Two-digit grasp as pointing movements by the opposing digits: evidence based on the uncontrolled manifold analysis

We used the concept of performance-stabilizing synergies to test predictions of two hypotheses on the organization of reach-to-grasp movements, one assuming separate control of the transport and aperture, and the other assuming that such actions represent pointing movements by the opposing digits. Healthy participants performed series of reach-to-grasp movements to a cylindrical object with and without targets on the object surface with the dominant and non-dominant hands. Joint motion was captured by a motion analysis system. Inter-trial variance in the joint configuration space was analyzed within the framework of the uncontrolled manifold (UCM) hypothesis, and synergy indices were computed for the thumb and index finger tips, hand, and aperture trajectories. The synergy index was higher for both digit-tip trajectories and the hand compared to the synergy index for the aperture during the first half of the movement. Analysis of the variance components showed that, compared to the values computed for the aperture, there was lower variance affecting the trajectories of each of the two digits (orthogonal to the UCM), while variance that did not affect those trajectories (within the UCM) was higher for the index finger and lower for the thumb. We interpret these findings as supporting the two-digit pointing hypothesis. They illustrate the power of the UCM-based method to distinguish between competing motor control hypotheses.