Calculating without reading: unsuspected residual abilities in pure alexia.

In pure alexic patients, clear-cut dissociations between impaired naming and preserved comprehension abilities can be found in the domain of number processing (Cohen & Dehaene, 1995). In the present study, we report a novel case of pure alexia with striking preservation of some calculation abilities. The patient was fully able to decide which of two numbers was the larger, or whether a number was odd or even, even with 2-digit numerals for which she made close to 90% reading errors. In arithmetic the patient, though unable to read aloud correctly the operands of visually presented problems, could still produce verbally the exact result of the very same problems. For instance, when presented visually with the subtraction problem 8 - 6, the patient read the problem aloud as "five minus four", but nevertheless produced the correct result "two." Such capacities for "calculating without reading" were observed in subtraction, addition, and division tasks, but not in multiplication tasks. We discuss how both the existence of residual abilities and the pattern of dissociations between operation types can be explained by current theories of the cerebral substrates of number processing (Dehaene & Cohen, 1995).