Biology 202
2000 First Web Report
On Serendip

We are aware that different parts of the brain are specialized to perform different functions. Over the years, scientists have devised increasingly sophisticated ways of exploring the geography of the brain, and they have succeeded in mapping it in considerable detail. One of the sources of information used is split-brain research, in which scientists study the effect of severing the corpus callosum, a bundle of millions of nerve fibers that connects the two halves of the brain and allows for communication between the hemispheres (3).(The operation is a rare one, done to treat intractable epilepsy.) By observing the performance of split-brain patients researchers have learned about the distinctive characteristics of the right and left hemispheres of the brain.

Surgeons treat some cases of epilepsy, when no other options are available, by cutting the corpus callosum, which connects the two cerebral hemispheres. Robert Sperry is a psychobiologist who conducted the landmark split-brain experiments. He discovered that human beings are of two minds. He found that the human brain has specialized functions on the right and left, and that the two sides can operate practically independently. In a normal brain, the stimulus entering one hemisphere is quickly transferred through the corpus callosum to the other hemisphere in order for the brain to function as a whole. In the split-brain patient the two hemispheres can not communicate. This procedure known as commissurotomy is done by opening the skull and exposing the corpus callosum between the two hemispheres. The surgeon then cuts through the corpus callosum, cutting the communication between the two hemispheres (4). This experiment demonstrated significant differences in the mental capabilities of the brain's two hemispheres. The left hemisphere was shown to be logical, analytic, quantitative, rational and verbal, whereas the right hemisphere was revealed to be conceptual, holistic, intuitive, imaginative and non-verbal.

The unusual behavior of the split brain patient has revealed many differences between the two brain hemispheres. Roger Sperry along with Michael Gazzaniga were the first to study the split brain in humans. Research showed that split brain patients present superiority on the right hemisphere when it comes to spatial tasks, such as arranging blocks. Researchers also showed drawings to the left and right hemispheres and the patient was asked to draw what he saw from both hemispheres. The conclusions were that the left-handed drawings were better drawn. Sperry and Gazzaniga also found that split-brain patients are less likely to talk about their feelings and emotions. In one of their patients, "Paul S." his right hemisphere was more developed in language ability before the operation. This is uncommon but does sometimes happen. The fact that Paul's right hemisphere was developed in verbal response enabled Sperry and Gazzaniga to interview both sides of the split brain. When the researchers asked the right side what he wanted to be, he answered an automobile racer while his left side stated he wanted to be a draftsman. Paul was asked other similar questions, which gave the researchers insight on the hidden differences between the hemispheres. Another patient also exhibited strange behaviors with his right and left hands. His right hand was trying to pull up his pants while the left hand was trying to pull then down. A similar incident occurred when a split-brain patient was having an argument with his wife. The patient was attacking his wife with his left hand while his right hand was defending her(5). These studies of split-brain abnormalities in patients as opposed to normal people are offering researchers insight on hemispheric differences and specialization. We can look at an example of callosal apraxia in both an intact brain and a split brain patient. Callosal apraxia, a type of limb apraxia is due to damage to the anterior corpus callosum. If a patient with limb apraxia is asked to raise both hands, the left side of the brain analyzes the verbal message and triggers the prefrontal cortex, which contains the memory of the movement. The information is then passed to the motor cortex, which controls the movement. In order for the right motor cortex to be signaled so the left hand can be raised the signals of the verbal command must be passed form the left to the right hemisphere through the corpus callosum. Therefore the right arm will be raised but the left arm will not. In a split-brain patient we observe something different. The patient shows a marked aparxia of the left hand to the verbal command. This is due to the fact that the right side, which controls the left hand, has less language understanding (5). Another interesting example of how the split brain affects the patient's perception of the outside world is seen in an experiment done by Roger Sperry and Ronald Meyers. An experiment was done by flashing a word so the right hemisphere of the brain would interpret the information. When the patient wrote down the word his left hand wrote down the correct word flashed. But when asked what he wrote done the patient did not know. Since the brain was split the information that was given to the right half could not relay the message to the left side (5).

Research was also done at the McLean Hospital by Fredric Schiffer MD, who studied two split brain patients. The McLean researchers studied the fact that each side of the brain controls the movement of the hand on the opposite side of the body. They devised a way for the left hand to answer for the right brain and for the right hand to answer for the left brain. Their experiment consisted of placing a row of five pegs in front of both the left and right hands of the split brain patients. These pegs represented a response, ranging from no response to an extreme response. The response was made by pointing simultaneously and independently with each hand immediately after the questions were asked. Dr. Schiffer observed that the left brain reported little or no anger toward a question asked about past bullies who had attacked him when he was young. But, his right brain was still upset about his early experiences. Another patient's two brains were asked questions about perceptions of himself. His right side perceived him as good while his left hemisphere thought he was insufficient. Dr. Schiffer and his staff found that each patient's right and left hemisphere seems to have its own opinions and emotions (6).

The end result of all the crossing over from one side of the body to the other is a bewildering network of interlacing nerve fibers that extends the total length of the human spinal cord. The key question is, of course, what purpose is served by such an arrangement. At this point neuroscientists have yet to formulate a really convincing answer. Experiments such as the split-brain enable scientists to grasp a better understanding of how the brain functions. To put the matter differently: The human brain has yet to discover this aspect of the mystery of its own functioning. There is little doubt, however, that we need both sides of our brain. In fact, one can state with equal validity that we are not just the right and left sides of our brain but simply we are our brain and wouldn't want to lose any part of it if we had a choice.

WWW Sources

2)Split Brain

3)How the Human Brain Developed and How the Human Mind Works

4)Splitting The Human Brain

5)Split Brain Consciousness

6)Split-Brain Study Finds New Evidence of Two Minds

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