Common encoding of novel dynamic loads applied to the hand and arm

Abstract

In manual action, the relationship between a given motor command and the ensuing movement depends on the dynamics of both the arm and hand-held objects. Skilled performance relies on the brain learning both these dynamics, and previous studies have examined how people adapt to novel loads applied to either the hand or the arm. In this study, we ask whether these different kinds of load are represented independently as a result of changes in cutaneous feedback and hand-arm coordination. We used a robotic apparatus that could either apply forces to an object held in the subject's hand or directly to the segments of the arm. We tested whether subjects could retain learning of a force field applied to the hand after subsequently experiencing the opposing field applied to the arm (or vice versa), or whether retrograde interference would be observed. In separate experiments, we used force fields and torque fields that were linearly related to either hand or joint velocities, respectively. Our finding of complete interference between opposing fields suggests that loads applied to the arm and hand are not represented independently by the sensorimotor system. This interference occurred despite markedly different cutaneous inputs that were directly related to the movement task. This result suggests that the brain represents dynamics independently of these sensory inputs. In addition, we found that the rate at which subjects adapted to a given force field, specified either in hand or joint coordinates, was independent of whether the forces were applied to the hand or arm segments.

Figure 1.

Configurations of the KINARM robot used to apply forces to the upper and lower arm segments via arm troughs (arm configuration shown to the left) or to the hand via a handle (hand configuration shown to the right). In the hand configuration, the elbow was supported by a cushioned air puck, and subjects grasped a handle. The upper and lower arm segments did not otherwise contact the apparatus.

Figure 2.

Mean hand-path area results for experiment 1, in which the imposed force fields depended on the velocity of the hand. The field was applied either to the hand via a handle (subscript h) or to the arm segments via an exoskeleton (subscript a). Data are shown for sessions 1 and 3 (first and second A). A block is a set of reaches to all eight targets. Each dot represents an average across subjects, and the vertical lines represent 1 SE. Exponential curves are fitted to each session of each group. The elevated dots at the right of each panel represent the average of two catch trials delivered in blocks 29 and 30.

Figure 3.

Joint-path area results for experiment 2, in which the imposed force fields depended on the angular velocities of the joints. Labels are as in Figure 2. deg, Degree.