the cortical surface. 149 , 150 , 151 , 152 These linguistic units can be represented through distinct patterns of cortical oscillations (usually in the high gamma range: between 70 and 150 Hz) involving more or less extended regions of the human cortex. For example, Mesgarani et al . have shown that different populations of neurons in the superior temporal cortex (STG) are selective for different phonetic features and are hierarchically organized around acoustic cues (e.g., manner of articulation was found to be a stronger determinant for neuronal selectivity than place of articulation, which is also a less discriminant acoustic cue than manner of articulation). 150

An extensive investigation of the cortical representations of words is less possible than with phonemes, given the exponential number of words in comparison to phonemes. Nevertheless, Pasley et al . 152 and Martin et al . 153 have shown that it is possible to decode the words that were heard or read (overtly and silently) by patients with relatively high accuracy by looking at cortical high-gamma activity, again predominantly recorded over the STG but also over the pre- and post-central gyri and higher-order cortical areas. These studies offer a rare window on the fine-grained spatiotemporal dynamics associated with linguistic properties and how they are represented in the brain. Similarly as for other percepts, the organization of the cortical representations of speech sounds seems hierarchical in that more simple acoustic features, such as tone, are represented in lower-level cortical areas, such as Heschl's gyrus, 154 and higher-order acoustic features, such as manner of articulation, are represented in higher-order cortical areas, such as the STG. 150 We can therefore imagine that for higher levels of abstraction, such as words or semantic categories, more extensive and associative regions will be involved, as has been shown, for example, in the visual domain. 155

The study of the neurobiological underpinnings of word retrieval—including how the spread of activation takes place from concepts to words and how the correct word is then selected—is still unfolding. Interesting parallels may be made from studies looking at the neuronal basis of decision making, where biologically plausible models such as the drift-diffusion model have been implemented and tested with neuronal data, such as the firing rates of single neurons. 156 We note that evidence accumulation has recently been proposed to be a plausible model for the lexical selection process using naming latency data. 157 Future studies will therefore need to investigate how such models may also serve to explain neuronal data associated with word retrieval.

This question is of direct clinical relevance for individuals with left-sided stroke-induced aphasia. Determining which aspect of language production can or cannot be compensated for by their intact right hemisphere is crucial for these patients, as this information could potentially guide treatment options. In addition, assessing the effect of focal brain injury on specific cognitive functions remains the most reliable way to understand causality of human brain function d . Therefore, if specific aspects of language production cannot be compensated for after left hemisphere stroke, it can be taken as evidence that this component of language critically relies on the left hemisphere. We will review evidence supporting the idea that processes involved in word retrieval may be differentially compensated for after left hemisphere stroke-induced lesions and will suggest hypotheses as to why this could be the case.

There is debate as to the role of the right hemisphere in compensating for left hemisphere stroke-induced language impairment. Some researchers have argued for such a role, 21 , 22 , 23 , 24 , 25 while others have suggested that right hemisphere recruitment is sub-optimal in comparison to peri-lesional recruitment, 26 , 27 , 28 or even maladaptive to recovery of language functions. 29 , 30 , 31 , 32 However, results can be very different depending on the extent of the left hemisphere lesion, 33 and the relative involvement of the right hemisphere in language may be dependent on time post-stroke. 34 In addition, age of stroke onset has been shown to have a strong influence on functional outcomes in studies performed in adults, 35 and even more clearly in studies comparing children to adults. 36 In this review, we focus on studies performed in adults. Perhaps one of the most compelling pieces of evidence for the role of the right hemisphere in language in adults are the cases of left hemisphere stroke patients whose language is further impaired after a second right hemisphere stroke, 21 , 25 suggesting that not only is peri-lesional tissue involved in compensating for the degradation of language function after injury to the left hemisphere, but that right hemisphere brain regions may play a causal role as well. Evidence supporting this idea also comes from intracarotid amytal injections (i.e., WADA test) in which right-sided anesthesia was found to affect remaining expressive language abilities in patients with left-sided hemisphere injury. 37 , 38 This brief overview highlights that the right hemisphere may play a causal role in compensating for some language deficits after left hemisphere stroke. What remains unclear is which aspect of language, including word retrieval, can or cannot be supported by the right hemisphere. Indeed, language cannot be seen as one unitary function that is either intact or uniformly damaged; instead, it is a sum of sub-processes that rely on distinct underlying physiological functions (or factors). 39 We will argue that efforts to understand why word retrieval can or cannot be compensated for by the right hemisphere could benefit from focusing on the subprocesses that word retrieval relies on (for similar approaches in language in general, see Refs. 19 and 40 ).

While communication functions, such as prosody (but see Ref. 41 ), 42 , 43 , 44 , 45 pitch, 46 and certain aspects of discourse-level processing, 47 have been claimed to be right lateralized, this review will only focus on single word retrieval. First, we will focus on the left hemisphere regions supporting word retrieval and the consequences of stroke-induced lesions to these regions. Second, we will discuss the right hemisphere regions engaged in word retrieval and their potential role in compensating for disruption of word retrieval caused by left hemisphere lesions. In these sections, we will review results from both the functional imaging literature in healthy individuals and stroke patients and lesion-symptom mapping approaches in stroke patients. Finally, in our discussion, we will propose hypotheses as to why the different subprocesses of word retrieval can or cannot be compensated for by the right hemisphere.

The role of the left hemisphere in word retrieval

Left hemisphere regions associated with word retrieval in the healthy brain.

As reviewed by Price, 18 many different left hemisphere regions of the frontal and temporal lobes have been associated with word retrieval in studies using functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). These regions include posterior regions in the left middle and inferior temporal gyri (MTG and ITG), 48 , 49 , 50 , 51 , 52 , 53 , 54 and, more rarely, the superior temporal gyrus (STG) 54 , 55 and left hippocampus; 55 , 56 , 57 the left superior, middle, and inferior frontal gyri (MFG and IFG); 56 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 and medial frontal regions, such as the pre-supplementary motor area (pre-SMA) 69 , 70 , 71 and the anterior cingulate cortex (ACC). 17 , 72 , 73 Such a broad spread of participating regions implies that word retrieval has multiple components, or may even interact with other cognitive domains. To tease this out, many of these studies have relied on the idea that word retrieval is a competitive process. 12 , 8 , 74 Thus, when a speaker aims to say the word apple , not only will that word become activated, e but so will its semantically related neighbors (e.g., pear, orange, banana). These semantically related words interfere in the process of selecting the correct word, a notion that is supported by a category of speech errors referred to as semantic errors (e.g., “put the milk back in the oven”) and also by experimental findings. 76 , 77 , 78 , 79 , 80 For example, in the picture–word interference paradigm, 76 participants have to name pictures on which a distractor word is superimposed ( Fig. 1 ). Performance is worse if the distractor word is from the same semantic category (e.g., picture of an apple with the distractor word pear ; Fig. 1A ) than when the distractor word is unrelated (e.g., picture of an apple with the distractor word car ; Fig. 1B ). 11 , 17 , 48 This effect is referred to as the semantic interference effect and is thought to reflect increased difficulty in word retrieval. Other paradigms eliciting semantic interference effects have been used in these studies, 50 , 59 , 80 as well as verb generation, 53 , 58 , 63 , 64 , 65 , 72 synonym/antonym generation, 67 verbal fluency, 56 , 60 , 68 , 69 simple picture-naming tasks, 49 , 51 , 52 and tasks comparing free versus constrained word generation. 70 , 71 , 73

Example stimuli for the picture–word interference paradigm. (A) Shown is a stimulus in which the distractor word is semantically related to the picture. (B) Shown is a stimulus in which the distractor word is semantically unrelated to the picture. Participants are instructed to name the picture as fast and as accurately as possible while ignoring the distractor word. Performance is typically worse for the type of stimuli shown in A than for the type of stimuli shown in B. This effect is referred to as the semantic interference effect.

When task difficulty is increased, brain regions that help resolve this difficulty are predicted to show increases in functional activation. Such contrasts have suggested that frontal and temporal regions may be differentially involved in sub-processes of word retrieval, such as lexical activation and lexical selection. Schnur and colleagues 59 reported that the blood oxygen level–dependent (BOLD) signal in both the left IFG and left MTG was sensitive to semantic interference, using the blocked-cyclic picture-naming paradigm introduced by Kroll and Stewart 78 ( Fig. 2 ). However, only activation in the left IFG positively correlated with the amount of errors made: participants with large left IFG activation in the semantically homogeneous condition (i.e., where there is more interference from semantically related alternatives; Fig. 2A ) made more errors in this condition compared to the semantically heterogeneous condition (i.e., where there is less interference from semantically related alternatives; Fig. 2B ). Such a correlation was not found for left MTG activation. The authors concluded that only the left IFG is necessary for the resolution of increased competition between semantically related alternatives in the paradigm they used, in agreement with what had already been suggested in verb generation tasks. 58 , 81 According to these studies, the left IFG would play a key role in lexical selection rather than lexical activation.

Example of stimuli used in the blocked-cyclic picture-naming paradigm in which pictures are presented either within semantically homogeneous (A) or heterogeneous (B) blocks. Pictures are repeated several times per block, usually five or six times. Participants are instructed to name the picture as fast and as accurately as possible. Performance is worse in homogeneous than in heterogeneous blocks. This effect is referred to as the semantic interference effect.

Piai and colleagues also compared left temporal versus frontal activity using the picture–word interference paradigm in both magnetoencephalography (MEG; using source localization) 11 and fMRI. 17 They found distinct responses to semantic interference in the following areas: activity in the superior frontal gyrus and ACC was larger for semantically related than for unrelated distractor words (they used the picture–word interference paradigm exemplified in Figure 1 ), whereas activity in the left temporal cortex, and more specifically, the anterior STG and posterior MTG and STG, was larger for unrelated than for related and identical distractor words ( Fig. 3 ), in agreement with previous reports. 82 On the basis of these results, the authors suggested that the left superior frontal/ACC activity reflects selection among competing alternatives, whereas the left temporal activity reflects lexical activation. The reduced activity in the temporal cortex in response to related compared to unrelated distractor words (i.e., facilitation effect) would be because of the greater semantic distance between the picture name and the distractor when unrelated. This effect has been interpreted with respect to semantic priming, similar to what is observed in the speech comprehension literature. 83 Importantly, the use of a time-resolved technique, such as MEG, enabled the localization of the effects in a time window compatible with a role of these regions in word retrieval (between 350 and 650 ms after stimulus presentation), which is not possible with fMRI. Increased ACC activity for high versus low selection nouns was also reported in the verb generation task, 72 in agreement with a possible role for this region in lexical selection among competing alternatives. However, semantic interference effects have also been reported in the left temporal cortex (middle and posterior portions of the left MTG or posterior STG) using the blocked-cyclic picture-naming paradigm. 59 , 55 Further investigation is needed to clarify which parts of the left temporal cortex are involved in lexical activation and selection, and at which point in time this occurs.

(A) Shown on the left is the estimated source based on whole-brain analysis for the semantic interference effect (more activity for semantically related than for unrelated distractor words in the picture–word interference paradigm) for total time–frequency power (i.e., phase locked and non-phase locked). On the right, dashed rectangles in the time–frequency plot indicate the spectrotemporal cluster of interest (4–8 Hz, 350–650 ms after stimulus presentation). In this cluster, a relative power increase was observed in the left superior frontal source only. (B) Shown on the left is the estimated source based on whole-brain analysis for the semantic facilitation effect (more activity for semantically unrelated than for related distractor words in the picture–word interference paradigm) for evoked brain activity (i.e., phase locked) in the significant temporal cluster (between 375 and 400 ms post-stimulus). Shown on the right is activity of the left temporal cortex averaged over the estimated sources for the different distractor types. Adapted, with permission, from Piai et al . 11

The role of the brain regions associated with word retrieval can also be dissociated on the basis of whether these regions have a more generic role, that is, whether they are also associated with other cognitive functions. Frontal regions in general have been associated with cognitive control processes in other domains and are not believed to be specifically associated with language 84 , 85 , 86 , 87 (although see Ref. 88 ). For example, Jonides and Nee have suggested the left IFG may be involved in resolving proactive interference between representations in working memory, 87 and Kan and Thompson-Schill suggested that this interference resolution process might be enabled through biased selection. 85 As reviewed by Ridderinkhof and colleagues, 86 , 89 the pre-SMA and ACC have been associated with response selection and monitoring outside of language. Thus, the increase in cognitive demands required for resolving semantic interference may call upon the domain-general cognitive control capacity of the frontal lobe. The distinction between domain specificity and generality is, however, less clear for left temporal regions.

This pattern of frontal versus posterior association with a specific cognitive process has been described more broadly by Fuster and colleagues (e.g., Refs. 90 and 91 ). Within the framework described by these authors, frontal brain regions, involved in execution, are linked to perceptual regions in the parietal, occipital, and temporal lobes to form “cognits,” which are different (although they can overlap partly) depending on the cognitive function involved. Thus, in the case of word retrieval, the posterior MTG and ITG could represent the perceptual component of the cognit, and the LIFG and pre-SMA/ACC could represent the executive component. The co-activation of these brain regions when retrieving words while speaking could be why it is sometimes difficult to dissociate the respective roles of these brain regions. The discussion in the next section, however, indicates that the deficits associated with damage to either of these brain regions do support this role distinction in the perception/action cycle.

Insights from stroke-induced aphasia on the causal role of left hemisphere cortical regions associated with word retrieval

In this section, we first review studies using different methodologies to identify which brain regions may be critical for word retrieval in aphasic individuals, including lesion–symptom correlations and voxel-based lesion–symptom mapping (VLSM) in chronic stroke patients, 92 and reperfusion functional imaging in acute stroke patients. VLSM is a statistical voxel-by-voxel analysis that infers which brain regions are critical for task performance. Reperfusion imaging is typically based on diffusion-weighted imaging measures obtained immediately after stroke and after a few days post-stroke (e.g., 3–5 days in Ref. 93 ). Reperfusion of a given cortical area is defined as hypoperfusion at day 1 and normal perfusion at follow-up. This technique infers which brain region is critical in regaining specific abilities in the first days post-stroke by correlating improvement in task performance between the two times of testing with the reperfusion measures. Second, we review functional imaging studies in chronic stroke patients that identify which brain regions may be involved in word retrieval recovery.

Even a cursory glance at the literature on aphasia will assert the importance of the left temporal lobe in word retrieval. Clinically, it is well known that aphasic persons with the most severe word retrieval deficits are those few patients with persisting Wernicke’s aphasia, subsequent to left temporal lobe injury. These individuals make pervasive paraphasic errors in which target words are substituted with incorrect words, making their remarks difficult to understand. For example, in describing a man flying a kite, one man said, “They have there/their young men, tree of the yellow that they use the marrows of the light of the wood.” Such clear demonstrations of word retrieval deficits are also reflected in their poor object or picture-naming abilities, where target names are substituted with other words and/or jargon. Importantly, these individuals also fail on most comprehension tasks, even single word comprehension, and may not recognize the correct word even when it is given to them. Likewise, their picture–word or word–word matching performance is also compromised. However, the same patients easily demonstrate what the object is used for, never misuse such objects, and in most cases, carry on leading normal lives, except for their severe communication deficit. As described by Dronkers and colleagues, 94 it is the lexical representations that are lost in this patient group, or, the ties between lexical representations and their underlying concepts.

Traditionally, such word retrieval deficits have been associated with the left posterior superior temporal gyrus (Wernicke’s area), but recent work has shown the importance of the left posterior middle temporal gyrus (pMTG) and underlying white matter in such a persisting disorder. 94 , 95 When large lesions occur in the pMTG and run deep into the fiber pathways that course beneath it, the effects of these lexical-semantic deficits are long lasting. In contrast, patients with posterior STG lesions tend to recover successfully within the first year post-onset of the disorder, though milder deficits may remain. Thus, the left pMTG is critical for lexical activation, as lesions here cause permanent impairments that do not resolve over time. f This evidence converges nicely with what has been suggested by the neuroimaging studies in healthy speakers. 11 , 17

The relationship between the pMTG and lexical activation has been confirmed numerous times in subsequent studies using VLSM in larger numbers of patients. For example, Baldo and colleagues showed how stroke-induced lesions to the left pMTG are associated with persisting picture-naming difficulties. 96 The authors argue that this difficulty stems specifically from word retrieval deficits, as they used verbal fluency scores as a covariate to partial out brain regions associated with speech output processes and control for visual recognition deficits. This is the same brain region found to be associated with persisting comprehension deficits at the word level in the chronic stroke patients described above ( Fig. 4 ). 92 , 94 As discussed below, patients with lesions restricted to the left prefrontal cortex (PFC), and especially the left IFG, do not have the same types of deficits.

(A) Shown are significant voxels (as obtained from a VLSM analysis) associated with impaired picture-naming performance. Here, the effect of speech production deficits was covaried out. All voxels shown in color exceeded the critical threshold for significance, and the colors reflect increasing t -values from 4.43 to 6.06 (shown in purple to red). (B) In red, the VLSM area was found to be associated with single word comprehension deficits. Adapted, with permission, from Baldo et al . 96 and Dronkers et al . 94

Reperfusion imaging in acute stroke patients has shown that both the left posterior MTG and inferior temporal/midfusiform gyri are critical for naming: reperfusion of these regions correlated with improved naming 3–5 days after initial scans. 93 This was also true for the posterior STG and left IFG but to a lesser degree. DeLeon et al . examined how deficits at different stages of speech production correlated with hypoperfusion in different cortical areas 97 and showed that hypoperfusion in the left posterior inferior temporal/midfusiform gyri correlated most strongly with impairment at the level of modality-independent lexical activation (i.e., the inability to name pictures in either the oral or written modality). The authors, however, did not differentiate lexical activation from lexical selection, and what they referred to as modality-independent lexical activation can be assimilated to word retrieval in our terminology. The importance of these regions for word retrieval has also been shown in patients with neurodegenerative diseases, such as in the semantic variant of primary progressive aphasia and semantic dementia, as well as epilepsy. 98 , 99

Lesions in the left frontal lobes, and particularly in the inferior frontal gyrus, have also been associated with word finding difficulty. 59 , 100 , 101 , 102 Importantly, these deficits are not found with unilateral right PFC lesions ( Fig. 5 ). 101 The deficits caused by left IFG lesions can be described as being of a different nature than those occurring after left temporal lobe lesions. Patients with lesions in the left IFG often know what they want to say but have trouble narrowing their search to the specific word. When given a choice between a few options or the onset of the target word, they can immediately identify the word they were looking for. g , 104 This differentiates them from patients with left MTG lesions, as shown by Schnur et al . who directly compared the performance of patients with left IFG lesions to that of patients with left MTG and STG gyri lesions in a task eliciting semantic interference (i.e., the blocked-cyclic picture-naming paradigm, in which pictures are repeated several times per block). 102 They showed that the semantic interference effect increased linearly across cycles, caused by increasing interference from semantically related alternatives in the homogeneous versus heterogeneous blocks, but only in patients with larger left IFG lesions. Thus, when a significant portion of the left IFG is damaged, overcoming the activation of semantically related words becomes progressively more difficult with the repetition of these semantically related neighbors. Patients with smaller left IFG lesions or left temporal lesions did not show this pattern. This, along with other evidence, 59 , 100 , 101 converges with the neuroimaging findings reported above for healthy speakers in suggesting that the left IFG is involved in overcoming interference caused by semantically related alternatives in the process of lexical selection. Left IFG lesions have been found to be associated with deficits in other processes, which has led several researchers to argue for a domain-general role of this brain region, in agreement with neuroimaging findings in healthy speakers. Thus, patients with left IFG lesions have been found to be impaired in the recent probes test that measures the ability to overcome proactive interference in working memory. 105 , 106 A number of researchers, including from our laboratory, have suggested that the left IFG plays a role in the anticipatory control of action. 87 , 101 , 107 Interestingly, recent results suggest that the left inferior frontal-occipital fasciculus (IFOF), which links the left IFG with posterior temporal regions, is engaged in both the resolution of semantic interference in picture naming and in working memory. 108 , 109 These conclusions again fit very well within the framework proposed by Fuster and colleagues, 91 where the perceptual component of the cognit (here, the posterior MTG/ITG) is critical for supporting the memory of words, whereas the executive component of the cognit (here, the left IFG) is critical for supporting the selection of these words for production. The coactivation of these brain regions and their interaction, possibly through the IFOF (or other temporal-frontal tracts, such as the arcuate fasciculus) would thus support efficient word retrieval in the healthy brain. Any injury to either part of this network is therefore expected to affect this cognitive function.

(A) Lesion overlapping of the seven left (top) and six right (middle) PFC patients included in the analyses. Left PFC patients' lesions are centered in both the inferior frontal gyrus and the middle frontal gyrus. Right PFC patients' lesions are centered in the middle frontal gyrus. (B) Semantic context effect in a blocked-cyclic picture-naming task on error rates. Values for semantically homogeneous blocks (HOM) are depicted by the solid lines and values for the semantically heterogeneous blocks (HET) are depicted by the dotted lines. Mean values for cycles 2–6 are presented (in this paradigm, pictures are presented several times per block). Standard deviations are represented by the vertical lines (only positive values are presented for the homogeneous condition and only negative values are presented for the heterogeneous condition, for visual clarity). The semantic context effect (difference between homogeneous and heterogeneous blocks) is larger in left PFC patients than in right PFC patients and controls. Adapted, with permission, from Ries et al . 101

Several studies examining the brain correlates of recovery from stroke-induced aphasia have shown that the recruitment of peri-lesional tissue in the left hemisphere is positively correlated with recovery, 26 , 27 , 28 , 110 , 111 and this is also true for word retrieval. 24 , 112 Perani et al . reported functional neuroimaging findings in aphasic patients performing verbal fluency tasks. 112 These patients had lesions in different sites, but importantly, in the three patients with good recovery, the activation foci involved predominantly perilesional or undamaged regions of the language-dominant hemisphere. (One patient with crossed aphasia had a focus of activation in the right hemisphere.) Weiller et al . tested six recovered Wernicke's aphasia patients with lesions in the posterior parts of the left superior temporal gyrus, large parts of the left MTG and angular gyrus, and large parts of the posterior arcuate fasciculus. 24 These patients were PET scanned while performing verb generation and word repetition tasks. The most rostral portion of the IFG and middle part of the MFG were the only regions that showed more activation in the verb generation than in the word repetition task, and patients showed enhanced regional cerebral blood flow (rCBF) in these regions compared to controls. This argues for a role of the left frontal region in compensating for word retrieval deficits caused by lesions to the left posterior superior and middle temporal cortices. Alternatively, the increased frontal activation could reflect an increased effort in cognitive control to try to select words as a consequence of reduced lexical activation in the left temporal lobe. As we review below, these patients also showed rCBF increases in the right hemisphere.

Medial frontal regions, including the ACC and SFG, show increased activation in patients with word retrieval difficulties compared to controls, 113 and this is also the case for other language functions such as sentence comprehension 34 , 114 and word repetition. 115 Because these brain regions are involved in word selection and action monitoring processes outside of language, 86 , 89 Garenmayeh and colleagues have suggested that upregulation of activity in these regions following stroke can be explained by the fact that patients recovering from stroke-induced aphasia rely on domain-general processes in order to compensate for language deficits. 40 As discussed below, the same interpretation has been proposed for increased right frontal activity in these patients.

To summarize our discussion of the left hemisphere, abundant evidence demonstrates its role in supporting word retrieval. Aphasic individuals with deep left temporal lesions, particularly involving the pMTG, have the most severe lexical activation deficits that do not recover over time. Left lateral frontal patients also show retrieval deficits, but these deficits tend to be more in lexical selection with a different pattern of errors and larger semantic interference effects. Individuals with right hemisphere injury do not demonstrate deficits in word retrieval, regardless of whether frontal or temporal lobes are involved, except in rare cases of crossed aphasia. Functional neuroimaging studies also show that left temporal, left lateral frontal, and medial frontal areas are associated with different aspects of word retrieval in healthy speakers. Here, the dissociation appears again: left temporal regions support lexical activation, while left frontal areas support lexical selection. The latter may relate to domain-general cognitive control mechanisms that affect other cognitive domains as well. Finally, functional neuroimaging in individuals recovering from left hemisphere-induced aphasia show predominantly perilesional activation but also activation in the lateral and medial PFC of the left hemisphere.

The role of the right hemisphere in word retrieval

Right hemisphere regions associated with word retrieval in the healthy brain.

Although the right hemisphere has not typically been the focus of neuroimaging studies of word retrieval, many fMRI studies often report right hemisphere activation. 55 , 59 , 65 This right hemisphere activation is often smaller and less robust than left hemisphere activation. Right PFC activity has been shown to increase when word selection difficulty is increased. 55 , 59 , 65 , 116 , 117 For example, Buckner et al . compared two tasks, 65 stem completion versus verb generation, and found anterior right frontal activation (in the vicinity of the anterior MFG and IFG) only in the verb generation task, which requires more selection than the stem-completion task. In addition, it has been suggested that age-related increases in the activity of this region are due to increased difficulty in word retrieval in older relative to younger participants. 118 Right hemisphere activation modulated by word retrieval difficulty has also been reported for the temporal lobe. Schnur and colleagues reported BOLD activation in the right superior temporal gyrus that was sensitive to the difficulty of word retrieval. 59 Finally, using perfusion fMRI, both left and right hippocampi were found to be sensitive to the difficulty of word retrieval. 55

According to a meta-analysis by Vigneau, 19 the participation of the right hemisphere in lexico-semantic processes, including word retrieval in language production, is low relative to the left hemisphere: 12 out of 34 contrasts looking at semantic associations (i.e., verb generation) were associated with bilateral activation, while 22 activated only left hemisphere regions. Only two clusters of right hemisphere activity associated with lexical-semantics were found in the right inferior frontal lobe. However, because these same clusters were found to be involved in other language processes (such as syntactic processing) and in tasks involving manipulation of verbal material in working memory, the authors argued for a non-specific involvement of these right frontal areas. This was also suggested by Basho et al ., who interpreted the activation observed in the right middle frontal gyrus and right anterior cingulate as being linked to sustained attention or working memory. 117 Thus, the right frontal activation found in language studies may not be specific to language, as also suggested by Geranmayeh and colleaugues. 40 As mentioned earlier, the same suggestion has been made for the left frontal activation also found in tasks looking at proactive interference resolution in working memory. 87

Insights from stroke-induced aphasia on the role of right hemisphere cortical regions associated with word retrieval

We are not aware of studies reporting word retrieval deficits following unilateral right hemisphere stroke-induced lesions, except in cases of crossed aphasia. 119 This suggests right hemisphere regions do not generally play a causal role in word retrieval or that left hemisphere contributions are sufficient to assume any lost ability. However, a possible compensatory role of right hemisphere regions, and particularly right frontal regions, in recovery from left hemisphere stroke-induced word retrieval deficits has been proposed. h , 22 , 24 , 104 , 112 , 122 , 123 , 124 , 125 , 126

Blasi et al . showed that the right frontal cortex may play a role in word retrieval learning in patients with left frontal lesions. 22 Specifically, the right frontal cortex was more activated in these patients than in controls in a word-stem completion task. Importantly, verbal learning evidenced by decreases in error rates and reaction times with the repetition of stimuli was accompanied by a decreased BOLD signal in the right IFG in patients with left frontal lesions but not in controls. In the controls, this pattern was observed in the left IFG and other regions. The patients with left frontal lesions were able to perform normally on a verbal learning task that typically engages the left frontal cortex, suggesting a compensatory role of the right IFG in verbal learning following left frontal infarct. The causal role of the right IFG in compensation for word retrieval deficits was tested by Winhuisen and colleagues, 125 using repetitive transcranial magnetic stimulation (rTMS) over the right IFG in aphasic patients with impaired verbal fluency, with the underlying assumption of creating transient dysfunction in this region. They reported a decrease in verbal fluency performance caused by rTMS to the right IFG in patients with limited perilesional recruitment determined from PET scans. This finding would argue for a role of the right IFG in compensating for deficits at the level of lexical selection. Conflicting rTMS results have also been reported, such that rTMS to the right IFG has facilitated aphasia recovery. 30 , 31 Differing results could be explained by the fact that Winhuisen et al . tested for right IFG activity (using PET) in addition to performing the rTMS study. 125 As suggested by Rijntjes et al ., providing information on the state of activation of right hemisphere regions pre-rTMS is critical in the interpretation of rTMS results. 127

Although the right frontal cortex may be able to play a compensatory role in word retrieval following left hemisphere lesions, it does not appear to be able to completely replace the functions of the lesioned left frontal lobe. Perani et al . reported that patients with poor performance in semantic verbal fluency had extensive left and right dorsolateral prefrontal activation. 112 Patients who recovered well showed activation in left IFG, a similar pattern to controls. The bilateral involvement in patients with poor recovery was interpreted as reflecting increased “mental effort” in the task, compared to patients who had a functioning left IFG. Furthermore, Buckner et al . reported right frontal cortex recruitment, with a peak in the right inferior frontal cortex, in a word completion task in a patient with left frontal damage (tested 1 month post-onset). 104 This region was activated to a greater extent in this patient compared to controls in the same task. This patient was, however, impaired in verb generation and other tasks involving generating more than the cued word. This suggests that the involvement of the right inferior frontal cortex was not able to completely overcome word retrieval deficits caused by the left frontal lesion. The authors suggest that this compensatory mechanism is unable to suppress more dominant responses while still allowing the selection of words under non-competitive conditions. The recruitment of right frontal areas in compensatory mechanisms for word retrieval deficits appears to depend on the extent of the left hemisphere lesion: in a study of two patients by Vitali et al ., only the patient with complete destruction of Broca's area showed an activation of the right homologous area post-training. 122 For the other patient, who had a smaller lesion partially sparing Broca's area, better performance was achieved post-treatment and was associated with left peri-lesional activation.

Finally, right frontal activation in word retrieval following left hemisphere stroke have been suggested to reflect more domain-general attentional recruitment, as has also been suggested in aphasia recovery in general 40 and in the studies performed in healthy speakers reviewed above. Weiller et al . found that patients with left temporal lesions had increased rCBF in right homologous areas (posterior STG, IFG, and MFG) compared to controls in both verb generation and word repetition, and also showed an additional area of activation in the right IFG that controls did not show. 24 The recruitment of the right IFG was related to intentional mechanisms or increased sustained attention for perception and comprehension of the stimulus nouns. Indeed, because this right inferior frontal activation was not stronger in verb generation than in word repetition, it was not interpreted as being involved specifically in word retrieval. Recruitment of right lateral and medial (pre-SMA) frontal regions in recovery from non-fluent aphasia can also be facilitated by certain types of aphasia therapies targeting nonspecific cognitive control processes. For example, Crosson et al . suggested that therapies focused on enhancing intention can increase the recruitment of these regions. 123 In addition, brain regions not typically associated with word retrieval may also be involved in recovery from word retrieval deficits in aphasia, 126 including the precuneus, right entorhinal cortex, thalamus, and left inferior parietal regions. 110 , 111

To summarize this section, right hemisphere activation, albeit weak, has been observed during word retrieval in the healthy brain. In brain-injured patients, the right frontal cortex appears to play a role in compensatory processes following word retrieval deficits, 22 , 125 particularly for lexical selection deficits. Otherwise, right frontal recruitment following left hemisphere lesions is usually suboptimal compared to perilesional recruitment. 104 , 112 , 122 This is consistent with the findings reported in the broader literature on aphasia recovery, including in the case of syntactic processing. 26 , 27 , 28 , 128 Indeed, right frontal regions that are activated following left hemisphere stroke cannot completely overcome word retrieval deficits, presumably because these right frontal regions are predisposed for other functions. 101 , 104 This also seems to be the case when left focal injuries occur early in life, as tested in individuals who have sustained a pre- or perinatal left hemisphere stroke. 129 Indeed, even when the injury occurs early in life, the right hemisphere seems unable to fully accommodate language functions. Finally, some studies suggest that right frontal activation may reflect more domain-general attentional recruitment in both patients and controls and is not specifically associated with linguistic processes. Instead, right frontal activation seems to be involved in cognitive control functions that have mostly been described in non-linguistic actions and that are eventually recruited when word retrieval is difficult. The precise role that these right frontal regions play to help compensate for language deficits after left hemisphere stroke needs to be specified in future studies. Indeed, different parts of the right frontal cortex have been associated with different cognitive control processes in actions in general (see Refs. 86 and 89 for reviews), but how and when they are involved when language functioning is damaged still needs to be investigated (however, see Ref. 130 for a possible role of the right IFG in linguistic response inhibition in healthy speakers).

Discussion: hemispheric asymmetries in word retrieval

Consistent with known clinical findings, the left hemisphere has a higher potential for word retrieval compared to the right hemisphere. However, right hemisphere regions, and especially right inferior frontal regions, may engage in compensatory mechanisms following stroke to the left hemisphere regions associated with word retrieval. The potential of right hemisphere regions to help compensate for word retrieval deficits following left hemisphere stroke-induced aphasia appears to be different depending on the specific sub-process that is disrupted. Thus, right hemisphere regions and, in particular, right frontal regions appear to be better (although not optimal) at compensating for lexical selection than lexical activation deficits. As discussed earlier, this finding may reflect recruitment of more domain-general processes rather than linguistic ones.

Why is there a left hemisphere bias for word retrieval?

As mentioned earlier, the enhanced role of the left compared to the right hemisphere for language in general has been known for over a century. Many studies have sought to understand the reasons for this left hemisphere bias. Here, we briefly review studies that hint at why word retrieval or the ability to link concepts to words is predominantly left lateralized.

In a series of experiments by De Renzi and colleagues on left hemisphere–lesioned patients that aimed to assess the ability of the left hemisphere in what was referred to as “associative thought,” 131 , 132 , 133 the authors were looking for non-verbal correlates of the ability to link different forms of an object to a unified concept (e.g., sound of a siren with the picture of its source, or picture of a clothed baby doll with an actual doll of a different form). Associative thought, as assessed by, for example, object-figure matching, was found to be more impaired after left rather than right hemisphere lesions. 131 In addition, Faglioni and colleagues found that performance on another test aimed to assess associative thought (i.e., sound–object matching test) was correlated with both the presence and the degree of aphasia: patients with greater language deficits performed worse. 132 More specifically, Saygin and colleagues tested for the domain specificity of the cortical regions involved in associative thought using a sound–object matching task closely matched with a word–picture matching task in stroke patients. 134 These authors found that similar cortical regions, including mainly the left posterior MTG and STG (i.e., Wernicke's area), contributed to performance in both tasks, using a VLSM analysis. It is thus tempting to think that this common brain substrate critical for amodal associative thought may be at the basis of the involvement of the pMTG in word retrieval and especially lexical activation. Although associative thought is not linguistic in nature, it is of primary importance in language and particularly in word retrieval. A stronger capability of the left hemisphere for associative thought may thus underlie its stronger capability for word retrieval. However, it is difficult to draw a causal link between the two capabilities on the basis of these studies. Indeed, one could argue that the reason why associative thought is supported by left hemisphere regions is because it also supports language, especially in the case of the pMTG (see Saygin et al . 134 for a stronger association of the pSTG with the sound–picture matching task). This chicken-or-egg type of problem recurs often in searching for the cause of the stronger potentiality of the left hemisphere for language.

Studies on split-brain patients have shown that the difference between the left and right hemispheres may be more quantitative than qualitative, including at the level of lexical semantics: Gazzaniga and Hillyard suggested that the right hemisphere can attach noun labels to pictures and objects, only not as well as the left hemisphere (note, however, that only very few split-brain patients showed this ability). 135

More recent research has tried to elucidate the basic physiological mechanism(s) underlying associative thought in auditory speech comprehension. i As suggested by Bornkessel-Schlesewsky and colleagues, dependency-based combinatorics may provide such a mechanism. 136 According to these authors and others, 137 cortical regions along the ventral stream are involved in identifying auditory objects from perceptual (e.g., phonemes) to conceptual units, and the most anterior portion of the temporal lobe is needed for accessing lexical semantics. This framework draws on the results of research performed in non-human primates in order to find common underlying principles of brain mechanisms involved in complex auditory processing. It provides a mechanistic explanation for the preponderant role of the anterior temporal lobe in lexical semantics as delineated by studies examining speech comprehension. 138 , 139 In addition, within this framework, the increased potentiality for lexical semantics of the left compared to the right hemisphere is naturally derived from the physiological asymmetries described for auditory regions ( Box 2 ). Indeed, it makes sense that the regions involved in identifying concepts from complex auditory patterns would be closely linked anatomically to the regions able to detect fast acoustic changes (themselves needed to identify phonemes and syllables). Referring to other dual-stream models of speech perception (e.g., Ref. 140 ) allows us to link these superior and anterior temporal regions to the other temporal regions thought to be crucial for word retrieval, such as the posterior MTG and ITG. In this model, the pMTG and inferior temporal sulcus are considered as a lexical interface linking semantic to phonological information, which would be in agreement with the strong association that has been established between the pMTG and verbal knowledge in both speech comprehension and speech production tasks (see Ref. 141 for a meta-analysis). Such a framework, however, still needs to be developed for language production and represents a promising avenue to further the understanding of its neurobiological basis.

Hemispheric asymmetries of cortical auditory areas

Asymmetries in human auditory areas (particularly in the planumtemporale) have been known since the 1960s. 158 In addition, left and right auditory areas have been shown to have unequal abilities in processing fast acoustic changes. 159 Research using dichotic listening experiments have suggested that fast acoustic changes are better perceived by the left than by the right hemisphere (Krashen in Ref. 160 ). In language, phonemes that differ only by the place of articulation are much harder to differentiate by the right than by the left hemisphere. 161 Place of articulation refers to where the obstruction occurs in the vocal tract. For example, /ba/ versus /da/ differ only in the place of articulation of the first phoneme, and /b/ is bilabial, whereas /d/ is apico-alveolar. Accordingly, cortical stimulation of the left STG has been shown to impair consonant discrimination, which relies on the ability to perceive fast acoustic changes, but not vowel discrimination, as vowels generally spread over a longer timeperiod. 162 Thus, temporal sequencing is better performed by the left than by the right hemisphere. This has been shown in linguistic and non-linguistic tasks, suggesting that this higher potential of the left compared to the right hemisphere in processing fast acoustic changes may be at the origin of the better ability of the left hemisphere in performing phonological processing (see also Giraud and Poeppel 163 for a neurobiological model based on asymmetrical left versus right hemisphere oscillation-based parsing). In agreement with this idea, patients with aphasia have (as a group) been shown to perform significantly worse than controls and rightbrain–lesioned patients in nonlinguistic tasks requiring fine temporal order judgments, 164 , 165 , 166 although it is not clear which types of aphasic patients have these deficits. A recent review of detailed neuroanatomical investigations of the human brain's cytoarchitectonics hints as to why the left and right cerebral hemispheres could have different temporal sequencing abilities: 167 the way that cortical minicolumns are organized is different in the left compared to the right hemisphere. In the left hemisphere, cortical minicolumns are more widely spaced and have less overlapping dendritic fields, allowing for more independent minicolumn function. This type of organization has been suggested to be optimal for higher-resolution processing, in the sense of detailed feature analysis. In the right hemisphere, on the other hand, minicolumns are more densely packed, which has been associated with more overlapping, lower-resolution, holistic processing. Interestingly, genetic factors explaining hemispheric asymmetries have been found in the fetal brain. 168 In addition, axons of neurons in the superior posterior temporal lobe have been found to be more thickly myelinated in the left than in the right hemisphere, supporting faster processing speed in the left than in the right hemisphere. 169 These anatomical differences could very well explain the increased ability of the left hemisphere in identifying fast acoustic changes that are critical to our ability to perceive speech accurately.

Hemispheric asymmetries are less clear concerning the size of the MTG and ITG in comparison with that of the planum temporale (see, for example, Ref. 142 and Box 2 ). However, studies on white matter as well as resting-state connectivity have revealed that the left MTG is connected through an extensive structural and functional connectivity pattern to other language regions. 95 A number of these tracts have been shown to be larger or more dense in the left than in the right hemisphere, including the arcuate fasciculus 143 , 144 and superior longitudinal fasciculus, 145 as well as more ventral pathways. 144

Finally, heterogeneous results have been reported on hemispheric anatomical asymmetries of frontal regions associated with lexical semantics. Leftward asymmetry was found for the pars triangularis in 9 out of 10 patients with left-lateralized language as determined by Wada testing. 146 This, however, does not seem to be true for all frontal regions as assessed in healthy adults. 142 Similar to the MTG, the reasons for the asymmetric linguistic impact of left versus right PFC lesions may be found in the asymmetry of pathways connecting the left frontal lobe to left posterior language regions. Indeed, the abovementioned pathways connecting the MTG to frontal regions, such as the IFG, have been found to be larger or more dense in the left hemisphere. 143 , 144 , 145 , 147 It is not clear whether the anatomical asymmetries reported are a cause or a consequence of the specialization of the left hemisphere for language.

Domain generality and the two sub-processes involved in word retrieval

As pointed out by neuroimaging studies in healthy and stroke patients and by the lesion–symptom correlation studies reviewed here, lexical activation and selection have been differentially associated with domain-general processes. Prefrontal regions found to be involved in lexical selection are also found to be involved in other cognitive processes. This is not clearly the case for regions associated with lexical activation. The domain-general aspect of prefrontal functions suggests that lexical selection processes should be more resistant to left hemisphere damage and should be better compensated for by right frontal regions than is lexical activation. Indeed, if lexical selection is enabled by brain regions that have a domain-general role, it may be easier for other brain regions in the right hemisphere to become involved if left hemisphere regions are lesioned, even if these right hemisphere regions are not as efficient in supporting the functions typically handled by the left hemisphere. In addition, we believe that a similar framework for understanding the role of the right hemisphere after left hemisphere stroke-induced language deficits may be applied to other components of language processing, such as at the sentence level, where multiple words have to be put together to form sentences.

Our hypothesis follows from the recent proposal by Geranmayeh and colleagues that domain-general networks play an important role in recovery from aphasia. 40 In their proposal, domain-general cortical regions play a role in language in challenging situations in healthy speakers. The same regions are also engaged in patients with aphasia, as these patients need to exert more cognitive effort to produce or comprehend language than do healthy speakers. A similar proposal has been made to explain the more prominent frontal bilateral pattern of activity observed in older compared to younger adults when performing cognitively demanding tasks (i.e., the scaffolding theory of cognitive aging). 148 We argue that part of the brain regions associated with normal language production, such as the left IFG and pre-SMA/ACC, may also play a role in domain-general cognitive-control processes. When these regions are damaged—and therefore when the processes they support are affected—it may be easier for other regions involved in domain-general cognitive-control processes, such as those involving the right frontal lobe or other parts of the medial frontal cortex, to support recovery.

Acknowledgements

This research was supported by a postdoctoral grant from the National Institute on Deafness and Other Communication Disorders of the National Institutes of Health under Award Number F32DC013245 to S.K.R.; NINDS Grant 2R37NS21135 and the Nielsen Corporation to R.T.K.; and Grants 10F-RCS-006 and CX000254 from the U.S. Department of Veterans Affairs Clinical Sciences Research and Development Program to N.F.D. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health, the Department of Veterans Affairs, or the United States government. This article chapter was also prepared within the framework of the Basic Research Program at the National Research University Higher School of Economics (HSE) and supported within the framework of a subsidy granted to the HSE by the Government of the Russian Federation for the implementation of the Global Competitiveness Program. We would like to thank the members of the Center for Aphasia and Related Disorders at the VA Health Care System in Martinez, CA, for their useful comments on earlier versions of this manuscript.

a We note, however, that very early reports before the Common Era had already associated loss of speech with paralysis of the right side of the body (Hippocrates, On Injuries of the Head , in finger, 2000, p. 30).

b Interestingly, whereas Paul Broca was confident in linking the ability of articulated language to the third frontal convolution, he was more cautious in linking it to the left hemisphere in particular: “And, quite remarkably, in all these patients the lesion was on the left side. I don't dare to draw a conclusion from that and wait for new facts” (ibid. “Et, chose bien remarquable, chez tous ces malades la lesion existait du cote gauche. Je n'ose tirer de la une conclusion et j'attends de nouveaux faits”). 3 The left lateralization of this function was confirmed by Gustave Dax who reported 87 cases of right hemiplegia with loss of speech, 53 cases of left hemiplegia without loss of speech, and only 6 violating cases. 4 , 5

c We are aware that word retrieval and selection are considered to be synonymous in some psycholinguistic models and that other psycholinguistic studies have argued otherwise. Here, we refer to word retrieval as a more general term including both lexical activation and lexical selection, similar to Oppenheim et al . 10 and Piai et al . 11

d Difficulties in word retrieval are also observed in other pathologies, such as the semantic variant of primary progressive aphasia or temporal lobe epilepsy. This review focuses on stroke patients because unilateral focal lesions are most informative with respect to the lateralization of brain function.

e Here, “activation” is to be understood in the sense of computational modeling, in which different words are represented as nodes or units that can have different activation values. 75

f Instances of such severe aphasia occur only very rarely in right hemisphere patients with crossed aphasia.

g These symptoms are much more mild than those described by Paul Broca who associated the loss of articulated speech to lesions in the same region. However, as was shown later, 103 the lesions of the initial cases described also extensively involved the underlying insula and white matter, explaining the severity of the symptom.

h Evidence for a compensatory role of the right temporal lobe, and particularly of the right posterior MTG and ITG, in word retrieval in language production has to our knowledge not been reported, even if a few studies have argued for a potential role of the right temporal lobe in recovery from left hemisphere stroke-induced lexical-semantic deficits in speech comprehension. 120 , 121

i This type of approach was initiated by Luria who looked at linguistic processes as abstract functions being built upon more basic physiological mechanisms, which he termed factors . 39 Luria classified aphasic syndromes according to the specific brain factor that was disrupted. For example, a symptom such as kinetic apraxia, which is a deficit in the temporal organization of speech movements, is explained by the factor “disintegrated kinetic melody of movement,” which is caused by a lesion in inferior premotor areas (secondary motor cortex, BA 44 and 45).

Conflicts of interest

The authors declare no conflicts of interest.

Left Brain vs. Right Brain

Left-brained people are supposed to be logical, analytical, and methodical, while right-brained people are supposed to be creative, disorganized, and artistic. But this left-brain / right-brain theory has been refuted by a large-scale, two-year study by researchers at the University of Utah. In other words, it is untrue that logical people predominantly use the left side of the brain and artistic people predominantly use the right. All people use both halves of the brain. However, the stereotypes associated with being left- or right-brained persist and continue to arouse curiosity.

This comparison explains some myths and facts about the topic and compares what are now only metaphors of left-brained and right-brained personality types.

Comparison chart

The theory of right brain vs. left brain dominance originates with Nobel Prize winning neurobiologist and neuropsychologist Roger Sperry. Sperry discovered that the left hemisphere of the brain usually functions by processing information in rational, logical, sequential, and overall analytical ways. The right hemisphere tends to recognize relationships, integrate and synthesize information, and arrive at intuitive thoughts.

These findings, while true, serve as the basis for the now-disproved theory that people who are logical, analytical and methodical are left-brain dominant, and those who are creative and artistic are right-brain dominant.

A study conducted at the University of Utah has debunked the myth. Neuroscientists analyzed over 1,000 brain scans from people between the ages of seven and 29. The brain scans did not show any evidence that people use one side of the brain more than the other. Essentially, the brain is interconnected, and the two hemispheres support each other in its processes and functions.

Lateralization of Brain Function

The human brain is split into two distinct cerebral hemispheres connected by the corpus callosum. The hemispheres exhibit strong bilateral symmetry regariding structure as well as function. For instance, structurally, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere, and functionally, Broca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers. Neuroscientist and Nobel laureate Roger Sperry has contributed significantly to the research of lateralization and split-brain function.

Brain Process and Functions

The left hemisphere of the brain processes information analytically and sequentially. It focuses on the verbal and is responsible for language. It processes from details into a whole picture. The left hemisphere's functions include order and pattern perception as well as creating strategies. The left hemisphere controls the muscles on the right side of the body.

The right hemisphere of the brain processes information intuitively. It focuses on the visual and is responsible for attention. It processes from the whole picture to details. The right hemisphere's functions include spatial perception and seeing possibilities in situations. The right hemisphere controls the muscles on the left side of the body.

Result of Damage

When people sustain an injury or have a stroke that's localized on one side of the brain, they have specific troubles. When the left hemisphere of the brain is damaged, people have difficulty speaking or understanding words either said or written down. They cannot see things on the right side of the body. It affects motor skills (limb apraxia [1] ) and they often move slowly and carefully.

People with damage to the right hemisphere of the brain often have trouble with visual perception and spatial orientation [2] , for example, getting a sense of how far or near an object is in relation to the body. They often neglect the left side of the body, and they're not able to see things on the left. These people are often impulsive and make poor decisions. They also have a short attention span, and their ability to read, process some elements of language [3] or learn new things is slowed down.

Application in WW-II

If a specific region of the brain, or even an entire hemisphere, is either injured or destroyed, its functions can sometimes be assumed by a neighboring region in the ipsilateral hemisphere or a corresponding region in the contralateral hemisphere, depending upon the area damaged and the patient's age.

Michael Gazzaniga , neuroscientist and a protégé of Sperry, talks about the specific case of WJ, a WWII veteran and epileptic patient as a result of war injury. He was the first veteran to undergo experimental split-brain surgery, which was successful. To quote Gazzaniga from his interview :

WJ was the first moment of excitement, he made a slow recovery from surgery, he was about 50 when he was operated on so I remember him visiting Caltech, coming up in a wheelchair in a protective helmet and all kinds of gear. Anyway we rolled him in to our testing room and these were really first days so it was very crude, we had the pipes that sent the water to the various labs and everything were open and exposed in the ceiling and so we literally threw a rope over them and hung this screen that you could back-project on, and then using a little gadget we could flash pictures to one side of a fixation point and accordingly, if you know how the visual system is hooked up, if you flashed it to the left of the fixation point that went exclusively to your right hemisphere, and if you flashed it to the right it went exclusively to your left hemisphere. It's just the way we're wired up.

The Stereotype

People who are analytical and logical and who pay attention to detail are said to be left-brain dominant, i.e., they use the left side of the brain more than the right side. Basic characteristics of left-brain thinking include logic , analysis, sequencing, linear thinking, mathematics, language , facts, thinking in words , remembering song lyrics and computation. When solving problems, left-brained people tend to break things down and make informed, sensible choices. Typical occupations include being a lawyer , judge, or banker.

People who are creative, artistic and open-minded are said to be right-brain dominant, and the right side of their brain is more dominant. Basic characteristics of right-brain thinking include creativity, imagination, holistic thinking, intuition, arts, rhythm, non-verbal, feelings, visualization, recognizing a tune and daydreaming. When solving problems, right-brained people tend to rely on intuition or a "gut reaction." Typical occupations include politics , acting, and athletics.

What's True

• The Laterlaization of Brain Function: It is true that the two lateral halves of the brain have neurons or receptors for different functions. There is evidence to show that some cognitive functions like speech and language are linked with the left hemisphere, while face recognition are in the right hemisphere. However, even for the specific functions to be executed, humans use the entire brain.
• There exist personality types who are predominantly more analytical than artistic.
• Everyone uses their brain holistically, regardless of whether they're analytical or creative.
• It is possible to be analytical/logical as well as artistic/creative and many people are.

What's Not True

• The fact that analytical people are governed by the left side of their brain or creative people are governed by the right side of their brain.
• Analytical people cannot be creative (or the other way round) because only one part of their brain is dominant.

Strengths and Difficulties

Left-brained people are supposed to be good at mathematics , reading, spelling, writing, sequencing and verbal and written language. They may have difficulty with abstract visualization.

Right-brained people are supposed to be good at multi-dimensional thinking, art, music , drawing, athletics, coordination and repairs. They remember faces, places and events. However, right-brained people may have difficulty understanding parts if they can't see the whole. They may also struggle with sequencing, organizing a large body of information and remembering names.

Of course, these are stereotypes and any individual can have strengths and weaknesses from either set. There could also be differences in the way the brain processes various categories of cognitive skills. e.g., both left-brained and right-brained people can be good at spelling but how they do it may be different. Left brains memorize the sequence of each letter in a word; right brains memorize the image of the whole word. You might see right brains raise their finger during spelling questions to draw out the word in mid air in front of their face to mentally visualize that whole word.

• Left Brain vs. Right: It's a Myth, Research Finds - Live Science
• Left Basic Characteristics of Left and Right Brain - UCMAS
• Which Way Do You Spin… Left Brain or Right Brain? - Lateral Action
• The rehabilitation of limb apraxia: a study in left-brain-damaged patients. - NIH.gov
• Strokes in the Left and Right Brain - University of Wisconsin-Madison
• Study Challenges Theory About Left Brain/Right Brain Behavior - HealthDay
• Are you a Right-Brain or Left-Brain Thinker? - University of Alabama (PDF)
• An Evaluation of the Left-Brain vs. Right-Brain Hypothesis with Resting State Functional Connectivity MRI - Plos One
• How an Epilepsy Treatment Shaped Our Understanding of Consciousness - The Atlantic

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Biopsychology: Hemispheric Lateralisation & Split Brain Research

Last updated 10 Apr 2017

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Lateralisation is the idea that the two halves of the brain are functionally different and that each hemisphere has functional specialisations, e.g. the left is dominant for language, and the right excels at visual motor tasks.

The two hemispheres are connected through nerve fibres called the  corpus callosum,  which facilitate interhemispheric communication: allowing the left and right hemispheres to ‘talk to’ one another.

Split-Brain Research

Sperry and Gazzaniga (1967) were the first to investigate hemispheric lateralisation with the use of split-brain patients.

Background: Split-brain patients are individuals who have undergone a surgical procedure where the corpus callosum, which connects the two hemispheres, is cut. This procedure, which separates the two hemispheres, was used as a treatment for severe epilepsy.

Aim: The aim of their research was to examine the extent to which the two hemispheres are specialised for certain functions.

Method: An image/word is projected to the patient’s left visual field (which is processed by the right hemisphere) or the right visual field (which is processed by the left hemisphere). When information is presented to one hemisphere in a split-brain patient, the information is not transferred to the other hemisphere (as the corpus callosum is cut).

Sperry and Gazzaniga conducted many different experiments, including describe what you see tasks, tactile tests, and drawing tasks.

In the describe what you see task, a picture was presented to either the left or right visual field and the participant had to simply describe what they saw.

In the tactile test , an object was placed in the patient’s left or right hand and they had to either describe what they felt, or select a similar object from a series of alternate objects.

Finally, in the drawing task , participants were presented with a picture in either their left or right visual field, and they had to simply draw what they saw. 

Conclusion: The findings of Sperry and Gazzaniga’s research highlights a number of key differences between the two hemispheres. Firstly, the left hemisphere is dominant in terms of speech and language. Secondly, the right hemisphere is dominant in terms of visual-motor tasks.

• Biopsychology
• Hemispheric Lateralisation
• Split Brain Research

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Home — Essay Samples — Nursing & Health — Human Brain — Left-Brain Versus Right-Brain Myth

Left-brain Versus Right-brain Myth

• Categories: Human Brain

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Words: 1739 |

Published: Feb 9, 2022

Words: 1739 | Pages: 4 | 9 min read

Table of contents

Origin of the myth and supporting evidence, evidence against the myth.

• Bandura, A. (1994). Self-efficacy. In V. S. Ramachaudran (Ed.), Encyclopedia of human behavior (Vol. 4, pp. 71-81). New York: Academic Press. (Reprinted in H. Friedman [Ed.], Encyclopedia of mental health. San Diego: Academic Press, 1998).
• Bruer, J. T. (2002). Avoiding the pediatrician's error: How neuroscientists can help educators (and themselves). Nature Neuroscience (Supplement), 5, 1031–1033.
• Dündar, S. and Gündüz, N. (2016), Misconceptions Regarding the Brain: The Neuromyths of Preservice Teachers. Mind, Brain, and Education, 10: 212-232. doi:10.1111/mbe.12119
• Gazzaniga, M. S. (2015). Tales from both sides of the brain: a life in neuroscience (1st ed.). New York, NY: Ecco.
• Gibson, C., Folley, B. S., & Park, S. (2009). Enhanced divergent thinking and creativity in musicians: A behavioral and near‐infrared spectroscopy study. Brain and Cognition, 69, 162–169.
• Lienhard, D. A. (2017, December 27). Roger Sperry’s split-brain experiments (1959–1968). Embryo Project Encyclopedia. Retrieved from http://embryo.asu.edu/handle/10776/13035.
• Lindell, A. K. (2006). In your right mind: Right hemisphere contributions to human language processing and production. Neuropsychology Review, 16, 131–148.
• Nielsen J.A., Zielinski B.A., Ferguson M.A., Lainhart J.E., Anderson J.S. (2013). An evaluation of the left-Brain vs. right-brain hypothesis with resting state functional connectivity magnetic resonance imaging. PLoS ONE 8(8), e71275. https://doi.org/10.1371/journal.pone.0071275
• Noggle C.A., Hall J.J. (2011) Hemispheres of the Brain, Lateralization of. In: Goldstein S., Naglieri J.A. (eds) Encyclopedia of Child Behavior and Development. Springer, Boston, MA
• Pines, M. (1973) Two astonishingly different persons inhabit our heads. The New York Times Sunday Magazine. from https://www.nytimes.com/1973/09/09/archives/we-are-leftbrained-or-rightbrained-two-astonishingly-different.html
• Reilly, J., Losh, M., Bellugi, U., & Wulfeck, B. (2004). Frog, where are you? narratives in children with specific language impairment, early focal brain injury and Williams Syndrome. Brain and Language, 88, 229–247.
• Runco, M. A. (2004). Creativity. Annual Review of Psychology, 55, 657–687.
• Society for Neuroscience (2009, June). Neuroscience Research in Education Summit: The Promise of Interdisciplinary Partnerships Between Brain Sciences and Education. University of California, Irvine. June 22–24, 2009. http://www.ndcbrain.com/articles/SocietyforNeuroscience-EducationSummitReport.pdf
• Sperry, R. W. (1961). Cerebral Organization and Behavior.  Science, 133, 1749–1757. http://people.uncw.edu/puente/sperry/sperrypapers/60s/85-1961.pdf
• Sperry, R.W. (1984). Consciousness, personal identity and the divided brain. Neuropsychologia, 22, 661-673.
• Waters, E. (2017). Waters, Elizabeth: The left brain vs. right brain myth. [Video file]. Retrieved from https://ed.ted.com/lessons/the-left-brain-vs-right-brain-myth-elizabeth waters#discussion

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Right Brain May Control Writing in Some Lefties, Study Shows

• by Andy Fell
• November 18, 1996

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NPR in Turmoil After It Is Accused of Liberal Bias

An essay from an editor at the broadcaster has generated a firestorm of criticism about the network on social media, especially among conservatives.

By Benjamin Mullin and Katie Robertson

NPR is facing both internal tumult and a fusillade of attacks by prominent conservatives this week after a senior editor publicly claimed the broadcaster had allowed liberal bias to affect its coverage, risking its trust with audiences.

Uri Berliner, a senior business editor who has worked at NPR for 25 years, wrote in an essay published Tuesday by The Free Press, a popular Substack publication, that “people at every level of NPR have comfortably coalesced around the progressive worldview.”

Mr. Berliner, a Peabody Award-winning journalist, castigated NPR for what he said was a litany of journalistic missteps around coverage of several major news events, including the origins of Covid-19 and the war in Gaza. He also said the internal culture at NPR had placed race and identity as “paramount in nearly every aspect of the workplace.”

Mr. Berliner’s essay has ignited a firestorm of criticism of NPR on social media, especially among conservatives who have long accused the network of political bias in its reporting. Former President Donald J. Trump took to his social media platform, Truth Social, to argue that NPR’s government funding should be rescinded, an argument he has made in the past.

NPR has forcefully pushed back on Mr. Berliner’s accusations and the criticism.

“We’re proud to stand behind the exceptional work that our desks and shows do to cover a wide range of challenging stories,” Edith Chapin, the organization’s editor in chief, said in an email to staff on Tuesday. “We believe that inclusion — among our staff, with our sourcing, and in our overall coverage — is critical to telling the nuanced stories of this country and our world.” Some other NPR journalists also criticized the essay publicly, including Eric Deggans, its TV critic, who faulted Mr. Berliner for not giving NPR an opportunity to comment on the piece.

In an interview on Thursday, Mr. Berliner expressed no regrets about publishing the essay, saying he loved NPR and hoped to make it better by airing criticisms that have gone unheeded by leaders for years. He called NPR a “national trust” that people rely on for fair reporting and superb storytelling.

“I decided to go out and publish it in hopes that something would change, and that we get a broader conversation going about how the news is covered,” Mr. Berliner said.

He said he had not been disciplined by managers, though he said he had received a note from his supervisor reminding him that NPR requires employees to clear speaking appearances and media requests with standards and media relations. He said he didn’t run his remarks to The New York Times by network spokespeople.

When the hosts of NPR’s biggest shows, including “Morning Edition” and “All Things Considered,” convened on Wednesday afternoon for a long-scheduled meet-and-greet with the network’s new chief executive, Katherine Maher , conversation soon turned to Mr. Berliner’s essay, according to two people with knowledge of the meeting. During the lunch, Ms. Chapin told the hosts that she didn’t want Mr. Berliner to become a “martyr,” the people said.

Mr. Berliner’s essay also sent critical Slack messages whizzing through some of the same employee affinity groups focused on racial and sexual identity that he cited in his essay. In one group, several staff members disputed Mr. Berliner’s points about a lack of ideological diversity and said efforts to recruit more people of color would make NPR’s journalism better.

On Wednesday, staff members from “Morning Edition” convened to discuss the fallout from Mr. Berliner’s essay. During the meeting, an NPR producer took issue with Mr. Berliner’s argument for why NPR’s listenership has fallen off, describing a variety of factors that have contributed to the change.

Mr. Berliner’s remarks prompted vehement pushback from several news executives. Tony Cavin, NPR’s managing editor of standards and practices, said in an interview that he rejected all of Mr. Berliner’s claims of unfairness, adding that his remarks would probably make it harder for NPR journalists to do their jobs.

“The next time one of our people calls up a Republican congressman or something and tries to get an answer from them, they may well say, ‘Oh, I read these stories, you guys aren’t fair, so I’m not going to talk to you,’” Mr. Cavin said.

Some journalists have defended Mr. Berliner’s essay. Jeffrey A. Dvorkin, NPR’s former ombudsman, said Mr. Berliner was “not wrong” on social media. Chuck Holmes, a former managing editor at NPR, called Mr. Berliner’s essay “brave” on Facebook.

Mr. Berliner’s criticism was the latest salvo within NPR, which is no stranger to internal division. In October, Mr. Berliner took part in a lengthy debate over whether NPR should defer to language proposed by the Arab and Middle Eastern Journalists Association while covering the conflict in Gaza.

“We don’t need to rely on an advocacy group’s guidance,” Mr. Berliner wrote, according to a copy of the email exchange viewed by The Times. “Our job is to seek out the facts and report them.” The debate didn’t change NPR’s language guidance, which is made by editors who weren’t part of the discussion. And in a statement on Thursday, the Arab and Middle Eastern Journalists Association said it is a professional association for journalists, not a political advocacy group.

Mr. Berliner’s public criticism has highlighted broader concerns within NPR about the public broadcaster’s mission amid continued financial struggles. Last year, NPR cut 10 percent of its staff and canceled four podcasts, including the popular “Invisibilia,” as it tried to make up for a $30 million budget shortfall. Listeners have drifted away from traditional radio to podcasts, and the advertising market has been unsteady.

In his essay, Mr. Berliner laid some of the blame at the feet of NPR’s former chief executive, John Lansing, who said he was retiring at the end of last year after four years in the role. He was replaced by Ms. Maher, who started on March 25.

During a meeting with employees in her first week, Ms. Maher was asked what she thought about decisions to give a platform to political figures like Ronna McDaniel, the former Republican Party chair whose position as a political analyst at NBC News became untenable after an on-air revolt from hosts who criticized her efforts to undermine the 2020 election.

“I think that this conversation has been one that does not have an easy answer,” Ms. Maher responded.

Benjamin Mullin reports on the major companies behind news and entertainment. Contact Ben securely on Signal at +1 530-961-3223 or email at [email protected] . More about Benjamin Mullin

Katie Robertson covers the media industry for The Times. Email:  [email protected]   More about Katie Robertson