INDEPENDENT CONTROL OF REFLEX AND VOLITIONAL EMG MODULATION DURING SINUSOIDAL PURSUIT TRACKING IN HUMANS

It is well known that during volitional sinusoidal tracking the long-l atency reflex modulates in parallel with the volitional EMG activity. In this study, a series of experiments are reported demonstrating seve ral conditions in which an uncoupling of reflex from volitional activi ty occurs. The paradigm consists of a visually guided task in which th e subject tracked a sinusoid with the wrist. The movement was perturbe d by constant torque or controlled velocity perturbations at 45-degree s intervals of the tracking phase. Volitional and reflex-evoked EMG an d wrist displacement as functions of the tracking phase were recorded. The relationship of both short-latency (30-60 ms) and longer-latency (60-100 ms) reflex components to the volitional EMG was evaluated. In reflex tracking, the peak reflex amplitude occurs at phases of trackin g which correspond to a maximum of wrist joint angular velocity in the direction of homonymous muscle shortening and a minimum of wrist comp liance. Uncoupling of the reflex and volitional EMG was observed in th ree situations. First, during passive movement of the wrist through th e sinusoidal tracking cycle perturbation-evoked long-latency stretch r eflex peak is modulated as for normal, volitional tracking. However, w ith passive joint movement the volitional EMG modulation is undetectab le. Second, a subset of subjects demonstrate a normally modulated and positioned long-latency reflex with a single peak. However, these subj ects have distinct bimodal peaks of volitional EMG. Third, the imposit ion of an anti-elastic load (positive position feedback) shifts the vo litional EMG envelope by as much as 180-degrees along the tracking pha se when compared with conventional elastic loading. Yet the long-laten cy reflex peak remains at its usual phase in the tracking cycle, corre sponding to the maximal velocity in the direction of muscle shortening . Furthermore, comparison of the results from elastic and anti-elastic loads reveals a dissociation of short- and long-latency reflex activi ty, with the short-latency reflex shifting with the volitional EMG env elope. Comparable results were also obtained for controlled velocity p erturbations used to control for changes in joint compliance. The unco upling of the reflex and volitional EMG activity in the present series of experiments points to a flexible relationship between reflex and v olitional control systems, altered by peripheral input and external lo ad.