The modulation of intermanual interactions during the specification of the directions of bimanual movements.

In two experiments bimanual movements with various combinations of target directions were studied by means of the timed-response procedure. The findings revealed an adaptive modulation of intermanual interactions during direction specifications depending on particular target directions. For symmetric movements intermanual correlations of movement directions are positive, indicating a symmetric coupling. For parallel movements the positive intermanual correlations, observed at short preparation intervals, turn into negative correlations as the time available for motor preparation increases. Biases of mean directions, that can be observed for movements to targets with different eccentricities, reflect one or the other kind of coupling, symmetrical for symmetric target directions and parallel for parallel target directions. These biases are static, that is, they are present at long preparation times, and they are phasically enhanced at shorter preparation intervals. The task-adaptive modulation of intermanual interactions is superposed on a basic symmetry bias.