Verbal memory decline in Alzheimer's disease: a multiple-processes deficit

Memory dysfunction is a primary diagnostic criterion and one of the earliest clinical manifestations of dementia of the Alzheimer type (DAT). The profile of progression of this memory deficit is assumed to represent a unitary decline. We studied the profile of verbal memory decline using an immediate free recall learning task in 18 DAT patients judged to be at 3 different stages of severity (mildly, moderately, and severely affected), using the serial-position function as the dependent measure. Results showed that the pattern of performance of DAT patients depended upon the severity of the disease. With mild dementia, the U-shaped curve was qualitatively similar to that observed in normals (presence of primacy and recency effects). With increasing severity of dementia, there was a consistent modification of the serial-position function with, in moderately affected subjects, a progression toward a unimodal curve devoid of a primacy effect and, later on, a decrease of the recency effect. These findings suggest that the serial-position function shows dynamic changes across stages of DAT. These dynamic changes may imply distinct disturbances of 2 or more learning-memory processes in this condition. The processes involved presumably reflect the temporal nature of the pathologic involvement of distinct neural systems, both structural and neurochemical.