Intrusion of the verbal code during the production of Arabic numerals: A single case study in a patient with probable Alzheimer's disease

In the present study, we report on the performance of a patient with probable Alzheimer's disease (AD) who made a lot of verbal code intrusions during the production of Arabic numerals. For instance, she wrote 3 mille instead of 3000 [trois mille] or 15ZE instead of 15 [quinze]. These errors of shift towards the unintended code are typical in cases of AD. Here, we suggest a classification of shift errors into three categories. We try to elucidate the origin of these errors on the basis of the analysis of the patient's performance in other numerical tasks, on the basis of the regularities observed in the errors, and on the basis of the evolution of shift errors with the advance of the disease. It appeared that a pattern of the sort we have observed could be better accounted for by the co-occurrence of two distinct deficits: an impairment of inhibitory processes on one hand and an impairment of the transcoding mechanism per se on the other