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Abstract:
Abstract: Gradients of attention have been the subject of
previous studies, but few have investigated the time course of
competitive attentional processes at different locations in the
visual field. We investigated the time course of extinction in
right posterior parietal patients. Patients were asked to identify
pairs of letters that appeared briefly in the right visual
hemifield, and which were separated in time by various stimulus
onset asynchronies (SOAs). Patients often extinguished the leftmost
stimulus, but only when the rightmost stimulus preceded it by
300-900 ms. Furthermore, this extinction was greatly reduced when
the stimulus pairs were presented more ipsilesionally (farther to
the right of fixation). Performance for stimuli appearing singly
was unaffected by location. This shows that extinction can occur
within the ipsilesional hemifield, and provides evidence for the
existence of a spatial gradient of attentional processing in the
left parietal lobe. We also found that extinction was maximal
during this same range of SOAs even when stimuli were presented in
the contralesional hemifield, or in opposite hemifields. These
experiments suggest that 1) extinction is maximal during a specific
time frame, 2) the attentional deficits of parietal patients may be
the result of unbalanced, mutually inhibitory competition between
neurons representing stimuli in different locations, and 3) this
competition can occur between neurons within the same cerebral
hemisphere.
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