[Reading numbers in pure alexia: effects of the task and hemispheric specialization]

Selective conservation of the ability to read Arabic numbers in patients unable to read words or even letters is a classical characteristic of pure alexia described by Dejerine (1982). We report our work on the capacity of two patients with pure typical alexia to process numbers. Our main finding was that these patients could count pairs of Arabic numbers correctly when the reading task was simple (example 2 4-->"two four") or when the task involved comparing sizes (example 2 4-->"four is bigger than two"). Inversely, these patients often made mistakes when asked to perform arithmetic operations (example 2 4-->"two plus four equals six"). Using these two numbers, there was a similar dissociation between excellent performance on comparison tests and severe deficiency in reading out loud. We interpret these findings with the hypothesis that both of the hemispheres can identify Arabic numbers, but that the visual systems on the right and left play a different role during different tasks. In pure alexia, a lesion in the left identification system leads to selective deficiency in linguistic tasks such as reading numbers out loud, recognizing numbers with several figures or mental arithmetic. The right identification system in intact and is sufficient for comparison or reading isolated Arabic numbers.