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