Visual feature discrimination in simultanagnosia: A study of two cases

Abstract

Case studies of two simultanagnosic patients are reported. Both patients present with difficulties in interpreting complex thematic pictures and in identifying multiple, briefly presented letters. Experiments 1-4 examine the patients' picture processing capabilities, and show that the patients are abnormally sensitive to stimulus exposure duration and to overlap between objects. Experiments 5-8 examine the patients' abilities to discriminate targets defined by a simple disjunctive feature relative to the background. Processing of targets defined by colour and size is shown to be abnormally affected by decreasing target exposure and target-distractor discriminability, though flat, efficient search functions occur with salient targets. Processing of targets defined by differences in form information is markedly impaired, even when the targets are very salient (lines differing in orientation by 45°). We discuss the relations between the patients' picture processing and their feature discrimination abilities, and the role of impaired feature discrimination in simultanagnosia.