Force Field Generalization and the Internal Representation of Motor Learning

TL;DR

This study investigates how internal representations of motor learning generalize across different directions, revealing asymmetries and the influence of limb mechanics that challenge previous models.

Contribution

It demonstrates that limb impedance and mechanics significantly affect generalization measurements, proposing a model that better accounts for these factors in motor learning.

Findings

Generalization is local and asymmetric across directions.

Limb impedance influences measurements of internal representations.

A new model accounting for limb mechanics explains data better.

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. Though it is the subject of frequent studies, 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 measurements of it. 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, at odds with previous explanations. 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.