No abstract

The following sequences of two single movements were examined in 10 patients with Parkinson's disease and compared with the performance of 9 normal subjects of similar age. Isometric opposition of thumb and fingers to a force of 30 N ('squeeze'), followed by isotonic elbow flexion ('flex') through 15 degrees with the same arm. 'Squeeze' with the left hand followed by 'flex' with the right elbow. Isotonic opposition of thumb and fingers ('cut') through 90 degrees followed by isotonic 'flex' with the same arm. Isotonic elbow 'flex' followed by isometric 'squeeze' with the same arm. All movements were self-paced. Subjects were given instructions to move as rapidly as possible and to start the second movement immediately after the end of the first. Patients were slower than normal when each single movement was performed separately. There was a further decrease in speed when two movements were executed sequentially. This was due to an increase in movement duration of each of the component movements, especially the second, and to an increase in the pause between the first and second movements. In both normals and patients, there was no correlation between the times taken to perform the first and second movements of any of the four sequences that were studied. Because of this we suggest that the two components of the sequence remained under the control of two separate motor programs. When performing the sequential tasks, normal subjects automatically chose an interval between the onsets of the two separate movements of about 230 ms, even in tasks in which the duration of the first movement was less than 200 ms. If normal subjects were instructed to begin the second movement with an interonset interval of less than 200 ms, the speed of the second movement was much slower. Patients with Parkinson's disease automatically chose a much longer interonset interval of 400-500 ms. In addition, they exhibited difficulty in switching from the first to the second movement in the sequence. We suggest that the problems exhibited by patients with Parkinson's disease when they try to perform two rapid sequential movements can be seen as a deficit in the capacity to switch from one motor program to another within an overall motor plan.

The average Bereitschaftspotential (BP) preceding a rapid, self-paced voluntary extension movement of the index finger was recorded from 6 scalp locations in 14 patients with Parkinson's disease who had been withdrawn from their normal drug therapy for at least 12 h before testing. The amplitude of the potential was measured at the peak negativity (N1) and 650 ms prior to this (NS1), and compared with that recorded in a group of 12 age-matched control subjects. The N1 amplitude was the same as in the normals, but the NS1 component was smaller in the patients, especially in midline leads. As a result, the rise in the BP between the peak NS1 and N1 component (termed NS2) was larger in the patient group. The NS1 component of the BP is thought to reflect preparatory activity in the supplementary motor area (SMA) of cortex. Since the basal ganglia provide a major source of afferent input to SMA, the reduction in NS1 in the patients probably results from inadequate basal ganglia activation of SMA. The larger NS2 component may reflect extra activity in other brain areas to compensate for the reduced SMA activity.