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.
The present study examines whether visual information indicating the center of mass (CM) of an object can be used for the appropriate scaling of fingertip forces at each digit during precision grip. In separate experiments subjects lifted an object with various types of visual cues concerning the CM location several times and then rotated and lifted it again to determine whether the visual cues signaling the new location of the CM could be used to appropriately scale the fingertip forces. Specifically, subjects had either no visual cues, visual instructional cues (i.e., an indicator) or visual geometric cues where the longer axis of the object indicated the CM. When no visual cues were provided, subjects were unable to appropriately scale the load forces at each digit following rotation despite their knowledge of the new weight distribution. When visual cues regarding the CM location were provided, the nature of the visual cues determined their effectiveness in retrieval of internal representations underlying the anticipatory scaling of fingertip forces. Specifically, when subjects were provided with visual instructional information, they were unable to appropriately scale the forces. More appropriate scaling of the load forces occurred when the visual cues were ecologically meaningful, i.e., when the shape of the object indicated the CM location. We suggest that visual instructional cues do not have access to the implicit processes underlying dynamic force control, whereas visual geometric cues can be used for the retrieval of the internal representation related to CM for appropriate partitioning of the forces in each digit.
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