Visual extinction in oculocentric coordinates: A selective bias in dividing attention between hemifields

Abstract

Visual extinction following unilateral brain damage manifests as a deficit in detecting contralesional stimuli that occur briefly together with more ipsilesional events, despite intact detection of isolated stimuli on either side. Here we report a patient with left-side extinction, MD, whose contralesional deficit arose only when concurrent visual stimuli appeared billaterally, one in each of the two retinal hemifields. By contrast, when the two visual events both appeared to the left of fixation within blocks of trials (i.e. within the contralesional hemifield), her detection peformance improved significantly. MD's extinction of left targets in bilateral trials, together with her good performance for concurrent targets within the left hemifield, was unaffected by having her deviate her gaze into the left or right hemispace; nor was it affected by having her fixate close to left or right targets, or by altering the spatial separation between targets. Moreover, MD was normal in detecting double targets when these were vertically aligned so that they appeared above and below fixation, thus ruling out a general problem in detecting concurrent stimuli. When visual events could appear on either side of fixation at random, however, so that she had to monitor both sides for potential targets, MD showed extinction for concurrent targets presented entirely within her left hemifield, as well as for those presented bilaterally. We suggest that MD's extinction is due to a deficit in oculomotor preparation for action that arises when visual targets can appear unpredictably on either side of fixation.