AN EVALUATION OF THE ROLE OF INTERNAL CUES IN THE PATHOGENESIS OF PARKINSONIAN HYPOKINESIA

Our animal studies suggest that the basal ganglia provide an internal non-specific cue to trigger movement and imply that Parkinson's diseas e involves a deficiency in this cueing mechanism. Indeed parkinsonian patients typically rely upon external visual cues. To assess the effec ts of such non-specific cueing mechanisms on movement, we examined pat ients' utilization of a variety of auditory cues. Ten patients sufferi ng from Parkinson's disease, and their matched controls, pressed butto ns at a series of two-way choice points sequentially down a pathway, b oth when the latter remained illuminated throughout its length, and wh en it had to be followed from memory alone. In other experimental cond itions, auditory cues were also provided, either contingent upon the p revious response, at its initiation (a medium level of advance informa tion) or at its completion (a low level of advance information), or as a series of regularly paced (non-contingent) auditory cues (from a me tronome). In addition to error data, we recorded down time (DT, time t o initiate each next response) and movement time (MT), time to execute each next response). However, both DT and MT measurements showed that parkinsonian patients were enormously disadvantaged by the absence of external cues. While contingent auditory cues were of some help, the performance of patients with Parkinson's disease was dramatically impr oved by the provision of non-contingent auditory information. Moreover , parkinsonian patients, unlike controls, were greatly affected by the length of individual sub-movements, especially in the absence of exte rnal cues. When the pathway to be followed remained illuminated, sub-m ovement length had little effect. We conclude that for well-learnt, pr edictable sequences the basal ganglia provide a non-specific internal cue that is necessary for switching between one movement and the next in a movement sequence, and also for development of preparatory activi ty for each sub-movement in the sequence.