Interhemispheric Visual Communication in Human Comissurotomy Subjects

Citation

Johnson, Larry E. (1980) Interhemispheric Visual Communication in Human Comissurotomy Subjects. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ccfk-jg81. https://resolver.caltech.edu/CaltechTHESIS:07162025-170515256

Abstract

Several problems concerning the transfer of visual information between the cerebral hemispheres in human forebrain commissurotomy patients were examined. These patients, 10-15 years post-surgery, were compared with normal and partially-split (splenium-intact) control subjects both on their ability to verbally categorize or name stimuli tachistoscopically presented unilaterally into either the left or right visual half-field (LVF or RVF), several degrees from the fixation point, and on their ability to compare or name simultaneously presented bilateral visual stimuli. The aim of these experiments was (1) to determine the extent to which stimuli presented to the left visual field could be orally described, (2) to learn whether visual information presented separately to each "disconnected" hemisphere could be compared, and (3) to explain the findings in context with previous neuroanatomical, physiological, and behavioral studies.

Using either a box tachistoscope or a back-projection screen, a wide variety of stimuli (brightness, colors, numbers, letters, patterns, schematic faces, and photographs of human faces) were flashed to one or both visual half-fields, and both the accuracy and the speed of several kinds of manual and verbal responses were measured.

Three of the four split-brain patients were able to orally categorize unilaterally presented stimuli ("yes"-"no", "odd"-"even") in both visual fields. In addition, all four patients responded as rapidly to LVF stimuli as to RVF stimuli in categorization experiments. The control subjects also accurately categorized stimuli and responded equally fast to LVF and RVF stimuli.

When the patients were asked to~ the unilateral stimulus, those in the RVF were easily named but the ability to name LVF stimuli was found to vary between patients for different stimuli and sample sizes. However, despite these differences in accuracy, all of the split-brain subjects responded significantly more slowly to LVF than to RVF stimuli, except for facial stimuli. Control subjects, on the other hand, continued to respond equally fast to stimuli in either visual half-field.

Finally, when the patients were required to compare a variety of bilaterally presented visual stimuli as same or different, there was again a wide range of abilities. In general, it was found that those patients who were best at naming stimuli were worst at cross-comparing them, and vice versa. One subject (LB) was able to name two bilaterally flashed stimuli, and yet was unable to compare them as same or different beforehand, while another (NG) could cross-match two stimuli by name identity as well as physical identity, but could not name the LVF stimulus.

These results most easily suggest the following hypotheses: (1) Some commissurotomy patients can make oral categorization responses to unilateral LVF stimuli, perhaps using their left hemisphere by way of midbrain pathways, while (2) naming LVF stimuli most likely requires a different mechanism, probably involving right hemisphere speech. In addition, (3) some patients can cross-compare stimuli between the two visual fields. In these cases, the oral responses likely come from left hemisphere verbal centers since these patients are also poorest at naming LVF stimuli.

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