RELIANCE ON EXTERNAL CUES FOR MOVEMENT INITIATION IN PARKINSONS-DISEASE - EVIDENCE FROM MOVEMENT-RELATED POTENTIALS

The aim of this study was to investigate the neurophysiological mechan isms underlying Parkinson's disease patients' increased reliance on ex ternal cues for the initiation of movement. Lateralized movement-relat ed cortical potentials were recorded in a noise-compatibility task wit h seven patients and seven age-matched control subjects. In this two-c hoice task, visual stimuli containing incompatible target and distract er elements, which simultaneously instructed for responses from both h ands, initially caused activation of the motor cortex controlling the wrong response hand. The incorrect response activation was of higher a mplitude in patients than in control subjects, causing a longer respon se delay relative to response times when target and distractors instru cted the same hand. In addition, hand-specific motor cortex activation started earlier in patients than in control subjects. These results i ndicate that visual stimuli exerted an earlier and stronger influence on movement initiation in patients than in control subjects. We hypoth esize that information from sensory stimuli relevant for the generatio n of a response can have rapid access to motor structures in Parkinson 's disease patients, thereby facilitating the initiation of movement. The findings may reflect a compensatory mechanism, but could also be r elated to excitability changes in the motor cortex intrinsic to the pa thophysiology of Parkinson's disease.