General slowing of lexical and nonlexical information processing in dementia of the Alzheimer type.

Individuals with very mild and mild dementia of the Alzheimer type (DAT) and age-matched controls performed three lexical and three nonlexical speeded information-processing tasks. The DAT group was slower than the control group on all six tasks. As predicted by general slowing (Nebes & Brady, 1992), most main effects of task condition were accompanied by Group × Condition interactions. That is, as task complexity increased, the response times (RTs) of the DAT group increased more than the RTs of the control group. Multitask regression analyses confirmed the existence of general slowing in DAT, such that the DAT group took approximately 1.8 times as long to process information as the controls on all six tasks. Importantly, lexical and nonlexical processing speed were equivalently affected by DAT. This pattern was observed both in very mild and mild DAT, although the degree of general slowing increased with the severity of the dementia.