Adaptation is a prominent feature of the human motor system and has been studied extensively in reaching movements. This study characterizes adaptation and generalization during isometric reaching in which the arm remains stationary and the participant controls a virtual cursor via force applied by the hand. We measured how learning of a visual cursor rotation generalizes across workspace 1) to determine the coordinate system that predominates visual rotation learning, and 2) to ascertain whether mapping type, namely position or velocity control, influences transfer. Participants performed virtual reaches to one of two orthogonal training targets with the applied rotation. In a new workspace, participants reached to a single target, similar to the training target in either hand or joint space. Furthermore, a control experiment measured within-workspace generalization to an orthogonal target. Across position and velocity mappings, learning transferred predominantly in intrinsic (joint) space, although the transfer was incomplete. The velocity mapping resulted in significantly larger aftereffects and broader within-workspace generalization than the position mapping, potentially due to slower peak speeds, longer trial times, greater target overshoot, or other factors. Although we cannot rule out a mixed reference frame in our task, the predominance of intrinsic coding of cursor kinematics in the isometric environment opposes the extrinsic coding of arm kinematics in real reaching but matches the intrinsic coding of dynamics found in prior work. These findings have implications for the design of isometric control systems in human-machine interaction or in rehabilitation when coordinated multi-degree-of-freedom movement is difficult to achieve.
Isometric reaching, in which the arm remains stationary and the user controls a virtual cursor via force input, is a motor task that has not been thoroughly compared to real reaching. In this study, we ask if isometric adaptation to a kinematic perturbation is similar to adaptation in movement, and if the type of isometric mapping (position or velocity control) influences learning. Healthy subjects made real and virtual reaches with the arm in plane. In some trials, the cursor was rotated counter clockwise by 45° to perturb the kinematic mapping. To assess adaptation, the angular error of cursor movement at 150 ms from movement onset was measured for each reach; error was averaged across subjects and a two-state learning mode was fit to error data. For movement and isometric groups, average angular error peaked at perturbation onset, reduced over 200 reaches, and reversed direction when the perturbation was removed. We show that subjects are able to adapt to a visuomotor rotation in both position- and velocity-based cursor control, and that the time course of adaptation resembles that of movement adaptation. Training of virtual reaching using force/torque input could be particularly applicable for stroke patients with significant movement deficits, who could benefit from intensive treatments using simple, cost-effective devices.
Haptic feedback located on the torso is proposed to enhance the state awareness of a user in virtual reality or during teleoperation while leaving the hands free for manipulation and communication. We provide haptic feedback on the torso by compressing a set of closed air pouches against the skin in order to render the sensation of air pressure when piloting a drone. The pouch devices are cable driven and integrated in a wearable soft exoskeleton, called the FlyJacket. A mechanical model and simulation of a pouch device were developed in order to determine appropriate parameters, including the air pouch inner pressure, its attachment point, and the cable position. Using the simulation results, a set of pouch devices were constructed and integrated into the soft exoskeleton on both sides of the upper chest and middle of the back. The mechanical performance of the constructed device is close to that predicted by the simulation. Application of the haptic device in a flight task in which the user controls a drone using upper body movements was demonstrated with a user study. Adding haptic feedback during a stabilization task reduced the user's workload and improved the state awareness of the user. Index Terms-Soft robot applications, haptics and haptic interfaces, wearable robots. I. INTRODUCTION H APTIC DEVICES that provide touch feedback to the fingers and hands have proven to be a compelling way of transmitting information and enhancing training and performance of a variety of tasks. However, many applications require Manuscript
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