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Simultanagnosia

A defect of sustained attention yields insights on visual information processing

Division of Behavioral Neurology and Cognitive Neuroscience, Department of Neurology (Dr. Rizzo), College of Medicine and Department of Speech Pathology and Audiology (Dr. Robin), College of Liberal Arts, The University of Iowa, Iowa City, IA.

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.

Address correspondence and reprint requests to Dr. Matthew Rizzo, Department of Neurology, The University of Iowa Hospitals & Clinics, Iowa City, IA 52242.

Supported by NIH grant PO NS 19632.

Received April 11,1989. Accepted for publication in final form August 23,1989.