Hand dominance and multi-finger synergies.

Recent studies of arm movement reaching to a target have provided support for the Dynamic Dominance model of handedness, which proposes that each hemisphere/limb system is specialized for controlling different features of performance: The dominant system for control of the trajectory, and the nondominant system for control of the steady state final position. We now examine a more general form of this hypothesis by investigating differences between the right and left hands of right-handed persons in their ability to stabilize the combined action of a set of fingers. The subjects produced very quick pulses and steps of force from a low background force level while pressing with four fingers of one hand. An index of force stabilizing synergy showed a strong multi-finger synergy during steady-state force production followed by a small anticipatory drop in the index prior to the force increase. Hand differences emerged during the force increase: The left (non-dominant) hand showed a significantly larger drop in the synergy index. While our findings support the idea that the dominant system is specialized for stabilizing quick changes in performance variables, we failed to support a nondominant specialization for stabilizing steady-state isometric force. This may be a ceiling effect due to the simplicity of the task for either hand, or it might indicate that this aspect of the dynamic dominance hypothesis does not generalize to isometric conditions.