Orientation control strategies and adaptation to a visuomotor perturbation in rotational hand movements
Computational approaches to biological motor control are used to discover the building blocks of human motor behavior. Models explaining features of human hand movements have been studied thoroughly, yet only a few studies attempted to explain the control of the orientation of the hand; instead, they mainly focus on the control of hand translation, predominantly in a single plane. In this study, we aimed to establish a basic understanding of the way humans control the orientation of their hands. We developed a quaternion-based score that quantifies the geodicity of rotational hand movements and evaluated it experimentally. In the first experiment participants performed a simple orientation-matching task with a robotic manipulator. We found that rotations are generally performed by following a geodesic in the quaternion hypersphere, which suggests that, similarly to translation, the orientation of the hand is centrally controlled. We also established a baseline for the study of human response to perturbed visual feedback of the orientation of the hand. In the subsequent second experiment we studied the adaptation of participants to visuomotor rotation that is applied on the hands rotation, and the transfer of the adaptation to a different initial orientation. We observed partial adaptation to the perturbation. The patterns of the transfer of the adaptation to a different initial orientation were consistent with the representation of the orientation in extrinsic coordinates. The results of the two experiments raise questions regarding the nature of central control of hand orientation. Discussion and intuitions from these results can be of benefit for many applications that involve fine manipulation of rigid bodies, such as teleoperation and neurorehabilitation. Author summaryDaily motor actions, as simple as pouring a glass of wine or as complicated as playing a violin, require coordinated activation of multiple muscles that synchronize to produce a precise motion of the hand. Controlled by sensorimotor areas in the central nervous system, our muscles can translate and rotate our hand from one posture to another. Our study focused on the very basis of the control of orientation: we attempted to reveal which variables are centrally controlled when we rotate our hand. The discovery that simple rotations are generally performed along a geometrically optimal path established a baseline for studying the response to rotation-based perturbations. By visually remapping the orientation of the hand, we found that humans perceive their hands orientation in visual, rather than joint-based coordinates. These findings have implications for the design of human-centered control systems for teleoperation, where visual distortions may occur, and for the design of rehabilitation devices for people with motor impairments.