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WIREs Cogn Sci
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The split‐brain

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Abstract Research on split‐brain individuals started to flourish approximately 70 years ago and has since then significantly contributed to our understanding of hemispheric specialization. This overview aims to capture the essential of its progress. Amongst other things, the disconnection syndrome is exposed through a description of its manifestations on sensory, motor, and cognitive functions. Ground work and recent studies on split‐brain individuals are integrated. Copyright © 2010 John Wiley & Sons, Ltd. This article is categorized under: Neuroscience > Clinical Neuroscience

Illustration of the two types of trials used in a traditional crossed‐uncrossed difference (CUD) paradigm. In uncrossed trials, the hemisphere receiving the visual stimulus is the same hemisphere providing the manual response. In crossed trials, the hemisphere receiving the visual stimulus transfers the signal to the other hemisphere providing the manual response. The CUD of split‐brain individuals is much larger than the very small CUD of normal individuals. Note that the interhemispheric pathway illustrated here goes through the motor cortex. There may also be interhemispheric transfer through the visual cortex.

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Dichotic listening in normal and split‐brain individuals.

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Test of tactile localization. Without visual support, stimulation is randomly delivered on one finger after which identification of the stimulated finger is carried out with the thumb of a given hand. In split‐brain individuals, intramanual responses usually reflect correct identification of the finger, whereas intermanual responses often fail.

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Illustration of chimerical figures that can be used to test hemispheric specialization in face recognition. One half of two faces are combined together to form a full face and each half is presented to different hemispheres. Consequently, identification takes place as a function of the hemisphere providing the response.

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Although split‐brain patients cannot name a lateralized stimulus presented to their right hemisphere, they can select it with their left hand (right hemisphere) amongst several objects.

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In split‐brain individuals, when lateralizing each stimulus in one hemisphere, the left hemisphere can properly read and name ‘flower’, whereas the right hemisphere is unable to name the word that lies in its hemifield (left hemispace).

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In split‐brain individuals, information does not transit through the corpus callosum anymore. As a consequence, because of hemispheric specialization, the two hemispheres are limited in the accomplishment of specific tasks. This can give rise to manifestations such as left unilateral agraphia. For example, split‐brain individuals can write the word ‘tree’ with their right hand because writing is a function controlled by the left hemisphere. Alternatively, they can draw a picture of a tree with their left hand, but cannot write the word ‘tree’, because drawing is a function predominantly controlled by the right hemisphere.

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Representation of motor control in a neurologically intact individual. Distal musculature such as hands and fingers are strictly controlled by the contralateral hemisphere, whereas proximal and axial musculature such as arms and shoulders are both controlled by the ipsilateral and the contralateral hemispheres.

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Representation of the fiber distribution through the corpus callosum. Figure courtesy of the Psychiatry Neuroimaging Laboratory, Brigham and Women's Hospital, Harvard Medical School (http://pnl.bwh.harvard.edu).

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(A) Magnetic resonance imaging (MRI) of a normal individual illustrating the four portions of the corpus callosum: the rostrum, the genu, the body, and the splenium on a sagittal plane. Note that on this plane, the corpus callosum constitutes the roof of the third ventricle. (B) Coronal section showing how the corpus callosum constitutes the roof of the lateral ventricles.

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MRI of (A) total split‐brain individual, (B) partial split‐brain individual, and (C) callosal agenesis individual.

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Neuroscience > Clinical Neuroscience

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