Hemisphere differences and interhemispheric relations with special reference to the functions of the corpus callosum
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This thesis contains reports of behavioural investigations carried out on an acallosal girl and her brain-damaged sister. These patients were compared on tasks involving tactile, auditory, and visual perception with a group of control Subjects matched for sex, I.Q. and age, as well as with normal Subjects of the same sex and age. The general aim of the experiments was (a) to investigate and compare the functions of the cerebral hemispheres and (b) to study interhemispheric relationships. Using normal Subjects as well as the acallosal patient special attention was directed towards furthering our understanding of the functions of the corpus callosum. Our results were compared with previous findings in an effort to resolve conflicting findings in the agenesis literature and to explain differences between the previously reported acallosal and surgical-bisected patients' behaviour. The acallosal was found to perform less efficiently than control Subjects on some bimanual and unimanual motor coordination tasks. This finding is consistent with earlier reports on agenesis Subjects. Some evidence was found to indicate that acallosals do not efficiently transfer learning of a tactile formboard task from one hand to the other The acallosal patient did not show an anomia of the left hand as do surgical-bisected patients. There is, however, an inefficiency in tactile cross-localization of light touch stimuli. Dichotic listening experiments used verbal stimuli to test the hypothesis that the acallosal girl would demonstrate equipotentiality of language processes in both hemispheres. Tonal patterns were similar presented dichotically. The acallosal Subject exhibited a greater right-ear advantage than the controls on the recall of digits. Studies of reaction time to a simple visual stimulus replicated findings of greater pathway differences for acallosals than for normals. Another visual perception study supported the finding that acallosals do not show interocular transfer of movement aftereffects. Tachistoscopic recognition tasks of alphabetical and facial stimuli were used on the acallosal, her sister, and normal controls. A developmental study of normals was also carried out. Thirteen year old normals and adults were found to have a left-hemisphere superiority for letters and right-hemisphere for faces. The seven and eight year olds did not show significant lateralization of either faces or letters. The acallosal showed a left visual field superiority for letters, a finding opposite to that for normals. These findings suggest that processes involved in recognition of letters and faces develop with increasing age in childhood. The acallosal Subject appears to have bilateral representation of language processes across sensory modalities in opposite hemispheres, presumably the result of compensatory development arising from lack of a corpus callosum.
Thesis, PhD Doctor of Philosophy
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