Differentiation of reaction time performance in Parkinson disease on the basis of motor symptoms

Reaction time (RT) deficits, wel1-correlated with cognitive dysfunction in many neurologic disorders, have not been clearly established in Parkinson disease (PD). These inconclusive findings may reflect a failure to separate PD subjects into subgroups based on motor symptoms. This hypothesis was tested with an analysis of simple RT and choice RT in 17 PD patients quantitatively differentiated on the basis of predominant bradykinesia or tremor. The bradykinetic subgroup was found to significantly differ from age- and education-matched controls in choice RT but not simple RT. Correlations across all PD subjects revealed a significant relationship between deficits in choice RT and increased severity of bradykinesia. Significant correlations were not obtained between bradykinesia and simple RT, or between tremor and either RT task. The nonsignificant effect of simple RT makes it unlikely that motoric or perceptual factors alone account for the impairment in choice RT. Instead, the data suggest that common neuropathologic mechanisms underlie bradykinesia and cognitive deficits revealed by the choice RT task. This relationship was not found for tremor. These results confirm the importance of taking into account the presence and severity of specific motor symptoms in analyses of cognitive functioning in PD.