Simultanagnosia. To see but not two see.

Simultanagnosia is a disorder of visual perception characterized by the inability to interpret complex visual arrays despite preserved recognition of single objects. We report a series of investigations on a simultanagnosic patient which attempt to establish the nature of this visual processing disturbance. The patient performed normally on a feature detection task but was impaired on a test of attention-requiring visual search in which she was asked to distinguish between stimuli containing different numbers of targets. She was not impaired on a visual-spatial orienting task. She identified single briefly presented words and objects as rapidly and reliably as controls suggesting that access to stored structural descriptions was not impaired. With brief, simultaneous presentation of 2 words or drawings, she identified both stimuli significantly more frequently when the stimuli were semantically related than when they were unrelated. On the basis of these and other data, we suggest that the patient's simultanagnosia is attributable to an impairment in the process by which activated structural descriptions are linked to information coding the location of the object.