VISUAL FEATURE DISCRIMINATION IN SIMULTANAGNOSIA - A STUDY OF 2 CASES

Case studies of two simultanagnosic patients are reported. Both patien ts 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 tha t 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 feat ure relative to the background. Processing of targets defined by colou r and size is shown to be abnormally affected by decreasing target exp osure and target-distractor discriminability, though flat, efficient s earch functions occur with salient targets. Processing of targets defi ned by differences in form information is markedly impaired, even when the targets are very salient (lines differing in orientation by 45-de grees). We discuss the relations between the patients' picture process ing and their feature discrimination abilities, and the role of impair ed feature discrimination in simultanagnosia.