Patterns of hypermetria and terminal cocontraction during point-to-point movements demonstrate independent action of trajectory and postural controllers

Robert A. Scheidt, Claude Ghez, Supriya Asnani
2011 Journal of Neurophysiology  
Scheidt RA, Ghez C, Asnani S. Patterns of hypermetria and terminal cocontraction during point-to-point movements demonstrate independent action of trajectory and postural controllers. We examined elbow muscle activities and movement kinematics to determine how subjects combine elementary control actions in performing movements with one and two trajectory segments. In reaching, subjects made a rapid elbow flexion to a visual target before stabilizing the limb with either a low or a higher level
more » ... f elbow flexor/ extensor coactivity (CoA), which was cued by target diameter. Cursor diameter provided real-time biofeedback of actual muscle CoA. In reversing, the limb was to reverse direction within the target and return to the origin with minimal CoA. We previously reported that subjects overshoot the goal when attempting a reversal after first having learned to reach accurately to the same target. Here we test the hypothesis that this hypermetria results because reversals co-opt the initial feedforward control action from the preceding trained reach, thereby failing to account for task-dependent changes in limb impedance induced by differences in flexor/extensor coactivity as the target is acquired (higher in reaching than reversing). Instructed increases in elbow CoA began mid-reach, thus increasing elbow impedance and reducing transient oscillations present in low CoA movments. Flexor EMG alone increased at movement onset. Test reversals incorporated the initial agonist activity of previous reaches but not the increased coactivity at the target, thus leading to overshoot. Moreover, we observed elevated coactivity in reversals upon returning to the origin even though coactivity in reaching was centered at the goal target. These findings refute the idea that the brain necessarily invokes distinct unitary control actions for reaches and reversals made to the same target. Instead, reaches and reversals share a common control action that initiates trajectories toward their target and another later control action that terminates movement and stabilizes the limb about its final resting posture, which differs in the two tasks. learning; feedforward control; feedback; planning THIS REPORT is the third in a series of studies in which we used cross-adaptation paradigms to isolate and characterize the elementary control actions comprising point-to-point reaching movements of the arm (a reach task) and out-and-back reversal movements made between the same spatial locations (a reversal task) Scheidt and Ghez 2007) . Those studies were undertaken to determine whether the entire realized trajectories of point-to-point reaching movements could be considered as resulting from the implementation of unitary control actions, a frequent assumption of learning models
doi:10.1152/jn.00763.2010 pmid:21849613 pmcid:PMC3214114 fatcat:54kwv23tsjfmnbuxwvszjiwam4