Cognitive slowing in Alzheimer's disease and geriatric depression

Response slowing on psychological tasks is found both in Alzheimer's disease and depression. However, the underlying cause for this slowing may be different in the two disorders. This research examined whether the behavioral slowing found in Alzheimer patients results from a reduction in their rate of cognitive processing, whereas the slowing in depressed geriatric patients reflects a purely motor retardation. This hypothesis was tested using a task in which subjects had simply to determine the number of dots present in an array (i.e., enumeration). In all four subject groups (Alzheimer patients, depressed geriatric patients, healthy old controls, and healthy young controls), response time increased linearly with array size. The slope of this linear function (reflecting rate of enumeration) was the same in the normal and depressed patients, but was significantly greater in the Alzheimer patients, suggesting the presence of a cognitive slowing in Alzheimer's disease, but not in depression.