Abstract

Objective: While cognitive deficits are well documented in Parkinson disease (PD), identifying a theoretical framework in which to interpret these findings has been less studied. Theories of cognitive aging suggest that processing speed is likely a critical factor in normal cognitive function that explains a large portion of the interindividual variance associated with age-related cognitive decline. We tested the hypotheses that measures of processing speed underlie deficits in working memory and inhibition in PD as well.
Methods: We measured cognitive function in a group of 77 medicated individuals with PD without dementia and 54 controls. Participants completed a battery of behavioral tests, including measures of processing speed (SDMT), working memory (Digit Span Backward), inhibition (DKEFS Color Word Interference), attention (Digit Span Forward) and depression (GDS). We compared performance across groups using analysis of variance. In addition, we used mediation analysis to examine the effect of processing speed on the relationship between age and both working memory and inhibition.
Results: Groups were similar for age, years of education, MMSE score and premorbid IQ. Performance on measures of processing speed and inhibition were significantly worse in the PD than control group. Furthermore, we found that processing speed mediated the relationship between age and inhibition in the PD and control groups. However, the decrease in working memory performance in the PD group was a statistical trend and the relationship between working memory and age was not mediated by processing speed in either group.
Conclusion: The pattern of cognitive aging consistent with the processing speed theory may be exacerbated in PD. However, while the working memory measure was correlated with processing speed, it was not correlated with age. The processing speed theory of cognitive decline provides a framework for hypothesis testing about the complex concept of bradyphrenia.

Keywords: Processing speed; Working memory; Inhibition; Executive function; Mediation analysis