As you saw in the previous section, the major function of the corpus callosum is to transfer information from one hemisphere of the cerebral cortex to the other. In the case study described there, we concluded that the damage to the policeman’s corpus callosum may have caused his behavioral and cognitive impairments. However, single case studies are not sufficient for inferring causation — that is, we cannot conclude from that single case study that damage to the policeman’s corpus callosum caused his difficulties. In this vein, it is interesting to note that some people born without a corpus callosum do not experience any behavioral, cognitive, or emotional problems. Treffert and Christensen (2005) noted that: “Neurologists joke that [the corpus callosum's] only two certain functions are to propagate seizures and hold the brain together” (p. 110).
The joking comment that one function of the corpus callosum is “to propagate seizures” refers to what sometines 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 (for more information, see epilepsy.com). 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-convulsant 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 lesioning 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; click here for a game based on the findings of split-brain experiments). 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:
- the localization of cognitive functions in the cerebral cortex;
- the specialization of each hemisphere for unique cognitive functions (that is, for laterality of functioning).
Nevertheless, there are individual differences in the location of cognitive functions in the cerebral cortex. For example, as mentioned in Section 3-8 (see Table 2 in that section WHERE IS THIS?), 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 nonmusicians use the right hemisphere (Evers, Dannert, Rödding, Rötter, & Ringelstein, 1999; Schlaug, 2001). 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, 2007).
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.
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 know as anarchic hand syndrome). Preilowski (1975) described some examples:
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 language-dominant hemisphere — the syndrome cannot be explained by damage to the corpus callosum.
Study Questions for Section 3-14
- What is split-brain surgery?
- When is split-brain surgery used?
- What can we infer about the localization of mental functions in humans based on studies of split-brain patients?
- What is a major limitation of the use of split-brain studies when generalizing the results to all humans?
- 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?
- 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?
- How would you describe alien-hand syndrome in your own words?
- What usually causes alien-hand syndrome?
- Based on what you’ve learned, what is your best guess about why alien-hand syndrome disappears quickly in almost all split-brain patients?
Banks, G., Short, P., Martinez, A. J., Latchaw, R., Ratcliff, G., & Boller, F. (1989). The alien hand syndrome: Clinical and postmortem findings. Archives of Neurology, 46, 456-459.
Dodge, N. (2007). The brain that changes itself: Stories of personal triumph from the frontiers of brain science. New York: Viking Adult.
Evers, S., Dannert, J., Rödding, D., Rötter, G., & Ringelstein, E. (1999). The cerebral haemodynamics of music perception: A transcranial Doppler sonography study. Brain, 122, 75–85. doi: 10.1093/brain/122.1.75
Gazzaniga, M.S. (1967). The split brain in man. Scientific American, 217, 24-29.
Gazzaniga, M. S. (1970). The bisected brain. New York: Appleton Century Crofts.
Gazzaniga, M. S. (1998, July). The split brain revisited. Scientific American, 279(1), 50-55. Retrieved August 1, 2009, from http://courses.dce.harvard.edu/~phils4/splitbrain.pdf
Kolb, B., & Whishaw, I. Q. (1996). Fundamentals of human neuropsychology (4th ed.). New York: Freeman.
Preilowski, B. (1975). Bilateral motor interaction: Perceptual-motor performance of partial and complete “split-brain” patients. In K. J. Zulch, O. Creutzfeldt, & C. C. Gailbraith (Eds.), Cerebral Localization. New York: Springer-Verlag.
Schlaug, G. (2001). The brain of musicians: A model for functional and structural adaptation. Annals of the New York Academy of Sciences, 930, 281-299. Retrieved September 22, 2o11, from http://www.musicianbrain.com/papers/Schlaug_NYAS_2001.pdf
Springer, S. P., & Deutsch, G. (1998). Left brain, right brain: Perspectives from cognitive neuroscience (5th ed.). New York: Worth.
Treffert, D. A., & Christensen, D. D. (2006, June/July). Inside the mind of a savant. Scientific American Mind, 17(3), 50-55.
Wegner, D. M. (2002). The illusion of conscious will. Cambridge, MA: The MIT Press.