Explicit and implicit contributions to learning in a sensorimotor adaptation task

Abstract

Visuomotor adaptation has been thought to be an implicit process that results when a sensory-prediction error signal is used to update a forward model. A striking feature of human competence is the ability to receive verbal instructions and employ strategies to solve tasks; such explicit processes could be used during visuomotor adaptation. Here, we used a novel task design that allowed us to obtain continuous verbal reports of aiming direction while participants learned a visuomotor rotation. We had two main hypotheses: the contribution of explicit learning would be modulated by instruction and the contribution of implicit learning would be modulated by the form of error feedback. By directly assaying aiming direction, we could identify the time course of the explicit component and, via subtraction, isolate the implicit component of learning. There were marked differences in the time courses of explicit and implicit contributions to learning. Explicit learning, driven by target error, was achieved by initially large then smaller explorations of aiming direction biased toward the correct solution. In contrast, implicit learning, driven by a sensory-prediction error, was slow and monotonic. Continuous error feedback reduced the amplitude of explicit learning and increased the contribution of implicit learning. The presence of instruction slightly increased the rate of initial learning and only had a subtle effect on implicit learning. We conclude that visuomotor adaptation, even in the absence of instruction, results from the interplay between explicit learning driven by target error and implicit learning of a forward model driven by prediction error.

Keywords: cerebellum; explicit; implicit; motor adaptation; motor learning; strategy.

Figure 1.

Experimental task. Top, Participants learned to overcome a 45° counterclockwise rotation while reaching to eight different target locations, separated by 45°. In the Instruction conditions, the workspace included numbered landmarks that flanked the target location. Before each movement, the participants verbally report where they planned to aim to make the cursor land on the target. In the No Instruction conditions, only the target location was presented. Vision of the hand in all conditions was occluded by a monitor that was mounted horizontally above the arm. For participants in the Endpoint Feedback conditions, the cursor disappeared at movement onset and reappeared as soon as the hand crossed a virtual ring, 7 cm from the start position. For participants in the Online Feedback conditions, the cursor remained visible throughout the reach. Bottom, In the baseline block, feedback was veridical (no rotation). In the second-baseline block, feedback was veridical and participants in the Instruction conditions reported their aiming location. In the rotation block, the cursor was rotated 45°. In the no-feedback block, the visual landmarks and cursor feedback were removed, and participants were instructed to aim directly to the target. In the washout block, veridical cursor feedback was restored.

Figure 2.

A, Target error (hand heading angle plus the 45° counterclockwise rotation) for Instruction-Endpoint (blue) and No Instruction-Endpoint groups (magenta). B, Target error for Instruction-Online (red) and No Instruction-Online groups (cyan). The rotation was present between 56 and 376 (dashed vertical lines). C, Angle of aiming location (landmark number multiplied by a spacing constant of 5.625°) for Instruction-Endpoint (blue) and Instruction-Online groups (red). Participants made verbal reports of aiming locations between movements 48 and 376. D, Left, Difference in the hand heading angle between the first eight trials of the rotation block and the last eight trials of the second-baseline block. Center, Difference in hand heading angle for the last eight trials of the rotation block and the last eight trials of the second-baseline block. Right, Difference in hand heading angle for the first eight trials of the no-feedback block and the last eight trials of the second-baseline block. Bar graphs represent the mean, while the circles are the individual participants.

Figure 3.

A, Probability of aim change during the report phase for the Instruction groups. B, Magnitude of aim change, the average change from trial (n) and trial (n − 1) across participants. C, Probability of aim change following a trial in which the cursor hit or missed the target. Instruction-Endpoint, blue; Instruction-Online, red.

Figure 4.

A, Target error (performance) is the sum of the explicit aiming direction and implicit learning of a forward model minus the perturbation. B, Implicit learning can be estimated by subtraction of the aiming direction (see Fig. 2C) from the target error. Instruction-Endpoint, blue; Instruction-Online groups; red.