Flexible explicit but rigid implicit learning in a visuomotor adaptation task

Abstract

There is mounting evidence for the idea that performance in a visuomotor rotation task can be supported by both implicit and explicit forms of learning. The implicit component of learning has been well characterized in previous experiments and is thought to arise from the adaptation of an internal model driven by sensorimotor prediction errors. However, the role of explicit learning is less clear, and previous investigations aimed at characterizing the explicit component have relied on indirect measures such as dual-task manipulations, posttests, and descriptive computational models. To address this problem, we developed a new method for directly assaying explicit learning by having participants verbally report their intended aiming direction on each trial. While our previous research employing this method has demonstrated the possibility of measuring explicit learning over the course of training, it was only tested over a limited scope of manipulations common to visuomotor rotation tasks. In the present study, we sought to better characterize explicit and implicit learning over a wider range of task conditions. We tested how explicit and implicit learning change as a function of the specific visual landmarks used to probe explicit learning, the number of training targets, and the size of the rotation. We found that explicit learning was remarkably flexible, responding appropriately to task demands. In contrast, implicit learning was strikingly rigid, with each task condition producing a similar degree of implicit learning. These results suggest that explicit learning is a fundamental component of motor learning and has been overlooked or conflated in previous visuomotor tasks.

Keywords: explicit learning; implicit learning; motor adaptation; motor learning; visuomotor rotation.

Fig. 1.

Task workspace and structure. A: Rotating condition: a circular array of numbered landmarks flanked each side of the target and rotated along with the target such that 1 and −1 were always adjacent to the target. This configuration was also used in experiments 2 and 3. B: Fixed condition: the numbered landmarks remained fixed relative to the workspace regardless of target location. Before each movement, participants were instructed to verbally report where they needed to aim to get their cursor on the target. C: task block structure.

Fig. 2.

Experiment 1 learning time courses. A: end-point hand angle for Rotating and Fixed landmarks. B: explicit learning: angle of aiming location (verbally reported landmark). C: implicit learning: subtraction of aiming direction from end-point hand angle. Vertical dashed lines denote when the rotation was introduced and removed. Movement epicycles represent the average of an 8-trial bin, and shading represents the 95% confidence interval of the mean.

Fig. 3.

Experiment 1 phases of interest for each block. A: average end-point hand angles for the Baseline-Report block epicycle (Baseline) and first (Early Rot) and last (Late Rot) epicycles of the Rotation block and the aftereffect for the first epicycle of the No-Feedback block (Aftereffect). B: explicit learning: average angle of aiming location for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. C: implicit learning: subtraction of aiming location from end-point hand angle for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. Bars represent the group mean of each 8-trial bin (epicycle), and circles represent the individual participants for the Rotating and Fixed conditions.

Fig. 4.

Changes in explicit aiming during the Rotation block. A: probability of aim direction change during the verbal reporting phase for the Rotating and Fixed landmark conditions. B: average change in aiming direction from trial n and trial n − 1 across subjects. C: win-stay/lose-shift: probability of aim change after a successful or unsuccessful trial. Subjects in the Fixed condition are more likely to switch aiming strategies whether hitting or missing the target. Data were averaged into 8-trial bins (epicycles), and shading represents the 95% confidence interval of the mean.

Fig. 5.

Experiment 2 learning time courses. A: end-point hand angle for One-Target, Two-Target, and Four-Target conditions. B: explicit learning: angle of aiming location (verbally reported landmark). C: implicit learning: subtraction of aiming direction from end-point hand angle. Vertical dashed lines denote when the rotation was introduced and removed. Movement epicycles represent the average of an 8-trial bin, and shading represents the 95% confidence interval of the mean.

Fig. 6.

Experiment 2 phases of interest for each block. A: average end-point hand angles for the Baseline-Report block epicycle and first and last epicycles of the Rotation block and the aftereffect for the first epicycle of the No-Feedback block. B: explicit learning: average angle of aiming location for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. C: implicit learning: subtraction of aiming location from end-point hand angle for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. Bars represent the group mean of each 8-trial bin (epicycle), and circles represent the individual participants for the One-Target, Two-Target, and Four-Target conditions.

Fig. 7.

Experiment 3 learning time courses. A: end-point hand angle for Fifteen, Thirty, Sixty, and Ninety degree rotation conditions. B: normalized learning: end-point hand angle divided by the size of the rotation for each group. C: explicit learning: angle of aiming location (verbally reported landmark). D: normalized explicit learning: average angle of aiming location divided by the size of the rotation for each group. E: implicit learning: subtraction of aiming direction from end-point hand angle. F: normalized implicit learning: subtraction of aiming direction from end-point hand angle divided by the size of the rotation for each group. Vertical dashed lines denote when the rotation was introduced and removed. Movement epicycles represent the average of an 8-trial bin, and shading represents the 95% confidence interval of the mean.

Fig. 8.

Experiment 3 phases of interest for each block. A: average end-point hand angles for the Baseline-Report block epicycle and first and last epicycles of the Rotation block and the aftereffect for the first epicycle of the No-Feedback block. B: explicit learning: average angle of aiming location for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. C: implicit learning: subtraction of aiming location from end-point hand angle for the Baseline-Report block epicycle and first and last epicycles of the Rotation block. Bars represent the group mean of each 8-trial bin (epicycle), and circles represent the individual participants for the Fifteen, Thirty, Sixty, and Ninety degree rotation conditions.