TREMOR AS A FACTOR IN PROLONGED REACTION-TIMES OF PARKINSONIAN-PATIENTS

Prolonged reaction times and tremor are symptoms of Parkinson's diseas e (PD). Recently, we showed the existence of a systematic phase relati onship between tremor-at-rest and the onset of voluntary motor respons es in PD patients. In the present study we investigated whether this p hase relationship contributes to the prolongation of reaction time in PD. Eight PD patients with prominent tremor-at-rest and five age-match ed controls performed rapid stimulus-evoked index finger abductions un der isometric conditions. Force and surface electromyogram (EMG) signa ls from first dorsal interosseus muscle were recorded and analyzed off -line by means of automatic routines. Reaction times of PD patients ex hibited a significant dependence of mean values and variability on the current tremor phase at the onset of the voluntary motor response. Re sponses with an onset of contraction during the beginning of an EMG tr emor burst were substantially delayed (on average 50 ms) and showed mo re variability in comparison to responses initiated at later times in the tremor cycle. This effect can be modeled by a simple gating proces s splitting the tremor cycle into two different system states that sup ports and inhibits the initiation of voluntary motor responses, respec tively. We conclude that attraction of voluntary motor initiation to t he tremor oscillator clearly contributes to prolongation of reaction t ime in PD.