The effects of training breadth on motor generalization

Abstract

To generate new movements, we have to generalize what we have learned from previously practiced movements. An important question, therefore, is how the breadth of training affects generalization: does practicing a broad or narrow range of movements lead to better generalization? We address this question with a force field learning experiment. One group adapted while making many reaches in a small region (narrow group), and another group adapted while making reaches in a large region (broad group). Subsequently, both groups were tested for their ability to generalize without visual feedback. Not surprisingly, the narrow group exhibited smaller adaptation errors, yet they did not generalize any better than the broad group. Path errors during generalization were indistinguishable across the two groups, whereas the broad group exhibited reduced terminal errors. These findings indicate that overall, practicing a variety of movements is advantageous for performance during generalization; movement paths are not hindered, and terminal errors are superior. Moreover, the evidence suggests a dissociation between the ability to generalize information about a novel dynamic disturbance, which generalizes narrowly, and the ability to locate the limb accurately in space, which generalizes broadly.

Keywords: motor generalization; motor learning; motor training; practice.

Fig. 1.

Experimental setup. A: subjects sat in front of a robotic manipulandum that rendered either a null or a curl field while they made reaches to targets presented on a monitor. B: subjects in the narrow group made point-to-point reaches with the center-out star pattern of targets (example reaches are shown, purple traces). Subjects in the broad group made reaches in 8 directions pseudorandomly chosen in a region more than twice as large as the narrow group (example reaches are shown, dark blue traces). All subjects were then tested for generalization, making point-to-point reaches in the same region with the same star pattern of targets (light blue traces).

Fig. 2.

Example reaches for a representative subject across the 5 blocks and 2 conditions (on 2 separate days). All reaches shown have their starting location translated to the same central location and were made without visual feedback. Reach colors are shaded according to target. For blocks 1 and 2, reaches were made in the training and generalization workspaces, respectively, in a null field (the last 5 reaches are displayed). For blocks 3 and 4, reaches were made in a curl field in their respective training workspaces (the 1st and last 5 are displayed). For the generalization block 5, all subjects made reaches in the same generalization workspace with the curl field they trained on (the last 5 reaches are displayed). Note that the reaches in the curl field are curved in opposite directions when the subject switches from the narrow group to the broad.

Fig. 3.

Summary of errors. The perpendicular and angular errors for movement paths (A and B) and movement termination errors (C and D) for reaches made without visual feedback. In each panel, for both experimental groups (blue and purple for the broad group and narrow group, respectively), the across-subject average measures (± SE) are displayed. For baseline blocks 1 and 2 (null field), averages are over the entire block. For the training blocks 3 and 4 (force field), averages are computed over 50-trial bins with additional averages computed during the 1st and last 100 trials (shaded regions). For the generalization block 5, averages are computed over all force field trials. Also displayed are the generalization results for the matching broad and narrow control groups (labeled control). deg, Degrees.

Fig. 4.

Bootstrapped exponential learning curves and their differences for the perpendicular and angular errors (A and B) and movement termination errors (C and D) for reaches made without visual feedback. Displayed are the median value curves (solid lines) and the 95% confidence intervals (shaded regions). Overlaid are the mean values (shaded lines) of the data. All groups began with indistinguishable error curves; the confidence intervals overlapped, and the 95% confidence interval for the bootstrapped differences overlapped 0. Soon thereafter, the errors diverged, and performance in the narrow group improved relative to the broad group.

Fig. 5.

Catch trial errors. The perpendicular and angular errors for movement paths (A and B) and movement termination errors (C and D) for reaches made without visual feedback. In each panel, for both experimental groups (blue and purple for the broad group and narrow group, respectively), the across-subject average measures (± SE) are displayed. For baseline blocks 1 and 2 (null field), averages are over the entire block. For the training blocks 3 and 4 (force field), averages are computed over 50-trial bins with additional averages computed during the 1st and last 100 trials (shaded regions). For the generalization block 5, averages are computed over all force field trials.