CAN DEFICITS IN SPATIAL INDEXING CONTRIBUTE TO SIMULTANAGNOSIA?

Abstract

Patient AMA suffered a head trauma that left her with several visual complaints, including a reading disability. AMA appears to suffer from simultanagnosia, as established with tasks such as naming briefly presented multiple stimuli or overlapping figures, describing the theme of complex scenes, and counting arrays of stimuli. Specifically, AMA has difficulty perceiving immediately successive stimuli and, in particular, multiple stimuli that appear at novel or unexpected locations. Her ability to encode spatial relations rapidly (of either the categorical and coordinate type) is markedly reduced. However, when a familiar target appears among multiple stimuli at expected (previously encoded) locations, AMA's performance can be within normal limits. These results suggest that this patient's simultanagnosia cannot be reduced to an inability to process multiple stimuli per se. Rather it is better characterised as (1) an inability to index new locations of multiple stimuli, and (2) a reduced efficiency in pattern analysis. The former deficit, in turn, may lead to difficulty in focusing on objects efficiently and using objects as landmarks or reference points. Damage to one or both of the above mechanisms could produce simultanagnosia and reading difficulty.