Cerebral Cortex Advance Access published online on April 14, 2009
Cerebral Cortex, doi:10.1093/cercor/bhp059
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Too Little, Too Late: Reduced Visual Span and Speed Characterize Pure Alexia
1 Department of Psychology, Center for Visual Cognition, Copenhagen University, DK-1361 Copenhagen, Denmark, 2 Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College London, WC1N 3BG London, UK
Address correspondence to email: randi.starrfelt{at}psy.ku.dk.
Whether normal word reading includes a stage of visual processing selectively dedicated to word or letter recognition is highly debated. Characterizing pure alexia, a seemingly selective disorder of reading, has been central to this debate. Two main theories claim either that 1) Pure alexia is caused by damage to a reading specific brain region in the left fusiform gyrus or 2) Pure alexia results from a general visual impairment that may particularly affect simultaneous processing of multiple items. We tested these competing theories in 4 patients with pure alexia using sensitive psychophysical measures and mathematical modeling. Recognition of single letters and digits in the central visual field was impaired in all patients. Visual apprehension span was also reduced for both letters and digits in all patients. The only cortical region lesioned across all 4 patients was the left fusiform gyrus, indicating that this region subserves a function broader than letter or word identification. We suggest that a seemingly pure disorder of reading can arise due to a general reduction of visual speed and span, and explain why this has a disproportionate impact on word reading while recognition of other visual stimuli are less obviously affected.
Key Words: fusiform gyrus • number reading • reading • theory of visual attention • visual word form area