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

Visual extinction following unilateral brain damage manifests as a deficit in detecting contralesional stimuli that occur briefly together with more i psilesional events, despite intact detection of isolated stimuli on either side. Here we report a patient with left-sided extinction, Mn, whose contra lesional deficit arose only when concurrent visual stimuli appeared bilater ally, 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 performance impr oved significantly. Mn's extinction of left targets in bilateral trials, to gether with her good performance for concurrent targets within the left hem ifield, was unaffected by having her deviate her gaze into the left or righ t hemispace; nor was it affected by having her fixate close to left or righ t targets, or by altering the spatial separation between targets. Moreover, Mn was normal in detecting double targets when these were vertically align ed so that they appeared above and below fixation, thus ruling out a genera l problem in detecting concurrent stimuli. When visual events could appear on either side of fixation at random, however, so that she had to monitor b oth sides for potential targets, Mn showed extinction for concurrent target s presented entirely within her left hemifield, as well as for those presen ted bilaterally. We suggest that Mn's extinction is due to a deficit in ocu lomotor preparation for action that arises when visual targets can appear u npredictably on either side of fixation.