THE ACCURACY AND PRECISION OF TIMING OF SELF-PACED, REPETITIVE MOVEMENTS IN SUBJECTS WITH PARKINSONS-DISEASE

In separate experiments, we studied the temporal accuracy and precisio n of self-paced, repetitive finger-tapping in two groups of 12 patient s with Parkinson's disease and a group of 12 controls matched to the p atients with respect to age and general cognitive state. One group (I) of patients was studied initially following 12-15 h abstinence from n ormal levodopa medication ('off') and again, subsequently, similar to 1 h after ingestion of a single normal dose ('on'). A second group (II ) of patients, each of whom had bilaterally asymmetrical neurological signs, was tested using 'worse' and 'better' hands separately. Within each session, subjects were tested repeatedly on a tapping task during which they were required to produce a regular series of self-timed in ter-tap intervals, the target duration (550 ms) of which had been esta blished previously during an initial period of tapping in synchrony wi th the beats of a regular metronome. We employed Wing and Kristofferso n's (1973) model of control of motor timing to partition the total var iance (TV) about the mean interresponse interval (IRI) produced during the self-paced phase of each run into separate components ['clock' va riance (CV) and 'motor-delay' variance (MDV)] attributable to hypothet ical 'clock' and 'motor-implementation' processes. Although the mean s elf-paced IRI of parkinsonian patients was generally shorter than that of controls, only during the 'on' medication condition (Group I) was it significantly so. By comparison with control values, and those obse rved during the 'on' medication condition, values of TV, CV and MDV in Group I were all significantly higher when subjects were 'off' medica tion. During the 'on' medication condition, only CV was significantly higher than the control value. In Group II, values of TV, CV and MDV a ssociated with use of the 'worse' hand were all significantly higher t han both control values and those associated with use of the 'better' hand Values of these variables when subjects used the 'better' hand di d not, however differ significantly from control values. The theoretic al import of these results is discussed in the light of several import ant procedural, statistical and computational issues and we conclude t hat TV, CV and MDV may all vary significantly as a function of the eff icacy of dopaminergic transmission in the basal ganglia.