Motor memories in manipulation tasks are linked to contact goals between objects

Abstract

Skillful manipulation requires forming memories of object dynamics, linking applied force to motion. Although it has been assumed that such memories are linked to objects, a recent study showed that people can form separate memories when these are linked to different controlled points on an object (Heald JB, Ingram JN, Flanagan JR, Wolpert DM. Nat Hum Behav 2: 300-311, 2018). In that study, participants controlled the handle of a robotic device to move a virtual bar with circles (control points) on the left and right sides. Participants were instructed to move either the left or right control point to a target on the left or right, respectively, such that the required movement was constant. When these control points were paired with opposing force fields, adaptation was observed. In this previous study, both the controlled point and the target changed between contexts. To assess which of these factors is critical for learning, here, we used a similar paradigm but with a bar that automatically rotated as it was moved. In the first experiment, the bar rotated, such that the left and right control points moved to a common target. In the second experiment, the bar rotated such that a single control point moved to a target located on either the left or right. In both experiments, participants were able to learn opposing force fields applied in the two contexts. We conclude that separate memories of dynamics can be formed for different "contact goals," involving a unique combination of the controlled point on an object and the target location this point "contacts."NEW & NOTEWORTHY Skilled manipulation requires forming memories of object dynamics, previously assumed to be associated with entire objects. However, we recently demonstrated that people can form multiple motor memories when explicitly instructed to move different locations on an object to different targets. Here, we show that separate motor memories can be learned for different contact goals, which involve a unique combination of a control point and target.

Keywords: motor control; motor learning; motor memory; movement planning.

Fig. 1.

A: robotic interface and virtual reality system used to simulate objects and force fields. B: Different Control Points experiment from Heald et al. (2018). Participants were required to move either the left or right control point, on the object, to the left or right target, respectively. The location of the target was linked to the direction of the force field. C: Single Control Point experiment from Heald et al. (2018). Participants were required to move a central control point, on the object, to the central target. The location of the lateral “target” was linked to the direction of the force field. CCW, counterclockwise; CW, clockwise.

Fig. 2.

Four conditions. A: Same Target condition. Participants were required to move the control point, located on the left or right of the object, to the target. The direction of the force field, clockwise (CW), or counterclockwise (CCW), was linked to the location of the active control point. The faded objects illustrate the rotation of the object, as it was moved forward. In every condition the rotation was completed in the first half of the movement. B: Same Target Check condition: Participants were required to move the central control point to the target. The direction of the force field was linked to the solid point located on the left or right of the object. C: Same Control Point condition: Participants were required to move the control point to the target located on the left or right. The direction of the force field was linked to the location of the target. D: Same Control Point Check condition: Participants were required to move the central control point to the target located straight ahead. The direction of the force field was linked to the location of the nontarget green circle (“context”).

Fig. 3.

Hand paths from nonchannel trials from representative participants in the Same Target and Same Target Check conditions. The paths shown are from selected blocks of trials, including the last baseline block of the preexposure phase (8), the first (9), 12th (20), and last (60) perturbation blocks from the exposure phase, and the first (61) and last (72) baseline blocks from the post-exposure phase. The representative participant in the Same Target condition improved throughout exposure and displayed after effects postexposure, while the participant in the Same Target Check condition did not. CCW, counterclockwise; CW, clockwise.

Fig. 4.

Adaptation in the Same Target and Same Target Check conditions. Top row: adaptation, in channel trials, as a function of trial for the representative participants in each condition. The gray areas on the left and right mark the preexposure and postexposure phases, respectively. Red and blue points represent trials with the clockwise (CW) or counterclockwise (CCW) field. Middle row: top row data averaged across the two channel trials (one per context) in each block-pair of 16 trials. Bottom row: group mean data for each condition corresponding to the middle row. Height of the shaded regions represents ± 1 SE.

Fig. 5.

Hand paths from nonchannel trials from representative participants in the Same Control Point and Same Control Point Check conditions. The paths shown are from selected blocks of trials, including the last baseline block of the preexposure phase (8), the first (9), twelfth (20), and last (60) perturbation blocks from the exposure phase, and the first (61) and last (72) baseline blocks from the postexposure phase. The representative participant in the Same Control Point condition improved throughout exposure and displayed after effects postexposure, while the participant in the Same Control Point Check condition only displayed rightward after effects. CCW, counterclockwise; CW, clockwise.

Fig. 6.

Adaptation in the same Control Point and Same Control Point Check conditions. Top row: adaptation, in channel trials, as a function of trial for the representative participants in each condition. The gray areas on the left and right mark the preexposure and postexposure phases, respectively. Red and blue points represent trials with the clockwise (CW) or counterclockwise (CCW) field. Middle row: top row data averaged across the two channel trials (one per context) in each block-pair of 16 trials. Bottom row: group mean data for each condition corresponding to the middle row. Height of the shaded regions represents ± 1 SE.