Failed suppression of direct visuomotor activation in Parkinson's disease

The response times in choice-reaction tasks are faster when the relative sp atial positions of stimulus and response match than when they do not match, even when the spatial relation is irrelevant to response choice. This spat ial stimulus-response (S-R) compatibility effect (i.e., the Simon effect) i s attributed in part to the automatic activation of spatially corresponding responses, which need to be suppressed when the spatial location of stimul us and correct response do not correspond. The present study tested patient s with Parkinson's disease and healthy control subjects in a spatial S-R co mpatibility task in order to investigate whether basal ganglia dysfunction in Parkinson's disease leads to disinhibition of direct visuomotor activati on. High-density event-related brain potential recordings were used to char r the cortical activity accompanying attentional orientation and response s election. Response time measures demonstrated a failure to inhibit automati c response activation in Parkinson patients, which was revealed by taking i nto account a sequence-dependent modulation of the Simon effect. Event-rela ted potential (ERP) recordings demonstrated that visuospatial orientation t o target stimuli was accompanied by signal-locked activity above motor area s of the cortex, with similar latencies but an enhanced amplitude in patien ts compared to control subjects. The results suggest that inhibitory modula tion of automatic, stimulus-driven, visuomotor activation occurs after the initial sensory activation of motor cortical areas. The failed inhibition i n Parkinson's disease appears therefore related to a disturbance in process es that prevent early attention-related visuomotor activation, within motor areas, from actually evoking a response.