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Split-Brain Surgery: A tale of two brains
The joking comment that one function of the corpus callosum is “to propagate seizures” refers to what
sometimes occurs in people with epilepsy caused by abnormal activity in the cerebral cortex. This abnormal
electrical activity may spread from its starting point in one hemisphere of the cortex and travel across the corpus
callosum, thereby involving both cortical hemispheres. In this case, sufferers often become unable to function,
lose awareness of the external world, and may convulse (if the motor cortex is affected). These seizures can last
several minutes, but their after-effects can last much longer. The symptoms often can be reduced or eliminated
with anti-convulsing medications; but some people are not helped by these medications. When seizures are
frequent and severe in these intractable cases, it sometimes helps to sever the connection between the two
cortical hemispheres by severing the corpus callosum. People who have had this operation are referred to as
split-brain patients.
By studying split-brain patients, researchers have discovered much about the separate functions of the two
cortical hemispheres (Gazzaniga, 1967; Gazzaniga, 1970; Springer & Deutsch, 1998; After the corpus callosum
has been cut, responses to stimuli depend on which hemisphere receives information.
A. With their right hemispheres, split-brain patients typically are unable to (a) use either written or spoken
language, and (b) name objects, letters, and numbers. Activity in the right hemisphere of the cerebral cortex
typically is important for:
orienting and navigating spatially,
recognizing faces,
imagining visual scenes and objects,
recognizing melodies and other components of music.
B. With their left hemispheres, split-brain patients typically are unable to (a) draw objects accurately, (b) copy
a design using blocks, or (c) recognize faces and other complex visual objects. Activity in the left hemisphere of
the cerebral cortex typically is important for:
producing and comprehending language,
solving math problems,
thinking logically and sequentially.
In short, studies of split-brain patients offer strong support for:
1. the localization of cognitive functions in the cerebral cortex;
2. the specialization of each hemisphere for unique cognitive functions (that is, for laterality of
Nevertheless, there are individual differences in the location of cognitive functions in the cerebral cortex.
For example, about 30% of left-handers have right-hemisphere dominance for language or bilateral (in both
hemispheres) dominance. And, in a case noted by Gazzaniga (1998), the speech of a left-handed split-brain
patient was associated with activity in the left hemisphere, whereas her ability to write was associated with
activity in the right hemisphere! There is no reason to think that other functions, such as musical abilities, do
not also show such variations among individuals. In fact, some research supports the claim that trained
musicians use the left hemisphere when listening to and analyzing music, whereas non-musicians use the right
hemisphere. In this case, experience seems to affect the lateralization of musical abilities.
The brain exhibits a great deal of plasticity: when the brain is damaged, neural structures often reorganize,
sometimes to such an extent that lost mental functions are regained. But in general, by the teenaged years,
extensive brain damage becomes much more likely to lead to a relatively permanent loss of the functions
associated with the damaged area; and definitely by adulthood, it becomes much more difficult for lost
functions to reappear. Nevertheless, there are exceptions. Gazzaniga (1998) reported that one split-brain patient
“developed the capacity to speak out of the right hemisphere — 13 years after surgery” (p. 53; also see Dodge,
The existence of plasticity suggests that the study of split-brain patients may be of limited use for making
generalizations about the localization of mental functions in the human brain. Epilepsy is caused by
abnormalities in the brain and because the brains of people with severe epilepsy have either been damaged or
developed abnormally, we should expect that plasticity has modified and reorganized their brains to some
degree. This might make the localization of functions in some epileptic brains significantly different from that
of people without severe epilepsy. In other words, people with severe epilepsy may have brains that tend to be
organized differently than the brains of people without epilepsy. Thus, we need to be careful about generalizing
from studies of epileptic and split-brain patients to all humans. On the other hand, brain researchers are
reasonably confident that much of the split-brain work has relevance for humans in general: research with
others (both those suffering brain damage and those with intact brains) has often supported the split-brain
findings. You should keep in mind that any one kind of evidence (such as the findings of split-brain research)
always has limitations that can be controlled for only by considering other kinds of evidence (such as the
findings of electrical-stimulation research). In short, researchers always need to consider evidence from many
different sources before placing much confidence in their generalizations.
Alien-Hand Syndrome
A fascinating phenomenon has been observed in a number of split-brain patients. For a short time after their
surgery, they seem to lose the ability to control their left hands — a problem called alien-hand syndrome (also
In one case the patient (W. J.) would repeatedly pick up a newspaper with his right hand and lay it down with
his left hand. This would be performed several times until finally the left hand threw the newspaper on the floor.
Another patient (R. Y.) was described by a physiotherapist: “He was buttoning his shirt with his right hand and
the left hand was coming along just behind it undoing the buttons just as quickly as he could fasten them.” … It
is of interest that the same patients … were able to use their left hand in a purposeful and cooperative manner
“when not thinking of what they were doing.” For example, they could pour coffee out of a pot held in the right
hand into a cup held by its handle with the left hand. (quoted in Kolb & Whishaw, 1996, pp. 344-45)
Banks, et al. (1989) described another patient who exhibited similar types of strangely antagonistic behaviors
between the two hands:
While playing checkers on one occasion, the left hand made a move [that the patient] did not wish to make, and
he corrected the move with the right hand; however, the left hand, to the patient’s frustration, repeated the false
move. On other occasions, he turned the pages of the book with one hand while the other tried to close it; he
shaved with the right hand while the left one unzipped his jacket; he tried to soap a washcloth while the left
hand kept putting the soap back in the dish; and he tried to open a closet with the right hand while the left one
closed it. (quoted in Wegner, 2002, p. 5)
This oppositional behavior between the right and left hands disappears quickly after the split-brain
surgery. Split-brain patients then are able to engage in virtually all the behaviors that anyone else can perform.
In fact, it takes special tests to demonstrate that their left and right hemispheres have been separated.
What is happening in these cases is that the patients’ left hemispheres, which control the right hand and which
also are language-dominant, are confused by the movements of the left hand, which is controlled by the
patients’ right hemispheres and which does not contain the language areas. In short, the “talking hemisphere” —
the left hemisphere — is telling us that it is not controlling the left hand, which, in fact, it isn’t: it is the nontalking right hemisphere that does so but, since it can’t talk to us, it can’t tell us that it is, in fact, making the left
hand move. One would think that, once the hemispheres are disconnected, alien-hand syndrome would be a
permanent problem, but it isn’t. There must be changes in the brain soon after surgery that again allows the two
hands to work in concert, another example of the brain’s plasticity.
In rare instances, however, people who have not had split-brain surgery develop alien-hand syndrome in either
their left or right hands, usually after experiencing a stroke. Because some of these people develop the
syndrome in their right hands, which is controlled by the motor cortex in their left hemispheres — the languagedominant hemisphere — the syndrome cannot be explained by damage to the corpus callosum.
Study Questions for Section 3-14
1. What is split-brain surgery?
2. When is split-brain surgery used?
3. What can we infer about the localization of mental functions in humans based on studies of split-brain
4. What is a major limitation of the use of split-brain studies when generalizing the results to all humans?
5. If you wanted to generalize the results of a study of the lateralization of language abilities to all humans,
would you use right-handed individuals, left-handed individuals, or both? Why?
6. Would a person born without a corpus callosum show the same unusual behaviors as a person whose
corpus callosum is destroyed during adulthood? Why or why not?
7. How would you describe alien-hand syndrome in your own words?
8. What usually causes alien-hand syndrome?
9. Based on what you’ve learned, what is your best guess about why alien-hand syndrome disappears
quickly in almost all split-brain patients?