[Parametric coupling in sequences of bimanual reversal movements with identical and different amplitudes]

In sequences of bimanual movements, the voluntary modulation of the amplitude of the one hand (inducing hand) induces an involuntary modulation of the amplitude of the other hand (dependent hand), the strength of which increases with increasing tempo. By means of a task in which subjects perform sequences of two short and two long reversal movements in alternation with the inducing hand, but constant short or long reversal movements with the dependent hand, we addressed two questions. The first question was concerned with differences in the effects of tempo on the involuntary amplitude modulation of short and long movements; the second question was whether the involuntary amplitude change fades away or is propagated when bimanual movements with certain target amplitudes are repeated. At low tempo the contralateral effect of voluntary amplitude changes on short-amplitude movements was stronger than the effect on long-amplitude movements, but at high tempo this difference was reversed. This result is not consistent with the assumption that contralateral amplitude modulation results from an overflow of efferent commands, which increases with the force of the movement; however, it is consistent with other findings on a transient coupling during amplitude specification (parametric coupling). The involuntary amplitude change was essentially propagated to the next movement in the sequence and did not fade away. This finding suggests that the assimilation of amplitudes that can be observed in bimanual sequences of movements with different, but for each hand constant, amplitudes could result from an effect of transient parametric coupling during the initial specification of amplitudes and need not necessarily be attributed to cross-talk at the level of motor commands or efferent innervation.