Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
9. When a surgeon severs the Corpus Callosum, the two hemispheres are no longer able to work in close communication while performing. The experimental set-up devised by Roger Sperry, Michael Gazzaniga, and Joseph Bogan (the surgeon) enabled them to measure the functional specialization of the two hemispheres. In the now classic studies of split-brain patients, a visual stimulus was presented in a single visual field and, therefore, destined for a single hemisphere. These visual stimuli were presented very briefly on a screen directly in front of the subject, to either the left or right of the central fixation point. Because the subjects' eyes were fixated straight ahead, the brief stimuli were processed selectively by the hemisphere opposite to the side of presentation. When pictures were presented in the right visual half field and processed by the left hemisphere, the subjects were able to name and describe the object in the picture. When the pictures were presented in the left visual half-field and processed by the right hemisphere, however, the subjects were not able to name or describe the object. This finding supported the idea that the left hemisphere is specialized for language.
In additional study, the subjects were asked to respond to the stimuli by pointing out the object presented. Using the left hand which was controlled by the verbally-silent right hemisphere, the subjects were able to point out the stimuli that were presented in the left visual half field. This finding indicated that the right hemisphere was processing the information, but unable to speak about it. Furthermore, these studies demonstrated a left hand and right hemisphere advantage for such task as assembling a puzzle or drawing, even in right-handed subjects. Additional visual-spatial tasks that are performed with greater accuracy and speed by the right hemisphere include the ability to distinguish color, arrange blocks to match a picture, and recognize faces. The major contributions of the so-called silent and non-dominant right hemisphere were beginning to emerge.
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After 30 years of research on the disconnection syndrome, Michael Gazzaniga, has written a number of papers that provide his theoretical interpretation of the results (see e.g. the Scientific American articles in the additional readings section). He has reached the conclusion that disconnection of the regions of the brain that underlie perception from those involved in verbal behavior, also disconnects them from conscious awareness. The left hemisphere, he argues, is the site of consciousness. It is this author's opinion that this interpretation is a return to the left hemisphere is "dominant" and right hemisphere is "non-dominant" perspective that characterized the earliest explorations in hemispheric lateralization of function, and may be a reflection of the perceptual and response-bias of the researcher. The data indicate that the right hemisphere may process the external and internal worlds holistically by interpreting and integrating multiple events across space in any given moment. This mode of processing may not be described easily using our sequentially organized (left hemispheric) language. Indeed, verbal awareness of events reaching the right hemisphere of a disconnected brain is nonexistent. Those, however, that are more geared to experiencing existence in the moment, are quite able to experience consciously the results of the right hemisphere in action!