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Abstract:
A fundamental issue in visual perception is whether visual
search occurs in serial or in parallel. To address this issue, we
recorded event-related potentials (ERPs) and examined the N2pc
component, an attention-related response observed at electrodes
contralateral to an attended object. If visual search proceeds as a
series of shifts of attention, then the N2pc component should shift
back and forth between the hemispheres as attention shifts between
the visual fields. In most visual search tasks, the search order is
unknown, making it difficult to measure the moment-by-moment focus
of attention. To circumvent this problem, we biased the order in
which attention might be shifted by presenting the target item in a
specific color on 75% of target present trials (this color was
called C75) and in a different color on the remaining 25% (this
color was called C25). When C75 and C25 were in different
hemifields, the N2pc was found to be contralateral to C75 from
200-300 ms post-stimulus and then switched hemispheres and became
contralateral to C25 from 300-400 ms. Similar results were obtained
in another experiment that did not rely on a probability
manipulation, but instead made use of subjects' intrinsic bias to
search the items closest to fixation first. These results indicate
that subjects rapidly reallocate attention among potential target
items during visual search, consistent with a high-speed serial
scanning mechanism.
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