Altered aiming movements in Parkinson's disease patients and elderly adults as a function of delays in movement onset
This study investigated the effect of lengthening the time the hand remains immobilized on an aiming movement performed by Parkinson's disease (PD) patients and elderly adults, and whether visual information could compensate for the effects of delay. In Experiment One, PD patients and elderly adults kept the limb in a static position for 1, 6, or 10 s prior to movement initiation, both with and without vision of the initial limb position and the movement trajectory. Compared to elderly adults, PD patients had increased movement times and jerk scores, and exhibited shorter primary submovements that erred in initial movement direction. Lengthening the time delay increased movement time, decreased mean acceleration, and decreased the distance covered in the primary submovement for both groups. Parkinsonian patients, however, exhibited reduced length of the primary submovement across delay compared to elderly adults. Occluding vision caused the movements of PD patients to deteriorate on all measures. Although the performance of both groups was enhanced when vision was available, vision was not able to fully counteract the effects of delay in either group. In Experiment Two, participants moved to a previously viewed target to examine movement accuracy. Systematic undershooting of the target as a function of delay was found for both groups. Parkinsonian patients exhibited greater undershooting of the target after the primary submovement both with and without vision. Visual feedback reduced the effects of delay for both the elderly and PD patients. It can be inferred from the results that the decay in position sense as a function of time produces impairments in incorporating the initial limb position in motor planning process.