Visual extinction as a spatio-temporal disorder of selective attention

STUDIES of normal behaviour have shown that the process of selection takes a finite time, one measure of which is the attentional dwell time, that is the period of interference produced by one attended stimulus on a subsequent one. Here we investigated the time for selection in FB, a neurological patient suffering from a visuospatial disorder of attention (unilateral extinction). FB was asked to identify two letters displayed in rapid succession either to the left (damaged), or to the right (intact) visual hemifield. By varying the interval between stimuli, we measured how long the first letter continued to interfere with accuracy on the second-that is the first letter's attentional demand over time. The results showed that the process of selection has an abnormal duration in the affected visual field, being at least twice as long as in the intact field. We suggest that the slowed visual processing for the contralesional object may contribute to the competitive bias against that object which is the hallmark of unilateral extinction. (C) 1998 Rapid Science Ltd.