Generalized cognitive slowing and severity of dementia in Alzheimer's disease: implications for the interpretation of response-time data

A meta-analysis was performed on the response times (RTs) of Alzheimer patients and normal subjects for 61 experimental conditions. Regression of the RTs of Alzheimer patients against those of young normals yielded a linear function: Alzheimer (time) = A+B Young (time), where B was a constant proportion of 2.3. As dementia severity increased, so too did the magnitude of this RT slowing, B rising from 2.0 to 2.9. The linearity of this function suggests that Alzheimer's disease produces a generalized slowing of cognitive processing. Since Alzheimer patients are proportionally slower than normals, the size of the absolute difference in RT between Alzheimer patients and normals increases as task complexity rises, irrespective of the exact cognitive operations being tested. It will be necessary to take this generalized slowing into account when interpreting differences in the effect that various experimental conditions have on the RTs of normals and Alzheimer patients.