Simultanagnosia: a defect of sustained attention yields insights on visual information processing.

Simultanagnosia, in which subjects report a piecemeal visual experience, offers an important probe of human attention. We studied 2 subjects with simultanagnosia following bilateral superior occipital strokes. Compared with controls, they could orient attention to spatial targets in visual, auditory, and mixed-modal conditions. A different task required immediate response to the appearance or disappearance at unpredictable intervals of any element in a random-dot CRT display. The subject tested could detect less than 50% of 1,600 events, and had increased "mirages" and prolonged reaction times. Undetected events occurred anywhere and formed temporal clusters. Application of signal-detection theory confirmed abnormal sensitivity and response bias (d' and beta). Yet performance improved when a valid cue introduced events in the random display. Our results suggest that simultanagnosia was related to an inability to sustain visuospatial attention across an array, corresponding to processing failure at a level of long-range (global) spatiotemporal interactions among converging inputs from early vision. The operations for orienting and sustaining attention may be dissociable at visual association cortex levels.