Beside the point: motor adaptation without feedback-based error correction in task-irrelevant conditions

Abstract

Adaptation of movement may be driven by the difference between planned and actual motor performance, or the difference between expected and actual sensory consequences of movement. To identify how the nervous system differentially uses these signals, we asked: does motor adaptation occur when movement errors are irrelevant to the task goal? Participants reached on a digitizing tablet from a fixed start location to one of three targets: a point, an arc, or a ray. For the arc, reaches could be in any direction, but to a specific extent. For the ray, reaches could be to any distance, but in a targeted direction. After baseline reaching to the point, the direction or extent of continuous visual feedback was perturbed during training with either a cursor rotation or gain, respectively, while reaching to either the ray (goal = direction) or the arc (goal = extent). The perturbation, therefore, was either relevant or irrelevant to the task goal, depending on target type. During interspersed catch trials, the perturbation was removed and the target switched back to the point, identical to baseline. Although the goal of baseline and catch trials was the same, significant aftereffects in catch trials indicated behavioral adaptation in response to the perturbation. Adaptation occurred regardless of whether the perturbation was relevant to the task, and it was independent of feedback control. The presence of adaptation orthogonal to task demands supports the hypothesis that the nervous system can rely on sensory prediction to drive motor learning that can generalize across tasks.

Fig. 1.

A: experimental task. During baseline and catch trials, subjects reached to move a cursor to a point target displayed 7 cm from the start position with continuous veridical visual feedback (left). During blocks of training with continuous perturbed visual feedback, subjects reached to move a cursor to a ray or an arc (right). Visual feedback was perturbed by rotating the cursor position (black) clockwise from the stylus/hand position (gray) relative to the start circle or by minifying the cursor position (black) from the stylus/hand position (gray) relative to the start circle. B: diagram of training schedule.

Fig. 2.

A: average handpaths from individual subjects for all baseline (black) and catch trial (gray) movements to the point during rotation training to the ray and arc. Squares indicate hand (stylus) position at 50 ms after peak velocity. B: mean (±SE) initial direction for all baseline (black) and catch trials (gray) during rotation training to the ray and arc across subjects. *P < 0.05; ***P < 0.0001 compared with baseline trials.

Fig. 3.

A: average handpaths from individual subjects for all baseline (black) and catch trial (gray) movements to the point during gain training to the ray and arc. Squares indicate hand (stylus) position at 50 ms after peak velocity. B: mean (±SE) initial extent for all baseline (black) and catch trials (gray) during gain training to the ray and arc across subjects. *P < 0.05 compared with baseline trials.

Fig. 4.

A: handpaths for each subject (n = 15) with no rotation (left) as well as early (trials 1, 5, and 9; middle) and late (trials 88, 92, and 96; right) in rotation training to the ray and arc. Handpaths to the ray before training are spread for visual purposes only. Arrow indicates direction of reach. Mean (±SE) perpendicular displacement is shown across subjects for each of the first and last 25 rotation training trials to the ray (relevant) and arc (irrelevant) (B) and across all early and late training trials (C). Significant mid-movement corrections occurred during early and late training only when subjects were reaching to the ray. ***P < 0.0001 compared with before training.

Fig. 5.

A: mean (±SE) deceleration duration across subjects for the last trial before gain training and for each of the first 10 gain training trials to the ray (irrelevant) and arc (relevant). Tangential velocity profiles inset above plots are from an individual subject reaching to the ray (dashed line) and arc (solid line) during the last trial before gain training and during the first trial of gain training. B: mean (±SE) deceleration duration averaged across all of the first 10 gain training trials compared with mean (±SE) deceleration duration to the ray and arc before training. ***P < 0.0001 compared with baseline.