An implicit plan overrides an explicit strategy during visuomotor adaptation

Abstract

The relationship between implicit and explicit processes during motor learning, and for visuomotor adaptation in particular, is poorly understood. We set up a conflict between implicit and explicit processes by instructing subjects to counter a visuomotor rotation using a cognitive strategy in a pointing task. Specifically, they were told the exact nature of the directional perturbation, a rotation that directed them 45 degrees counterclockwise from the desired target, and they were instructed to counter it by aiming for the neighboring clockwise target, 45 degrees away. Subjects were initially successful in completely negating the rotation with this strategy. Surprisingly, however, they were unable to sustain explicit control and made increasingly large errors to the desired target. The cognitive strategy failed because subjects simultaneously adapted unconsciously to the rotation to the neighboring target. Notably, the rate of implicit adaptation to the neighboring target was not significantly different from rotation adaptation in the absence of an opposing explicit strategy. These results indicate that explicit strategies cannot substitute for implicit adaptation to a visuomotor rotation and are in fact overridden by the motor planning system. This suggests that the motor system requires that planned and executed trajectories remain congruous in visual space, and enforces this correspondence even at the expense of an opposing explicit task goal.

Figure 1.

Task conditions. Each frame shows the start circle (S) and three of the eight surrounding targets. The bull's-eye pattern indicates the target proper (T_P), and the two open circles are the neighboring targets, 45° away. The arrows indicate the direction of hand and cursor movements (H and C, respectively). A, Baseline. B, Early rotation (45° CCW). C, Late rotation. D, Washout. E, Rotation plus strategy. F, Strategy only. In A–D, subjects aim for T_P. In E and F, subjects aim for T_N.

Figure 2.

Time course of directional error (mean ± SE; in degrees) at the endpoint for each group. A, Rotation plus strategy. B, Rotation. C, Strategy. Roman numerals indicate changes in experimental conditions (see Results for details).