Number processing in pure alexia: The effect of hemispheric asymmetries and task demands

The relative sparing of arabic numerals, in patients who fail to read words or even letters, is a classical feature of pure alexia originally observed by Dejerine (Comptes Rendus des Seances de la Societe de la Biologie 1892; 4 61-90). We report a study of number processing abilities in two patients suffering from typical pure alexia. Our main finding was that number identification performance varied considerably with task demands. Both patients could name pairs of digits, when they were engaged in a simple naming task or for the purpose of magnitude comparison. In contrast, they frequently misidentified the very same digits when treating them as the components of multldigit numerals, or as the operands of addition problems. Wlth two-digit numerals, a similar dissociation was shown between excellent comparison and severely impalred reading aloud. Finally, the variation of performance with task demands was shown not to prevail with spelled-out numerals. These findings confirm that some patients with pure alexia are able to process up to a semantic level symbolic stimuli that they cannot read aloud. We speculate that both hemispheres possess effective diglt identification abilities, which are differentially called on depending on the task.