Distributing vertical forces between the digits during gripping and lifting: the effects of rotating the hand versus rotating the object

During pinch grip we partition the vertical tangential forces at the digits according to the friction at the grip surfaces, and the mass distribution of the object. However, we cannot predictively partition the vertical forces to adjust to new frictional conditions after viewing a 180-deg rotation of an object with different textures at each grip surface. Hence, the processes that lead to predictive force partitioning may not access object representations, thereby suggesting that these processes are digit-specific. If this is true, then we should fail to predictively partition our fingertip forces when we rotate our hand. We tested this prediction by comparing the effects of object rotation with hand rotation for repeated lifts of an object that had one slippery grip surface and one rough grip surface. Subjects did not predictively redistribute the vertical tangential forces upon grasping the rotated object. Following object rotation, the vertical tangential force trajectories during the first 100 ms after contact indicated that 12/15 subjects failed to anticipate the reversed digit-friction relationships. All subjects appropriately partitioned the vertical tangential forces between the digits by the second lift after object rotation, confirming previous reports that sensory signals update the memory associated with lifting the object. In contrast, after hand rotation, 13/15 subjects anticipated the new digit-friction relationships and upon grasping the object immediately generated a steep rise in the vertical force trajectory at the rough surface. They also delayed the initial rise in vertical tangential force at the digit encountering the low-friction surface by approximately 65 ms. Thus, anticipatory partitioning of vertical fingertip forces is not strictly digit-specific. Internally driven motor plans can access the relevant memories or internal models for predictively partitioning the vertical tangential forces. It is not clear if this process involves rotating internal representations of fingertip force directly, or if the forces are derived after internally rotating a representation of the object. In contrast to the robust effects of vision on reach kinematics, or on wrist and finger configuration, visual signals about object rotation and orientation apparently do not influence vertical tangential fingertip forces.