Abstract
When learning a new motor behavior, e.g. reaching in a force field, the nervous system builds an internal representation. Examining how subsequent reaches in unpracticed directions generalize reveals this representation. Although often studied, it is not known how this representation changes across training directions, or how changes in reach direction and the corresponding changes in limb impedance, influence these measurements. We ran a force field adaptation experiment using eight groups of subjects each trained on one of eight standard directions and then tested for generalization in the remaining seven directions. Generalization in all directions was local and asymmetric, providing limited and unequal transfer to the left and right side of the trained target. These asymmetries were not consistent in either magnitude or direction, even after correcting for changes in limb impedance. Relying on a standard model for generalization the inferred representations inconsistently shifted to one side or the other of their respective training direction. A second model that accounted for limb impedance and variations in baseline trajectories explained more data and the inferred representations were centered on their respective training directions. Our results highlight the influence of limb mechanics and impedance on psychophysical measurements and their interpretations for motor learning.
Highlights
Learning a new motor behavior entails building an internal model [1,2,3]
To control for changes specific to reach directions, we present curves that depict how adaptation indices vary as the training direction rotates away from a fixed test target
The results were used to build standard generalization curves and new generalization curves that control for changes in reach direction
Summary
Learning a new motor behavior entails building an internal model [1,2,3]. It enables us to generalize what we have learned to new, unpracticed circumstances. Experimental studies exploit this ability to measure the internal representation of a motor task. The resulting movements and forces subjects apply in these new directions are used to infer their representation of the learned force field This standard approach is frequently used to examine the internal representation in a variety of motor tasks including movements with force [8,9,10] or visual perturbations [11, 12]
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