Split Brain
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1A VIEW OF THE WORLD FROM A SPLIT-BRAIN PERSPECTIVEbyDahlia W. ZaidelDepartment of Psychology, University of California at Los Angeles, Los Angeles, CA 90024, USAAcknowledgements: This work was supported by NIH Grant NS 20187.Outline1. Introduction2. Boundaries of reality and conscious awareness3. Unity of consciousness4. Some clinical background5. Basic facts about left and right6. Pathology and hemispheric specialization7. Elements of the paradox8. Relevant early experimental work9. One person despite a split brain10. One hemisphere in control11. Subcortical integration12. Sense of humor13. Sex14. Telling personal stories15. What is not considered normalMemoryEmotional reactionsDaily lifeLaboratory
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216. Dreams17. Attitudes towards the left hand18. ConclusionIntroductionThis chapter is not an account of dramatic personality changes following brain surgery.Instead, most of the changes are subtle and require special laboratory tests to emerge. But it istrue that the daily lives of split-brain patients stand in sharp contrast with their performance inlaboratory tests and defy some simple, unitary understanding of how the mind is organized in thebrain. These patients are cases with complete commissurotomy in the Bogen-Vogel, Caltechseries. As a group, their behavior represents some of the most fascinating phenomena inneurology and understanding them has provided a challenge to students of neuropsychology andneuroscience as well as philosophy.The contrast creates a paradox for the following reasons: The left and right hemispheresof the brain (the neocortex) are normally connected to each other via several different bundles offibers but here these rich fiber systems were sectioned surgically, separating the hemispheres.The result is two halves of the brain, originally designed by millions of years of evolution to beanatomically connected, now processing information nearly independently from each other whilehaving different functional specializations. The different hemispheric functions have come to beconsidered complementary and as such to represent the ideal for normal human behavior. Andyet, in daily life, the patients appear to behave as if there was no evolutionary purpose to thismajor forebrain neuronal connection between the hemispheres. Certain functions considered bysome to be specialized in the right hemisphere such as voice modulation or prosody appearunimpaired. Left hemisphere functions such as speech and language comprehension also appearunimpaired. Previously learned functions that require bilateral interaction such as, cooking,cycling, swimming, or piano playing appear unchanged, and have remained so until now, as longas 30 years post-surgery in some cases. Neither have there been major changes in personality or
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3mannerisms, or in general intelligence. There are no psychiatric symptoms such as hallucinations,dellusions, fugue states, or multiple personalities. Each patient behaves as one with a singlepersonality and unified consciousness. Thus, we must look at what is amiss in order todistinguish between the apparent and the real. Clues to the paradox were revealed in laboratorystudies and those are discussed in the following sections of this chapter.Boundaries of reality and conscious awarenessBoundaries of consciousness and of reality are recognized through deviation from thatwhich is accepted and considered normal. What is normal in the split-brain patients behaviorwill be described first.Some of the many dimensions of conscious awareness include orientation to space andtime, knowledge of human biological and sociological context, intentionality, and so on. Insplit-brain patientsas a group, all these dimensions of awareness seem intact. Orientation withrespect to where they are located at any given moment is fully acknowledged and known as istime of day, month, or year. Memory for past personal events that occurred before surgeryappears intact and knowledge of national or international historical events is at a level thatwould be expected given their educational background. Knowledge of current events is faultyand is most likely due to their poor recent memory and the lack of interest in reading followingsurgery. Knowledge of sociological good or evil and, depending on the personality andintelligence level of the patient, “taking sides”, all appear normal. Moreover, intentions areexecuted normally. Thus, if they want to touch a person or a table, they do so and they feel thedifference in their sensation. If they wish to listen to music, they turn the radio on and respondappropriately to the sounds. These are only a few examples.In laboratory tests where information is lateralized to only one hemisphere and alateralized motor response is required, either hand can do so, even when the left hand response iscontrolled by the right, non-speaking hemisphere. This is demonstrated in specially- designedtests where the answer is hidden from view and the response is nevertheless provided by either
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4hand. For instance, if the examiner requests, “after feeling all the choices behind the screen,decide on the correct answer, and tap on it with your finger” The patients can carry out theinstructions effortlessly (see D. Zaidel, 1990a for examples). In daily life, either the left or theright hand reaches out to touch or to pick things appropriately. In other words, intentionality isnot restricted to the dominant, speaking hemisphere and