Specificity of internal representations underlying grasping.

The present study examines anticipatory control of fingertip forces during grasping based on the center of mass (CM) of a manipulated object. Subjects lifted an object using a precision grip while the fingertip forces and the angle about the vertical axis (roll) were measured. The object's CM could be shifted to the left or right of the object's center parallel to the grip axis without changing it's visual appearance. Subjects performed 20 lifts with the CM in the center, left, and right side of the object, respectively. Subjects were instructed to lift the object while preventing it from tilting. Within three to five lifts, subjects were able to asymmetrically partition the load force development before lift-off such that it was higher in the digit opposing the CM. This anticipatory load force partitioning prevented the object from rolling sideways at lift-off. To determine whether the internal representation underlying the anticipatory control is specific to the effectors used to form it, subjects performed five lifts with the right hand with the CM on one side. Following these lifts, they rotated the object 180 degrees around the vertical axis and performed one lift with the same hand or they translated the object to the left side of the body (with or without rotating it) and performed one lift with the left hand. Despite subjects' explicit knowledge of the new weight distribution, they were unable to appropriately scale the load forces at each digit, resulting in a subsequent large roll of the object. The findings suggest that within a few lifts subjects achieve a stable internal representation which accounts for the object's CM and is used to scale the fingertip forces in advance. They also suggest that this representation, which is used for anticipatory control of fingertip forces, is specific to the effectors used to form it. We propose that multiple internal representations may be used during the anticipatory control of grasping.