Frequency-induced changes in interlimb interactions: increasing manifestations of closed-loop control.

In bimanual coordination, interactions between the limbs result in attraction to in-phase and antiphase coordination. Increasing movement frequency leads to decreasing stability of antiphase coordination, often resulting in a transition to the more stable in-phase pattern. It is unknown, however, how this frequency-induced loss of stability is engendered in terms of the interlimb interactions underwriting bimanual coordination. The present study was conducted to help resolve this issue. Using an established method (based on comparison of various unimanual and bimanual tasks involving both passive and active movements), three sources of interlimb interaction were dissociated: (1) integrated timing of feedforward signals, (2) afference-based correction of relative phase errors, and (3) phase entrainment by contralateral afference. Results indicated that phase entrainment strength remained unaffected by frequency and that the stabilizing effects of error correction and integrated timing decreased with increasing frequency. Their contributions, however, reflected an interesting interplay as frequency increased. For moderate frequencies coordinative stability was predominantly secured by integrated timing processes. However, at high frequencies, the stabilization of the antiphase pattern required combined contributions of both integrated timing and error correction. In sum, increasing frequency was found to induce a shift from predominantly open-loop control to more closed-loop control. The results may be accounted for by means of an internal forward model for sensorimotor integration in which the sensory signals are compared to values predicted on the basis of efference copies.