Date of Award


Degree Type


Degree Name

Doctor of Philosophy


When tested using behavioural methods strabismic humans and strabismic animals exhibit similar deficits in visual performance. There is often a decrease in visual acuity in the deviating eye, abnormal spatial localization, vergence eye movements, and poor stereopsis and depth perception. These similarities have suggested that strabismic humans may share many of the anatomical and physiological abnormalities which are found in the visual systems of strabismic animals. Among these abnormalities is a disruption and shrinkage of the fibres which run through the posterior portion of the corpus callosum interconnecting the sensory visual regions of the cortex.;In the present series of experiments, response times to presentations of visual stimuli were used to examine the conduction of information through callosal fibres. Response times of normal and strabismic humans were compared under two conditions: when the cerebral hemisphere initially stimulated by the presentation of the stimuli and controlling the motor response were the same, and when the hemispheres were different. This provided an estimate of cross-callosal transmission time for the two groups of subjects.;The cross-callosal transmission times for the strabismics were found to be consistently longer than those of the normals when simple unimanual responses were required. As predicted, this difference was found only when the stimuli were presented within 5 degrees from central fixation, an area which projects to a region of the sensory visual cortex where callosal fibres originate and terminate. The difference was not found when stimuli were presented outside this central area. When a decision about the stimulus was required before a response could be performed, the mean duration and variability of the estimates increased and provided a less useful measure of possible delays in cross-callosal transmission.;The results of the present study suggest that strabismic humans may have disruptions or abnormalities in those callosal fibres which are involved in the interhemispheric transmission of visual information between the sensory regions of the visual cortex. The abnormalities result in slower cross-callosal conduction times, and may be similar to the disruptions in posterior callosal fibres seen in strabismic animals. In addition to delays in cross-callosal transmission, strabismic humans many also show slower overall response latencies when stimuli are presented at eccentricities outside the central part of the visual field. This suggests that strabismics may have an additional, independent within-hemisphere processing delay.



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