Haptic information stabilizes and destabilizes coordination dynamics

Goal-directed, coordinated movements in humans emerge from a variety of con straints that range from 'high-level' cognitive strategies based oil percep tion of the task to 'low-level' neuromuscular-skeletal factors such as diff erential contributions to coordination from flexor and extensor muscles. Th ere has been a tendency in the literature to dichotomize these sources of c onstraint, favouring one or the other rather than recognizing and understan ding their mutual interplay. In this experiment, subjects were required to coordinate rhythmic flexion and extension movements with an auditory metron ome, the rate of which was systematically increased. When subjects started in extension on the beat of the metronome, there was a small tendency to sw itch to flexion at higher rates, but not vice versa. When subjects: were as ked to contact a physical stop, the location of which was either coincident with or counterphase to the auditor) stimulus, two effects occurred. When haptic contact was coincident with sound, coordination was stabilized for b oth flexion and extension. When haptic contact was counterphase to the metr onome, coordination was actually destabilized, with transitions occurring f rom both extension to flexion on the beat and from flexion to extension on the beat. These results reveal the complementary nature of strategic and ne uromuscular factors in sensorimotor coordination. They also suggest the pre sence of a multimodal neural integration process-which is parametrizable by rate and context - in which intentional movement, touch and sound are boun d into a single, coherent unit.