Early, untreated Parkinson's disease patients show reaction time variability.

While Parkinson's disease (PD) is associated with motor slowing, less attention has been paid to variability in performance on motor and cognitive tasks. To examine reaction time latencies and intraindividual variability in untreated patients with PD compared to healthy controls. Twenty-nine (19 men/10 women) patients with untreated PD and 16 controls (8 men/8 women) were examined using measures of simple reaction time (SRT) and choice reaction time (CRT) in addition to cognitive measures of executive function (Trail Making Test; adaptive digit ordering). Latencies and intraindividual variability were compared between groups. Partial correlation coefficients, adjusting for age, sex and education were used to examine the relationship between RT measures and motor or cognitive measures. Patients and controls did not differ with respect to age or sex distribution. Education and cognitive status differed between groups, but no subject was demented or clinically depressed. After adjusting for age, sex and education, significant group differences were found in latencies (2-choice RT and 8-choice RT) and intraindividual variability scores (all CRT conditions). Latencies did not differ significantly after adjusting for finger tapping rate. In the PD group neither the motor nor the executive measures correlated significantly with any of the reaction time measures. We conclude that CRT intraindividual variability and latencies are increased in untreated PD.