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Abstract:
Visual extinction is classically displayed when stimuli are
applied simultaneously to both visual fields. Under these
conditions of Double Simultaneous Stimulation (DSS), items in the
contralesional field fail to reach awareness. Recently it has been
shown (Di Pellegrino, Basso and Frassinetti, Neuropsychologia,
1997) that extinction is reduced if targets are asynchronous,
either with a contralesional or an ipsilesional lead. This is
surprising given that patients with unilateral brain damage
estimate events in the contralesional field to occur later than
simultaneous ipsilesional events. In this study we presented visual
stimuli in both visual fields at the same time as estimating the
delay of the contralesional field. Patients may show greatest
extinction when the stimuli are presented at the same time, with a
monotonic reduction with either the contralesional or ipsilesional
field delayed. Nonetheless, these same patients estimated
contralesional events to be delayed with respect to ipsilesional
events. The fact that extinction was not greatest in these patients
at subjective simultaneity suggests that it is objective
simultaneity, rather than subjective simultaneity that is important
in the generation of extinction. These results suggest that the
subjective delay of contralesional events may be the result of
extinction, but not a cause of extinction.
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