Split Brain Studies

 Some people who suffered epileptic seizures were able to find relief from these seizures by having a commissurotomy in which the neural pathways between the two hemispheres were severed. These “split-brain” patients had two independent hemispheres. However, the neural pathways from the eyes are so wired that each hemisphere receives inputs from both eyes. So under normal circumstances each hemisphere is aware of what is happening in the other hemisphere, so the person acts and behaves quite normally. Michael Gazzaniga has been quite creative in designing experimental situations in which stimuli are selectively input to each hemifield of the eye so that the two hemispheres receive different inputs and are oblivious of what the other hemisphere has received. For example if the command “Walk” was sent to the right hemisphere, the individual would begin walking. However, when asked to explain why he was walking, which went to the verbal left hemisphere, the individual would be perplexed and would try to think up some reason, such as I was going to get a soda. In another experiment a picture of a snow-covered house was presented to the right hemisphere while a picture of a chicken’s claw was presented to the left hemisphere. The participant was instructed to pick a line drawing of an object that related to the picture he saw. The right hand, which is controlled by the left hemisphere, chose a rooster to match the chicken claw, while the left hand, which is controlled by the right hemisphere picked a snow shovel to match the winter scene. Confronted with the problem of his two hands pointing to different drawings he consulted his left hemisphere, which had no knowledge of the winter scene, which offered the explanation that he picked the snow shovel to clean out the chicken coop. This was an after the fact rationalization offered confidently and honestly to make sense of his choice. Gazzaniga proposed that the left hemisphere contains an interpreter that is constantly drawing on general knowledge and past experience to try to make sense of our cognitive world. Gazzaniga and Elizabeth Phelps showed split-brain subjects pictures of sequences of everyday activities such as going to work. They later tested memory for these sequences. They were also asked about items that had not been presented such as a man fixing a television. There were also pictures of activities that fit the schema of a days work, sitting up in bed, brushing teeth, but which had not been presented. Although the left hemisphere often falsely recognized novel incidents that were consistent with the stereotype, the right hemisphere rarely ever did.[1]

There is a resemblance here between this type of rationalization and the stereotypical biases that were discussed earlier (See the blog, “Seven Sins of Memory). The left brain interpreter needs to rely on rationalizations, inferences, and generalizations to try to relate the past to the present. It is likely that this contributes to consistency, change, hindsight, and egocentric biases. The right hemisphere needs to serve as a check on the workings of the left hemisphere, as the right hemisphere appears to be more closely aligned with what really happens in the external world.


[1] Schacter (2001).  op cit. pp 157-158

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