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

The relative sparing of arabic numerals, in patients who failto read words or even letters, is a classical feature of pure alexla orlglnallyobserved by Dejerlne (Comptes Rendus des Seances de la Societé de la Biologie1892;4: 61-90).Wereport a study of number processlng abilities ln two patients sUfferlngfrom typical pure alexla. Our main flndlngwas that number Identificationperformance varled considerably wlthtask demands. Both patients could name pairs of digits, when they were engaged ln a simple namlng task or for the purpose of magnitude comparlson. ln contras " they frequently misldentifledthe very same digits when treating them as the components of multldlgltnumerals, or as the operands of addition problems. Withtwo-dlgitnumerals, a slmilar dissociation was shown between excellent comparison and severely impalred readlng aloud. Flnally,the variation of performance withtask demands was shown not to prevail withspelled-out numerals. These findings conflrm that some patients withpure alexla are able to process up to a semantic level symbolicstimuli that they cannot read aloud.Wespeculate that both hemispheres possess effective digit~dentiflcationabilitles, whlch are differenth:Ïlly called on depending on the task.