Simultanagnosia: effects of semantic category and repetition blindness.

When confronted with two identical stimuli in a very brief period of time subjects often fail to report the second stimulus, a phenomenon termed "repetition blindness". The "type-token" account attributes the phenomenon to a failure to individuate the exemplars. We report a subject, KE, who developed simultanagnosia (the inability to see more than one item in an array) as a consequence of bilateral parietal lobe infarctions. With presentation of two words, pictures or letters for an unlimited time, KE typically reported both stimuli on less than half of trials. Performance was significantly influenced by the semantic relationship between items in the array. He reported both items significantly more frequently if they were semantically related; in contrast, when presented either identical or visually different depictions of the same item, he reported both items on only 2-4% of trials. Performance was not influenced by the visual similarity between the stimuli; he reported visually dissimilar objects less frequently than visually similar but different objects. We suggest that KE's bilateral parietal lesions prevent the binding of preserved object representations to a representation computed by the dorsal visual system. More generally, these data are consistent with the claim that the posterior parietal cortex is crucial for individuating a stimulus by computing its unique spatio-temporal characteristics.