Motor Memories in Manipulation Tasks are Linked to Contact Events Between Objects

Abstract

Humans have the remarkable ability to learn and perform motor skills, often involving the manipulation of different objects or tools, such as swinging a hammer or a tennis racket. The objects that we manipulate are usually associated with different dynamics, which determine the movement of an object as a result of the force applied. Skillful manipulation requires forming and recalling memories of these object dynamics. Although it has been assumed that such memories are linked to objects, per se, it was recently shown that people can form separate memories, for opposing dynamics, when these are linked to different locations, or ‘control points’, on an object (Heald, Ingram, Flanagan, & Wolpert, 2018). In the previous study, participants controlled the handle of a robotic device to move a virtual rectangular object with circles (control points) on the left and right sides. In different trials, they were instructed to move either the left or right control point to different targets. When these control points were paired with opposing force fields, adaptation was observed even though the required movement was constant, a situation that typically cannot be learned. In this previous study, both the controlled point and the target location changed between contexts. The aim of the current study was to assess whether one or both of these factors is critical for learning. The first experiment was similar to our previous study, except that the bar automatically rotated as it was moved forward such that the left and right control points, controlled in different contexts, moved to a common target. In the second experiment, the bar was aligned vertically with a single control point at the far end. Again, the bar rotated as it was moved forward such that the control point moved to a target located on either the left or right in different contexts. We found that, in both experiments, participants learned opposing force fields applied in the two contexts. We conclude that separate memories of dynamics can be formed for different ‘contact events’ involving a unique combination of the controlled point on the manipulated object and the target object this point contacts.