Dissociable processing of temporal structure in repetitive eye-hand movements in Parkinson's disease

Movement takes place in multiple dimensions, including the temporal dimension, which itself may be made up of dissociable aspects. From this perspective, the present research tests three hypotheses: first, that the generation of regular, repetitive movement frequency is neurophysiologically dissociable from the generation of appropriate phase relations, or latencies, of such movements with respect to external stimuli; second, that the frontostriatal system is critically involved in the control of latency and not frequency and third that while the control of latency is closely linked to the effector motor system (e.g. eye, hand, etc.) the control of movement frequency is a more centralized function. These three hypotheses were investigated in nine Parkinson's disease (PD) patients with asymmetric akinetic-rigid syndrome and in nine age-matched control subjects in the context of repetitive eye-hand movements generated in response to regularly displaced visual stimuli and then the continuation of these movements in the immediate absence of the stimuli. PD patients demonstrated increased latencies for pointing movements, particularly with the affected hand, while their ability to follow and then reproduce the movement frequency remained largely intact. Interestingly, saccade latency was improved for controls and PD patients when pointing with the Less-affected hand and impaired with the More-affected hand. In contrast, saccade frequency was unaffected in these pointing conditions. These results support the hypothesis that movement frequency and latency controls are dissociable, with the frontostriatal system playing an important role in latency but not frequency control. The fact that pointing and saccade latency displayed a hand-effect, while movement frequency did not, also tends to support the hypothesis that latency control is linked to specific motor systems, including their interaction, whereas frequency control is more centralized.