Cognitive slowing in Parkinson's and Alzheimer's patients: distinguishing bradyphrenia from dementia.

The contribution of cognitive slowing to the slowed performance of patients with Parkinson's disease (PD) is a matter of long-standing debate. In this study, we contrasted the performance of PD patients on two reaction-time tasks with the performance of Alzheimer's disease (AD) patients, young normal subjects, and elderly normal subjects. Both nondemented and demented PD patients showed cognitive as well as motor slowing, and the extent of cognitive slowing varied with overall cognitive status. Moreover, by comparison with the cognitive slowing in AD patients, cognitive slowing in PD patients was disproportionate to their general level of cognitive performance. We suggest that this disproportionality be used to differentiate the concepts of bradyphrenia and nonspecific cognitive slowing.