Learning feedback and feedforward control in a mirror-reversed visual environment

Abstract

When we learn a novel task, the motor system needs to acquire both feedforward and feedback control. Currently, little is known about how the learning of these two mechanisms relate to each other. In the present study, we tested whether feedforward and feedback control need to be learned separately, or whether they are learned as common mechanism when a new control policy is acquired. Participants were trained to reach to two lateral and one central target in an environment with mirror (left-right)-reversed visual feedback. One group was allowed to make online movement corrections, whereas the other group only received visual information after the end of the movement. Learning of feedforward control was assessed by measuring the accuracy of the initial movement direction to lateral targets. Feedback control was measured in the responses to sudden visual perturbations of the cursor when reaching to the central target. Although feedforward control improved in both groups, it was significantly better when online corrections were not allowed. In contrast, feedback control only adaptively changed in participants who received online feedback and remained unchanged in the group without online corrections. Our findings suggest that when a new control policy is acquired, feedforward and feedback control are learned separately, and that there may be a trade-off in learning between feedback and feedforward controllers.

Keywords: arm-reaching; feedback control; feedforward control; online correction; visuomotor transformation.

Fig. 1.

Experimental setup. A: visual information was displayed on a horizontal virtual reality display above the hand. Top middle and ±20° rotated white squares indicate central and lateral targets, respectively, and bottom white square indicates the starting position. The distance from starting target to each target was 15 cm. B: during cursor displacement trials, the robotic arm applied a damped spring force to the subject's hand when the hand was deviated in the lateral direction from the midline. C: each block consisted of six mini-blocks containing 20 trials each. A mini-block consisted of 12 reaches to the lateral target and 8 reaches to the straight-ahead target. Six of these were trials with force channel, and two trials were free movement with endpoint feedback. The participants performed two blocks of non-mirror-reversed (MR) feedback as baseline phase (240 trials in total, Normal 1). In the endpoint, or online, feedback group, the following 16 blocks of MR feedback were divided into four training phases (1,920 trials in total, MR 1 to MR 4), containing four blocks each. In the control group, all training phases were performed with non-MR feedback (1,920 trials in total, Normal 2–5). After the training phases, participants performed two non-MR blocks as a washout phase (240 trials in total, Normal 2 or Normal 6).

Fig. 2.

Online correction during the first phase of MR learning. A: movement trajectories from a representative participant from the endpoint (left column) and online (right column) feedback groups. The top panels indicate the first 20 trials of MR 1, the middle panels indicate the middle 20 trials of MR 1, and the bottom panels indicate the last 20 trials of MR 1. Red lines indicate individual trajectories for trials aiming to the red target, and blue lines indicate individual trajectories for trials aiming to the blue target. B: standard deviation of the position perpendicular to the main movement direction as a measure of online movement corrections. Solid gray line indicates the endpoint feedback group, and solid black line indicates the online feedback group. Dashed horizontal lines indicate baseline average of each group.

Fig. 3.

Changes in feedforward control. Solid gray line indicates the endpoint feedback group, and solid black line indicates the online feedback group. Dashed horizontal lines indicate baseline average of each group. The shaded region indicates ± 1 SE across participants. A: changes in reaction time. B: changes in the absolute initial angular error.

Fig. 4.

Temporal profiles of visuomotor responses to cursor displacement. Force on x-axis produced in response to cursor displacements in Normal 1 (solid black), MR 1 (blue), MR 2 (purple), MR 3 (magenta), MR 4 (red), and Normal 2 (dotted black) is shown. The solid line indicates the mean responses across participants, and the shaded colored regions represent the SE. The dashed box indicates the time window (251 ms to 350 ms) that is used for the analysis shown in Fig. 5. The endpoint feedback group is shown on the left, the online feedback group is shown in the middle, and control group is shown on the right.

Fig. 5.

Magnitude of averaged visuomotor responses. Lines indicate mean force on x-axis produced in response to cursor displacements across all participants during the 251- to 350-ms interval after the onset of the perturbation. Solid blue line indicates the endpoint feedback group, solid red line indicates the online feedback group, and solid black line indicates control group. Dashed horizontal lines indicate baseline average of each group. The shaded colored region indicates ± 1 SE across participants.