READING NUMBERS IN PURE ALEXIA - EFFECTS OF THE TASK AND HEMISPHERIC-SPECIALIZATION

Selective conservation of the ability to read Arabic numbers in patien ts unable to read words or even letters is a classical characteristic of pare 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 24 --> ''t wo four'') or when the task involved comparing sizes (example 24 --> ' 'four is bigger than two''). Inversely, these patients often made mist akes when asked to perform arithmetic operations (example 24 --> ''two pins four equals six''). Using these two numbers, there was a similar dissociation between excellent performance on comparison tests and se vere deficiency in reading old loud. We interpret these findings with the hypothesis that both of the hemispheres can identify Arabic number s, 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 iden tification system leads to selective deficiency in linguistic tasks su ch as reading numbers old loud, recognizing numbers with several figur es or mental arithmetic. The right identification system! in intact an d is sufficient for comparison or reading isolated Arabic numbers.