Finger movements during reach-to-grasp in the monkey: amplitude scaling of a temporal synergy

To reduce the complexity of controlling hand-shaping, recent evidence suggests that the central nervous system uses synergies. In this study, two Rhesus monkeys reached-to-grasp 15 objects, varying in geometric properties, at five grasp force levels. Hand kinematics were recorded using a video-based tracking system. Individual finger movements were described as vectors varying in length and angle. Inflection points (i.e., stereotypic minima/maxima in the temporal profile of each finger vector) exhibited a temporal synchrony for individual fingers and in the coupling across fingers. Inflection point amplitudes varied significantly across objects grasped, scaling linearly with the object grasp dimension. Thus, differences in the vectors as a function of the objects were in the relative scaling of the vector parameters over time rather than a change in the temporal structure. Mahalanobis distance analysis of the inflection points confirmed that changes in inflection point amplitude as a function of objects were greater than changes in timing. Inflection points were independent of the grasp force, consistent with the observation that reach-to-grasp kinematics and grasp force are controlled independently. In summary, the shaping of the hand during reach-to-grasp involves scaling the amplitude of highly stereotypic temporal movements of the fingers.