Can deficits in spatial indexing contribute to simultanagnosia?

Patient AMA suffered a head trauma that left her with several visual compla ints, including a reading disability. AMA appears to suffer from 'simultana gnosia, as established with tasks such as naming briefly presented multiple stimuli or overlapping figures, describing the theme of complex scenes, an d counting arrays of stimuli. Specifically, AMA has difficulty perceiving i mmediately successive stimuli and, in particular, multiple stimuli that app ear at novel or unexpected locations. Her ability to encode spatial relatio ns rapidly (of either the categorical and coordinate type) is markedly redu ced. However, when a familiar target appears among multiple stimuli at expe cted (previously encoded) locations, AMA's performance can be within normal limits. These results suggest that this patient's simultanagnosia cannot b e 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 defi cit, 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 t he above mechanisms could produce simultanagnosia and reading difficulty.