The fate of global information in dorsal simultanagnosia

Abstract

We report a patient with simultanagnosia and Bálint-Holmes syndrome, following bilateral parieto-occlpital lesions, who exhibits a selective disturbance of global processing in everyday situations and in clinical tasks. For hierarchical Navon stimuli, where global letters are formed by the layout of smaller local letters, she could report only local shapes. She could identify the global form via proprioceptive input, when her finger was passively moved to trace the global shape, provided her eyes were closed. However, with her eyes open, the local visible form dominated once again even when the global shape was traced. This demonstrates dominance of pathological vision over intact proprioception, and shows that local capture persists even when the global information can still be processed by the patient, through another modality. This raises the possibility that some visual global processing might likewise still take place despite the local capture. In accordance wlth this notion, additional experiments showed that the patient was faster at naming local shapes if the global shape had the same identity rather than an incongruent identity. Moreover, a visual-search task found that parallel searching for unique features was preserved across the visual field. Taken together, our experiments suggest that the dominance of the local scale for KB is not due to a total inability to code any global information, but rather to an attentional bias towards salient local details following her brain damage.

Key Words:

• Balint syndrome
• local/global processing
• attention Visual search
• proprioception implicit information processing
• brain damage human