How does a Split Brain See the World?

Arash Afraz
Department of Psychology
Harvard University

Corpus callosum is the major interhemispheric cortical commissure 
in human brain.  Cutting posterior parts of the corpus callosum 
leads to serious deficits in integration of visual stimuli among 
left and right hemifields. Although a typical patient with 
posterior commissurotomy cannot compare two visual stimuli 
presented to left and right visual fields, it is still possible 
for some higher level visual functions to act among the hemifields. 
Here, I want to summarize some investigations about 
interhemispheric attentional interactions in a split brain patient 
whose posterior and middle parts of corpus callosum is resected 
surgically. Although the patient cannot compare visual stimuli 
presented to left and right hemifields above chance level, she 
still shows a strong interhemispheric "crowding" interaction. 
Also, she can perform an attentive object tracking task while an 
attended object crosses the midline. On the other hand, in a 
visual search task, she behaves like a classical split brain 
patient, showing no interhemispheric interaction. Finally, I want 
to suggest that "space selection" interactions can take place 
among the vertical midline based on anterior cortical commissures 
whereas, "object selection" interactions can be performed only 
based on rich interhemispheric connection through posterior 
corpus callosum.