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Split brain

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Split Brain is a lay term used to describe individuals that have had their corpus callosum severed. The corpus callosum is the primary pathway for cortical connections between the hemispheres of the brain and when it is severed, most of the communication between the right and left sides of the brain ceases. The effects of this procedure provide some of the most striking counterexamples to claims made by dualists and proponents of non-materialist neuroscience.

Functional asymmetry and cortical lateralization[edit]

One of the most distinct anatomical features of the animal brain is its division into two mirrored hemispheres. Both hemispheres are predominately symmetrical from the central axis (a large crack called the longitudinal fissure) but significant asymmetries do exist. These asymmetries are found both at the gross anatomical level as well as the much finer structural level of individual neurons and their connections. A great deal of research has revealed that the two hemispheres are functionally asymmetric and focused on processing different types of information. The more a given function is located on one hemisphere over another the more lateralized it is said to be.

The classic lateralization is in language recognition and production. Over 90 percent of people show significant left-hemisphere dominance for language. This is mediated somewhat by other factors, such as handedness (only 70 percent of left-handed individuals are left hemisphere dominant for language) and gender (males are significantly more lateralized in general than females), but by far the majority of people demonstrate this consistent lateralization. The right hemisphere seems to be predominantly focused on visiospatial processing, such as guiding, grasping, and dexterous manipulations of objects, and in facial recognition. The right hemisphere also seems to be used primarily for recognizing emotional cues. Larger patterns also exist, with the left hemisphere seeming to be optimized for processing information that comes in very rapidly with frequent changes, while the right hemisphere is better optimized for information that stays relatively consistent over time or changes much more slowly.

Another significant difference between the hemispheres is the connections from sensory input and to muscle output. The body itself is divided down the middle and each half reports the sensory information coming in to, and takes its orders for response from, only one hemisphere. The connections cross at various points between the brain and the organs and muscles such that the right side of the body sends its input and receives its output from the left-hemisphere and the left side of the body from the right hemisphere.

In normally functioning individuals the two hemispheres share information through the corpus callosum, so that all information is eventually made available to all areas of the brain. There can be some interesting time delay effects between hemispheres due to the need to cross-communicate, but this is on the order of milliseconds.

Corpus callosotomy[edit]

In some extreme cases individuals will have their corpus callosum surgically severed. This is most often in response to intractable epileptic seizures that have debilitating effects. The procedure used to be much more common and modern surgical techniques can often limit the need to sever the whole corpus callosum and focus on only certain parts. Patients who have had the surgery often do not show significant loss of cognitive ability and can live fairly normal lives. However, researchers have shown that these patients show a range of odd linguistic and behavioral responses in a wide variety of laboratory tests. Different information can be given to both hemispheres simultaneously by tapping into the fact that the right eye goes to the left hemisphere and the left eye to the right. Responses from each hemisphere can be measured by having the left or right hand perform some task. The tests basically show that patients are no longer a “whole” individual but seem to have become two separate, often competing, people. It is not difficult to get the left and right hand to engage in battles for control over a task. Information can be sent to the right hemisphere that clearly affects decision making, but the person fails to report any conscious recognition of that information.

These fascinating results show that the mind is clearly a product of the brain and its connections and structure. The results fall directly out of predictions from materialist theories. However, non-materialist dualism has no working hypothesis that can satisfactorily explain these results.

Examples of split brain effects[edit]

Patient N.G. and object recognition[edit]

“Split-brain patient N.G. sits in front of a screen with a small black dot in the center. She is asked to look directly at the dot. A picture of a cup is briefly flashed to the right of the dot. N.G. reports that she has seen a cup. A picture of a spoon is flashed to the left of the dot. N.G. says she saw nothing. She is then asked to reach under the screen and select, using only touch, the object she has seen, among several objects. She manipulates each object and selects the spoon. When asked what she is holding she says 'pencil.'”[1]

Patient W.J. and right vs. left hand conflict[edit]

“W.J. was the first patient to have his brain split in recent years. ..Following his surgery in 1961, his delightful personality remained unchanged but a most remarkable phenomenon happened. … Previously he could write dictated sentences and could carry out any kind of command` such as .. drawing geometric shapes with his right hand. But after his surgery, he could not arrange four red and white blocks in a simple pattern with his right hand. … At the same time W.J.’s right hemisphere was a whiz. When blocks were presented to his left hand, he quickly and adeptly arranged them into the correct pattern. .. After the first testing session … investigators arranged to film W.J. carrying out tasks. … During filming, W.J.’s right hand attempted to arrange the blocks and his left hand kept trying to intervene. W.J. had to sit on his left hand so the inadequate but dominant right hand could at least try. ..For the film’s final scene, everyone decided to see what would happen if both hands were allowed to arrange the blocks… As soon as Mind Right, working through the left hand, began to arrange the blocks correctly, Mind Left would undo the good work. The hands were in competition. The specializations of each hemisphere were different, and growing out of that were the behaviors of each half of the brain.”[2]

Patient P.S. and the burning house[edit]

P.S. was shown two pictures of houses, aligned down a central axis. The houses were identical in all ways, except one house had its left side on fire. The visual information from the left side goes only to the right side of the brain so the language dominant left-hemisphere does not have access to it. P.S. reported that there was no difference at all between the two houses. However, when asked in which of the two she would live, P.S. consistently chose the house that was not burning. This test has been performed many times but what is even more interesting is when patients are asked why they prefer one house over another.

"P.S. deemed 'silly’ the task of choosing between two ‘identical’ houses. However, other studies provide this information and report a variety of responses at debriefing. For instance, a patient described by Manning and Kartsounis (1993) chose the non-burning house confabulating that it had an extra fireplace...Another patient described by Bisiach and Rusconi (1990) consistently chose the ‘burning’ house, considering it more ‘spacious’ on the burning side, where the contour of the flames actually enlarged the shape of the house, an example of choice based on an implicit misinterpretation of the difference. In a group of 13 neglect patients (Doricchi et al. 1997), responses motivating correct implicit choices of the ‘non-burning house’ were equally distributed into two categories: ‘there is no specific reason for my choice', and 'the two houses are the same anyway’"[3]


  1. Kolb, B., & Whishaw, I. (1985). Fundamentals of human neuropsychology. Freeman, New York.
  2. Gazzaniga, M., Ivry, R., & Mangun, G. (2002). Cognitive Neuroscience: The Biology of the Mind. WW Norton & Company.
  3. di Neuropsicologia, C., & Santa, F. Left Unilateral Neglect as a Disconnection Syndrome. Cerebral Cortex.

See also[edit]