It has long been proposed that cognitive aging in fluid abilities is driven by age-related declines of processing speed. Although study of between-person associations generally supports this view, accumulating longitudinal between-person and within-person evidence indicates less strong associations between speed and fluid cognitive performance. Initial evidence also suggests that cognitive flex...

Parkinson's disease (PD) is attributable primarily to depletion of dopamine in the basal ganglia, but the full effects of this depletion are unknown. It is well known that PD involves motor slowing, and although it is not easy to distinguish between the motor and cognitive components of behavior, clinical observations suggest that cognitive processing may also be compromised. However, it remain...