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Hemispheric Specialization

The brain has two halves or hemi­spheres. Viewed from above, as in the following diagram, the two sides look like mirror images. This is called bilateral symmetry.

The word bilateral means two-sided, while symmetry means the two sides look the same. Bilateral symmetry refers to the fact that the brain consists of two halves that are nearly mirror images of each other.

the brain from above
The brain seen from above

Almost all the animals on earth show bilateral symmetry. The exceptions, such as starfish and jellyfish, are circular. They have radial symmetry (identical parts radiating from the center) rather than bilateral symmetry.

What is bilateral symmetry? What is the main alternative to bilateral symmetry? What is crossover organization?

In animals with bilateral symmetry, the right hemisphere generally controls the left side of the body, while the left hemisphere controls the right side of the body. This feature is called crossover or contralateral organization.

In humans and many other species, one side or the other may be dominant in motor activity (movement). In humans this is known as handedness.

About 13% of the human population is naturally left-handed. For the other 87%, the right arm works better for skilled hand movements.

The two hemispheres usually cooperate, but they do not necessarily contribute equally to every task. Language usually involves the left hemisphere more than the right.

What is lateralization?

Spatial tasks (such as reading a map) usually involve the right more than the left. When one hemisphere is more involved in an activity than the other, this is called hemispheric specialization or lateralization (LAT-er-al-iz-A-shun) from the word "lateral" meaning "side."

The Wada Test

Some of the best evidence relating to hemispheric specialization comes from the Wada test: a procedure carried out before brain surgery. In the Wada test each hemisphere is put to sleep at different times by injecting anesthetic into a major artery that leads only to that hemisphere.

While one hemisphere is anesthetized, the other remains awake and conscious. Once half the brain is anesthetized, doctors can interview the other half (if the patient is capable of talking) or give instructions to perform simple tasks.

This helps to determine what skills or abilities might be confined to one hemisphere. Typically, each hemisphere is tested on a different day before surgery.

What is the Wada test?

The Wada test reveals that 95% of right-handers and 70% of left-handers speak fluently only when the left hemisphere is awake. Among left-handers, 15% can speak using only their right hemispheres (when the left hemisphere is asleep).

They cannot speak when only the left hemisphere is awake. Another 15% of left-handers can speak when either side is awake.

How does the Wada test confirm lateralization of functions?

One of the interesting findings from the Wada test relates to emotion. When the left hemisphere is put to sleep, leaving the right active, patients typically became sad and worried.

When the right hemisphere is anes­thetized, it leaves the left awake. Then patients typically are happy and

Several different theorists, starting with Kinsbourne in the 1970s, suggested the two hemispheres might act as a "push/pull" control system, similar to an accelerator and a brake. The left hemi­sphere (mostly in the prefrontal area behind the eyes) might contribute more to approach tendencies. The right side (especially the amygdala in the midbrain) contributes more to worry and avoidance.

This would be consistent with the Wada test evidence. Patients with the left hemisphere active (but the right hemisphere asleep) were cheerful and unconcerned about upcoming brain surgery. The patients with the right hemisphere active were more worried.

How is the brain like a "control system" for approach and avoidance?

In most people, the right hemisphere is more expressive of emotion. Conse­quently, most people are more expressive on the left side of the face, controlled by the right half of the brain. People usually sneer or wink on the left.

If you hold a piece of cardboard over half your face and look in the mirror, you might find that the right half of your face looks relatively expressionless compared to the left. When you experiment with happy and sad expressions, the left half of your face will show more emotion, if you are typical.

Can normal people learn to activate one or another hemisphere, so that the entire hemisphere is more dominant than the other one, in mental activity? This comes dangerously close to "pop psychology."

In the best-selling book Drawing on the Right Side of the Brain (1993) a California art teacher named Betty Edwards claimed to be able to train people to use their right hemispheres to produce art. Her technique was to have right-handed people use their left hands while making sketches.

The book produced a small industry of workshops based on the idea, which was widely and uncritically accepted. To me, it is a fine example of a "fun idea" and therefore quite likely to be untrue or, at best, oversimplified and misleading. However, a lot of people endorse the idea; the book is in its third edition and still a best-seller on Amazon.

There are some well-documented cases of people able to control levels of hemi­spheric activity. Gott, Hughes and Whipple (1984), writing in Neuropsych­ologia, described a normal woman who could "turn on" one hemisphere or the other at will.

They provided evidence from biological and behavioral tests. All showed that brain activity went from one hemisphere to the other.

The subject was a 31-year-old right-handed female. From an early age she noticed two distinct patterns of mood and activity in herself. At the age of 16 she learned to control the states so she could switch them voluntarily.

The woman called one of the states "me" and the other "it." Left hemisphere activity correlated with "me." "Me" was used for logical discussions, planning, writing, arithmetic, playing Scrabble, reading for information, or confronting new people and situations. The patient called this her business woman side.

In the case of the woman who could "switch on" either hemisphere, what skills were associated with activity of each side?

The other state, called "it," was correlated with right hemisphere activity. The woman referred to this as her gardening personality. During this state she was more likely to be relaxed, comfortable, or in sexual moods.

When "it" was dominant she improved her ability at drawing, sports, and playing music. She improved at spatial tasks such as map reading and felt more at ease socializing. The researchers verified the performance differences with laboratory tests.

The foregoing case history was documented with brain measurement techniques. It is possibly one-of-a-kind, but it was well-documented by neuroscientists. By contrast, many of the commonly accepted ideas about right-brain and left-brain differences are not well-documented science.

The best-supported distinctions about right and left hemisphere specialization have already been mentioned. About 95% of right-handed people have language in the left hemisphere.

It is also true that the right hemisphere contributes more to emotional expression and spatial processing (such as reading maps). Most of the other speculations that appear in popular sources (for example, that the right brain is artistic or holistic while the left brain is analytic or drab and ordinary) are not supported by research.

Which ideas about right-brain and left-brain activity are well supported by research evidence, and which are not?

As long ago as 1978 the topic of hemispheric specialization was being denounced as "fad of the year" (Goleman, 1978). Michael Corballis of the University of Auckland wrote an article titled "Laterality and myth" in 1980.

Ten years later Corballis reviewed a book titled "The rise and fall of hemispheric specialization." Corballis lamented the fact that the idea of hemispheric specialization was still widely accepted (Corballis, 1990). He did not like the book, incidentally. He wrote "the best thing about it is the title."

I wrote Corballis in 1998 to see if he had changed his views:

Dear Dr. Corballis:

I enjoyed your article "Laterality and Myth" in 1980 plus your review of "The rise and fall of hemispheric specialization" in 1990, in which you lamented the fact that the idea of hemispheric specialization was still widely accepted. (You wrote, "The best thing about it is the title.") Are you on a ten-year cycle, planning another review for 2000?

On a more serious note, do you have a sentence or two I can share with my introductory psychology students regarding your current views of lateralization? One still sees the old mid-70s-ish "right brain/left brain" stuff in ads and pop psychology sources... [Personal communication, July 21, 1998]

Dr. Corballis responded, in part:

"Your message makes me realize, with horror, that I do seem to be on a ten-year cycle! I entirely agree with your assessment: language is in the left hemisphere, dammit, and emotional and spatial skills do seem to have a right-hemispheric bias."

Dr. Corballis also sent a pre-publication draft of a chapter (later published in 2000, continuing his ten year cycle) in which he detailed the mythology about lateralization. He noted that claims about superiority of the right or left brain for different types of mental processing go back to the mid-1800s and are continually changing. In his summary he wrote:

"...The supposed creative, intuitive skills of the right hemisphere are far from proven and are based more on speculation and the power of myth than on any incontrovertible scientific evidence."

Maybe the problem with hemispheric specialization is not that it is a complete lie but that it exaggerates and over­empha­sizes the differences that exist. That irritates professionals like Corballis who see references to "right and left brain thinking" in the media.

The truth is that the brain is full of specialized areas that are activated by particular tasks. In normal thinking, including intuition and artistic activity, multiple areas in both hemispheres contribute to the activity.


Corballis, M. C. (1980) Laterality and myth. American Psychologist, 35, 284-295.

Corballis, M. C. (1991). Left brain, right brain [Review of The Decline and Fall of Hemispheric Specialization by Robert Efron]. Science, 252, 575-576.

Corballis, M. C. (2000). How laterality will survive the millennium bug. Brain & Cognition, 42, 160-162.

Edwards, B. (1993) Drawing on the Right Side of the Brain. New York: TarcherPerigee.

Goleman, D. (1977). Split-brain psychology: Fad of the year? Psychology Today, Oct, 89-151.

Gott, P. S., Hughes, E. C., & Whipple, K. (1984). Voluntary control of two lateralized conscious states: Validation by electrical and behavioral studies. Neuropsychologia, 22, 65-72.

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