Abstract:
This thesis examines the nature of subcortical processes in vision, as manifest
in subjects lacking the major forebrain commissure, the corpus callosum. One subject
(L.B.) with full forebrain commissurotomy, one (R.B.) with callosal agenesis, as well
as normal confiols were tested in two sets of experiments: one on the effects of
luminance and equiluminance on interhemispheric transfer time, the second on the
amount of information that can be transferred subcortically in increasingly complex
tasks. The stimuli were presented in each visual field, and subjects responded with
one or the other hand. Reaction times (RT) for uncrossed conditions where the
responding hand is ipsilateral to the visual field of stimulus presentation were
subtracted from crossed conditions where the responding hand is contralateral to the
visual field of stimulus presentation. This difference between crossed and uncrossed
RTs (CUD) was taken as a measure for interhemispheric transfer time. R.B.'s CUD
generally fell in between the CUD of the normal controls and L.B.. In the first set of
experiments stimuli comprised circular disks which either varied in luminance against
a black background or formed a grey circle on an equiluminant yellow background.
Overall, simple RTs increased as the luminance of the stimuli relative to the
background decreased, but the CUD was not systematically affected. It remained
largely unaffected under equiluminance, suggesting that cortical processing takes
place before interhemispheric transfer. The results support previous evidence that
interhemispheric transfer, even in the split-brain, depends on visually insensitive
pathways.
