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Biology 202
2002 Second Paper
On Serendip

Of A divided Mind: Human Responses to the Human Split Brain

Hilary Hochman

When neuroscientists first made direct contact with the right hemisphere of the brain, during neurological tests of "split brain" human subjects, it was as if they had found intelligent, albeit speechless, life on Mars. At a time when brain imaging techniques were crude or nonexistent, the only way to observe and communicate with the brain's right hemisphere unimpeded by the left hemisphere was by testing split brain subjects (1). The right hemisphere, previously supposed mute, illiterate, mentally retarded, and completely subordinate to the left hemisphere, had a mind of its own (1). While it could not speak, it could respond to commands and questions via its contralateral control of the left hand. It had different abilities and even opinions and emotional states than the neighboring left hemisphere (2). These discoveries led to a model of hemispheric specialization of normal human brain function, with an analytic, verbal, problem solving left hemisphere and a visuospatial, synthetic, creative right hemisphere (1, 2). The formation of this model in turn offers insight into the brains of the observers as well as the observed. The observers' behavior supported some of their own hypotheses about the human brain, split or unsplit.
The term "split-brain" is commonly used to describe a person whose corpus callosum has been surgically severed (3). The corpus callosum, comprised of approximately 200 million neuronal fibers connecting the left and right hemispheres of brain, exists only in mammals' brains, and is largest in human brains (1, 3). Until the 1960's neuroscientists were unsure what purpose the corpus callosum served (3). By observing deficits in split brains' functions, scientists could better assess the corpus callosum's function (1).
Roger Sperry and his colleagues pioneered the operation severing the corpus callosum, known as callosal commisurectomy, in the 1960s, as a last ditch effort to control the seizures of life threateningly severe epilepsy by creating a fire wall to prevent electrical impulses from traveling between hemispheres (1). This treatment was successful, and after recovering from the surgery, the split-brain patients appeared normal in every day interactions and even during a routine physical exam (1). However, Sperry and his colleagues, after extensive and specific neurological tests of split brain patients, posited that the corpus callosum communicated stimuli and responses between the two hemispheres, each specialized for different cognitive functions (1).
Using a tachistoscope, Sperry delivered visual stimuli to a single visual field of the subject (1). He discovered that, with the exception of olfactory stimuli, the hemispheres of the brain receive sensory stimuli and exercise motor control contralaterally (1, 3, 5). Thus, when a word or picture of an object was flashed to the right eye, the patient was able to name the object or read the word. However, when a picture of an object was flashed to the left eye, the patient could not name it and in fact would deny seeing the object. Simultaneously, the left hand could point to the object or pick it out of a group of concealed objects (1). Thus, if a strong smell such as garlic or ammonia were presented to the right nostril, whose olfactory receptors connect directly to the right hemisphere, the patient would grimace with disgust while verbally denying that she smelled anything. However, her left hand could point to the object the right nostril had smelled, as the patient continued to say she smelled nothing (1, 5). .
Thus, with the primary information pathway between the two hemispheres severed, subjects apparently could not transfer information received as sensory stimuli between hemispheres. The researchers believed, however, that the two hemispheres attempted to communicate through external cues (4, 6). For example, when a subject was told to retrieve with his left hand a pencil palpated with his right hand, the right hand jabbed the left hand with the pencil (6). Researchers commonly observed subjects in similar tests saying aloud the name of the object touched with the right hand; the right hemisphere can sometimes understand many individual words and will then know what object to retrieve (4, 6). Researchers often saw subjects using facial expressions, such as frowning and smiling, in apparent attempts to cue the hemisphere that was attempting to complete a task (6).
These and similar experiments led Sperry to conclude that the corpus callosum served as the information pathway between the human brain's two hemispheres (1, 2). The results also suggested that the left hemisphere had exclusive control of language, and consequentially a split brain person could only speak about sensory input of which the left hemisphere was aware (1, 2, 4). Likewise, the researchers concluded that the hemispheres attempted to exchange information through external cues when internal pathways were severed (6). However, contrary to initial expectations, the right hemisphere could understand and respond to simple language, possessed a fund of general information, and had the same memories as the left hemisphere (1, 2).
Based on tests of a small pool of split brain subjects, a somewhat rigid functional portrait of the right and left hemispheres was developed (7). The right and left hemispheres were assigned respective areas of competence in a detail that sometimes rose to the level of assigning each hemisphere a personality: the left brain was analytic, language competent, and sequential in its thought; able to solve complex problems. The right hemisphere, by contrast, was visuospatial, synthetic, more emotionally attuned and creative (2,7). In sum, the researchers succumbed to the temptation to extrapolate general principles from a small set of individual observations.
However, as experiments continued, as researchers developed equipment able to deliver more information for longer periods of time to a single hemisphere (4)., as the effects of time on the split brain subjects began to manifest, and as more split brain subjects became available for study, the mapping of the brain became at once more detailed and less definite (2,7). Researchers discovered that the location of brain functions sometimes varied from patient to patient. The most dramatic example was a split brain patient who had speech in the left hemisphere, but writing in right hemisphere (2,8). . Younger patients, who were twelve to fourteen years old at the time of the operation showed significantly less lateralization of function than did older patients whose hemispheres had decades in which to specialize before commisurectomy (10). Finally, some experimental data suggested that split brain subjects developed and/or made greater use of subcortical connections between the hemispheres to integrate sensory input and motor output (11,12).
All of these findings provided evidence that individual brains can vary widely and respond to changes in conditions by developing and/or using different neural networks as well as coping strategies external to the brain (2, 5, 7). However, there was little discussion of the possibility that the frequent testing of the right hemisphere in isolation may have helped stimulate it to develop language. Likewise, requesting that test subjects integrate stimuli separately but simultaneously delivered to each sphere may have helped stimulated the subjects' brains to develop or use existing subcortical connections to compensate for lack of callosal connections. Attention focused not on the potential contribution of the experiments to the results, but rather on the left hemisphere's responses to behavior in response to stimuli received exclusively by the right hemisphere.
The left hemisphere of a split brain has no experience of stimuli delivered exclusively to the right hemisphere, yet the left hemisphere attempts to craft explanations for behavior initiated by the isolated right hemisphere. For example, a split brain subject's left eye received a command to stand. The person stood, but when asked why she stood up, she responded, using the language center of the left hemisphere, that she wanted a soda (7). . Likewise, when the left and right hemispheres were each asked to pick an appropriate picture to accord with an image flashed only to that hemisphere, the left selected a chicken to match the chicken claw in the picture it saw, while the right hemisphere correctly chose a shovel to remove the snow it saw. When asked why the person chose those images, he replied that the claw was for the chicken, and the shovel was to clean out the shed. (5). Similarly, the left hemisphere will generate false memories to fill in blanks in a sequence it is attempting to recall, while the right brain will not. (5).
Michael Gazzaniga, one of the original members of Sperry's laboratory and now head of the cognitive neuroscience program at Dartmouth College, suggests that this ability of the left hemisphere to confabulate is the product of its unique efforts to interpret actions and experience (2, 5, 7). The interpreter attempts to organize experience into patterns and responds to these patterns (2, 5, 7). The interpreter is driven to create explanations for experience and individual phenomena; when it does not know the facts, it makes its best guess (2, 5, 7).
The behavior of neuroscience researchers as they examined split brain subjects can also be construed as supporting the existence of this interpreter, offering explanations and finding patterns in an effort to make sense of their observations. The study of split brains presents a version of the light in the refrigerator problem: as Kandel points out, split brain studies, while essential in identifying the role of the corpus callosum, show much more about how separate hemispheres operate in its absence than they do about how the hemispheres function when connected (3).. Once the hemispheres are split, they function in isolation, with lateralization of function emphasized unless and until the brain develops adaptive strategies. Indeed, the split brain experiments may have themselves contributed to the development of different functions and coping strategies in and between the split brain's hemispheres. Yet eagerness to find a pattern, to interpret experience, however limited, perhaps impelled by the left hemisphere's interpreter, moved researchers to reject one hypothesis only to enshrine another equally rigid and equally, albeit differently, distorted picture of reality. The researchers' initial inclination was to assume that the first split brain patients' brain functions were representative not only of all split brain patients but of the hemispheres' functions in a brain with an intact corpus callosum. The left brain interpreter may well have been at work in the experimenters as well as in their subjects.


1) Nobel e-Museum, the official web site of the Nobel Foundation, filled with articles and information about Nobel laureates.

2) Gazzaniga, Michael, "The Split Brain Revisited," pp. 50-55, Scientific American, 1998 from theWeb page of Dr. Gary Laver, an assistant professor of psychology at California Polytechnic State University. The Web page is not of great interest but the article is free.

3) Kandell, Eric, Schwartz, J., and Jessel, T., Principals of Neural Science, 4th Ed., McGraw-Hill: NY, 2000.

4) Nobel e-Museum Zaidel, Eric, Zaidel, D., and Bogen, J., "The Split Brain," at Dr. Joe Bogen's home page, Dr. Bogen worked with Sperry in the 60s and now runs his own laboratory. This page is filled with interesting articles, as well as a proposal for teaching neuroanatomy using a walk-through human brain.

5) Gazzaniga, Michael, "Organization of the Human Brain," pp. 947-956, Science, Sept., 1989. Available on the Bryn Mawr College library server through

6) Puccetti, Ronald, "The mind with a double brain," The British Journal for the Philosophy of Science, p. 675-692, Dec. 1993. Available on the Bryn Mawr College library server through

7) Connors, Diane, "Michael Gazzaniga (neuroscientist) (interview)," pp. 99-106, Omni, Oct., 1993.

8) Strauss, Evelyn, "Writing, speech separated in split brain," pp. 827-828, Science, May, 1998. Available on the Bryn Mawr College library server through

9) Bogen, J.E., Dezure, R., Tenhouten, W.d., and Marsh, J.F., "The Other Side of the Brain IV. The A/P Ratio," on Dr. Joe Bogen's Web page.

10) Gazzaniga, M.S., Eliassen, J.C., Nisenson, L., Wessinger, C.M., Fendrich, R., Baynes, K., "Collaboration between the hemispheres of a callosotomy patient. Emerging right hemisphere speech and left hemisphere interpreter," pp. 1255-632, Brain; a Journal of Neurology, Aug., 1996.

11) Sergent, J., "A new look at the human split brain," pp. 1375-1392, Brain; a Journal of Neurology, Oct., 1987.

12) Corballis, Michael, "Visual Integration in the Split Brain," pp. 937-959, Neuropsychology, 1995.

Other Web sites of interest:

) A good overview of the split brain, some basic neuroanatomy, and split brain experiments.

) This is the Web page for an Introductory Neuropsychology course at MCP Hahnemann. In addition to a good overview of the effects of splitting the corpus callosum, it provides a good basic overview of other aspects of neuropsychology.

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