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Kunlin Wei, Gary Wertman and Dagmar Sternad

An asymmetric bimanual task was investigated in which participants performed a rhythmic movement with their dominant arm and initiated a second movement with their nondominant arm at a random phase of the continued oscillations. The objective was to examine whether different constraints existed between rhythmic and discrete movements and, more generally, whether rhythmic and discrete movements can be regarded as two different movement primitives. Participants performed rhythmic forearm rotations at 1 of 4 prescribed metronome periods. After a random interval, a trigger signaled to initiate either a discrete or rhythmic movement with the left forearm as fast as possible while continuing the oscillations. Analyses extracted the mutual influences that the two movements exerted on each other and contrasted discrete-rhythmic and rhythmic-rhythmic coupling. (a) The initiation of the rhythmic movement was constrained to occur in-phase with the ongoing rhythmic movement, while the discrete movement could be initiated at any arbitrary phase. (b) Reaction time of the initiated rhythmic movement scaled with the oscillation period, while the discrete movement's reaction time was invariant across periods. (c) Peak velocity of the initiated movement scaled with the oscillatory period in both tasks but more strongly in the discrete movement. (d) Synchronization of EMG bursts of both arm flexors was evident in both tasks but more strongly in the rhythmic-rhythmic combination. The results are interpreted as support for the hypothesis that discrete and rhythmic actions are two different control regimes, and coupling occurs at a higher level in the central nervous system.