Effects of end-goal on hand shaping.

The aim of the present study was to determine whether hand shaping was affected by planning of an action subsequent to object contact. Ten subjects (5 females and 5 males, ages 19-33) were requested to reach toward and grasp a convex object between the thumb and the four fingers of the right hand and to perform one of the following actions: 1) lift up the object; 2) insert the object into a niche of a similar shape and size as the object, or 3) insert the object into a rectangular niche much larger than the object. Flexion/extension at the metacarpal-phalangeal and proximal interphalangeal joints of all digits were measured using resistive sensors embedded in a glove. Although all experimental conditions required grasping the same object, we found different covariation patterns among finger joint angles across conditions. Gradual preshaping of the hand occurred only when planning object lift or when the end-goal required object placement into the tight niche. In contrast, for the larger niche, gradual preshaping was not evident for the ring and the little finger. Further, reaching movements were faster for movements ending with the larger niche than for the other movement conditions. The present results suggest that hand shaping takes into account end-goal in addition to object geometry. We discuss these findings in the context of forward internal models that allow the prediction of the sensorimotor consequences of motor commands in advance to their execution.