Monkey hand postural synergies during reach-to-grasp in the absence of vision of the hand and object.

Understanding how the CNS controls reach-to-grasp will require behavioral and neurophysiological studies of reach-to-grasp in the monkey, including the evaluation of whole-hand grasp with explicit force requirements. In this study, monkeys performed a reach-to-grasp task in which the size, shape, and orientation of the objects were varied. The monkeys were required to grasp each object at five force levels based on visual feedback. Seventeen positions on the wrist and hand were monitored to quantify kinematics. Hand shaping began with initiation of reach and continued throughout the reach, matching object properties even without vision of the hand or object. Grasp aperture scaled to object size. Singular value decomposition analysis of the marker positions identified two dominant hand postures. The first eigenvector or "eigenposture" consisted of an open hand configuration midway between flexion and extension that explained >93% of the variance. The second eigenposture consisted of hyperextension of all joints that accounted for another 4-5% of the variance. The two eigenpostures were similar across force levels and between monkeys. Reach kinematics consisted of a U-shaped hand path with a bell-shaped velocity profile. Trajectory and speed were independent of grasp force and object properties. In summary, hand shaping during the reach occurred without vision of the hand or object, and hand kinematics were not dependent on grasp force. Furthermore, the reach was independent of grasp force and object properties. These observations imply that the kinematics of reach-to-grasp and grasp force are controlled independently. Similar to humans, monkeys may use a simplifying strategy to reduce the degrees of freedom of the hand during reach-to-grasp.