Method Participants

Three groups were tested. One group consisted of five patients with Parkinson's disease (age = 68.9 years, SD = 3.9; education = 16 years, SD = 1.5; time since diagnosis = 11.4 years, SD = 5.9). All patients were rated to have mild to severe (2 to 4) Parkinson's symptoms on the Hohn and Yahr scale. They were all on their regular dopaminergic replacement medication program at the time of testing.

The cerebellar group (N = 7; age = 65.9, SD = 10.1; education = 13.6, SD = 2.4) consisted of two patients with bilateral cerebellar degeneration and five patients with unilateral lesions due to either stroke (two) or tumor (three). Three of the unilateral patients had left-sided lesions, and two had right-sided lesions. These patients were tested in a chronic condition, at least 2 years after their neurological incident.

A control group of six elderly participants was also tested (age = 69.5, SD = 5.2; education = 14.0, SD = 1.8). These individuals reported no history of significant neurological disease or injury and were selected to match the patients in terms of age and education. Procedure

Responses were made on a peripheral response device linked to a PC. The taps required flexion movements of the right or left index fingers on a piano-type key (2 x 10 cm), mounted parallel to the top surface of the response device. The tone generator in the PC was used to create the auditory metronome, with the pitch of the pacing tones fixed at 500 Hz.

The experimenter initiated the trial, triggering the onset of the auditory metronome. Participants were instructed to begin tapping when they had a good sense of the target pace, attempting to tap along with the metronome. Once the first response was detected, the metronome continued until it had completed another 200 cycles. At this point, the tones ceased. Feedback was then provided, indicating the target interval, the mean interval produced, and the variability of the interresponse intervals. The next trial began after a short rest period.

Two independent variables were manipulated. First, the target rate was either 500 or 900 msec. Second, participants used either the right hand alone, the left hand alone, or both hands, tapping bimanually. This chapter will only report data from the unimanual conditions.

The conditions were tested in separate blocks of four trials each, and the participants completed two blocks for each of the six conditions. Thus, the data set for the analyses consists of eight trials of approximately 200 intervals each. The order of the blocks was randomized across participants, although a complete counterbalancing was not possible given the small number of participants in each group. Rate was manipulated across sessions with half of the participants starting with testing at 500 msec and the other half starting with 900 msec. A short practice trial consisting of 20 paced intervals was included prior to the first block of each condition.

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