Early and Late Visual Deficits in Simultanagnosia

Abstract:
Abstract: Simultanagnosia is a disorder characterized by the inability to interpret complex visual arrays despite preserved recognition of single objects. We report the results of a series of experiments on four patients with simultanagnosia secondary to bilateral posterior infarcts. Anatomical and experimental data suggest that the deficits observed in these patients can be grouped in two different categories. Two patients showed early processing deficits as revealed by the presence of illusory conjunctions; they also were impaired in the binding of visual features to form objects as well as in the allocation of attention in space. The second group of patients did not produce illusory conjunctions but exhibited an inability in binding more than one object feature to a location. Anatomical data collected for clinical purposes (CT and MRI scans) revealed different patterns of brain damage in the two groups. Patients with earlier visual deficits showed bilateral posterior parietal/occipital lesions whereas patients with the binding deficit showed bilateral parieto-temporal lesions. These data suggest that simultanagnosia is a heterogeneous disorder which may be attributable to impairments at different levels of visual processing and have implications for accounts of normal visual processing.