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Abstract:
Many studies report hemispheric specialization on visual
tasks. A similar dichotomy exists for the upper and lower
hemifields. Following recent experiments in our lab, we question
the standard account of hemispheric specialization (Enns &
Kingstone, 1997). We found asymmetries in visual search only when
both hemispheres were stimulated; not when either hemisphere was
stimulated alone. This suggests the hemispheres are not specialized
per se, but instead compete for common resources. The present study
tested the generality of the competition hypothesis. Is hemisphere
and hemifield specialization only present during competition? 44
observers searched for targets (either schematic faces or the
letter N) in 3 types of brief visual displays: left-right (all
items left or right of fixation), up-down (all items above or below
fixation), and whole (items distributed equally across all
quadrants). We measured search efficiency by target localization
accuracy as a function of display size (total number of items
ranged from 2-24). Specialization theory predicts visual items
preferred by one hemisphere or hemifield will be favored in all
three types of visual displays. Competition theory predicts
hemisphere effects only in whole and up-down displays (conditions
that produce competition between hemispheres); and hemifield
effects only in whole and left-right displays (conditions that
produce competition between hemifields). All results supported the
competition hypothesis. We propose that cortical competition occurs
when information is compared between regions not directly connected
in V1.
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