Adaptation Paths to Novel Motor Tasks Are Shaped by Prior Structure Learning

Visuomotor experiment. A, Subjects were instructed to make fast straight movements between a starting position and a single target (there were no other targets used throughout the experiment). One group of subjects experienced horizontal, and another group vertical, visuomotor rotations, with the angle of rotation randomly chosen from the set {±30, ±20, ±10°} in blocks of five trials (structure learning period). For this and subsequent figures, hand directions were transformed into azimuth-elevation coordinates; the blue (red) dots show ideal hand directions corresponding to different horizontal (vertical) visuomotor rotations. The crosses correspond to visuomotor rotations used for probing after 3 days of training. Below, A subject is holding 3D manipulandum and wearing 3D glasses. B, Paradigm scheme, showing 3 days of experiment (see Materials and Methods). The shaded area shows the exposure to horizontal/vertical rotations, with its width corresponding to the maximal rotation angle (first 10°, then 20°, then 30°). Four different geometric symbols represent four different diagonal rotations used for probing (4 times each). C, Initial hand directions measured at 200 ms after movement onset, on the fifth trial of every structure learning block on the third day of training before probing, in blue for the horizontal group and in red for the vertical one (group averages). The small empty ellipses show SEM, and the large filled ones show SD (N = 30 for each ellipse). The average amount of adaptation for each group is shown as a percentage value. D, The same as above, but for the late movement hand positions, measured at 400 ms after movement onset.

Adaptation paths are bent toward the learned structure. A, Movement trajectories during the first trial of every probing block, in blue for the horizontal and in red for the vertical group (group averages). Each trajectory starts near (0, 0) because the diagonal rotation was switched on unexpectedly for subjects, and ends near the corresponding cross because subjects had to reach the target to finish the trial. Trajectories are taken from 200 ms after movement onset until the moment when cursor entered the target. The colored bands around average trajectories show SEM; N = 24 for each trajectory. B, Late movement hand positions (400 ms after movement onset) across probing trials, with color (dark to light) showing trial number (1–5) during a probing block. Hand positions for every probing direction and every trial number are significantly different between groups (20 comparisons) (for a scheme of statistical testing, see Fig. 4F): p < 0.001 for 17 pairs, p < 0.01 for 1 pair, p < 0.05 for another 2 pairs. The dashed lines show trajectories corresponding to exponential fits calculated separately for the adaptation of azimuth and elevation (for details, see Materials and Methods).

Adaptation paths are bent toward the learned structure (continued). A, Across-trial adaptation of late movement hand positions: the same data as presented in Figure 3B, but flipped to the first quadrant and averaged over directions (group averages). Ellipses show SEM (N = 96 for each ellipse), with color (dark to light) showing trial number (1–5) during a probing block. The difference between groups is significant for every trial (p < 10⁻¹⁰). The dashed lines show trajectories corresponding to the exponential fits, with small dots on the dashed line showing the positions on trials 1–5, according to the fit. The inset shows learning speeds for both groups with light (dark) bars corresponding to azimuth (elevation) learning speeds. Learning speed is defined as amount of error (from 0 to 1) corrected on each subsequent trial. The asterisks show statistical significance (for details, see Materials and Methods), with three asterisks meaning p < 0.001. B, The same as A, but for initial movement directions (200 ms after movement onset). For trials 2–5, difference between groups is significant with p < 0.001. Two asterisks in the inset mean p < 0.01. C, The same as B, but for initial directions of backward movements. Here, the data were flipped to the third quadrant only for convenience, to show that these are backward movements. Difference between groups is significant for each trial with p < 10⁻⁵. D, Movement trajectories during the first trial, averaged over directions. The black ticks show the time point when the difference between groups becomes significant with p < 0.05. E, For every subject, we computed the degree of bending, calculated as a normalized area between late movement adaptation path and the straight diagonal path. These values are shown for six subjects in the vertical and six subjects in the horizontal group. The horizontal lines show mean value for each group (printed nearby in respective colors) together with SEM. F, Scheme of statistical testing for differences between groups, used in A–D. The data were first projected on a line perpendicular to the perturbation, and then Mann–Whitney–Wilcoxon rank sum test was performed. This way, we were only assessing the difference in bending of adaptation paths, and not in the learning speed.

Cursor jump experiment. A, Subject makes a movement to the target in a “force channel” (blue), with the manipulandum exerting very strong returning forces preventing any deviations from the z-axis. At 3 cm from the origin, cursor was for 230 ms displaced in one of the diagonal directions but still moved in parallel to the z-axis according to the hand movement. The force channel allows to record forces that subjects exert on the channel walls in reaction to the cursor jump. B, Average reaction force at 300 ms after the jump onset for all four jump directions (group averages). Ellipses show SEM (N_HOR = 192 and N_VERT = 168 for each ellipse). The asterisks indicate the statistical significance of the difference between groups, with two asterisks meaning p < 0.01 and three asterisks meaning p < 0.001 (for a scheme of statistical test, see Fig. 4F). C, The same data as on the previous panel, flipped to the first quadrant and averaged over directions (p < 10⁻¹⁷; N_HOR = 768 and N_VERT = 672). The angular values show deviation of the average force responses from the diagonal. The inset shows these angular values for individual subjects (obtained via circular median).

Timing of force response to cursor jumps. A, Exemplary subject; horizontal force exerted as a reaction to the cursor jumps in the top-left/bottom-left (solid line) and in the top-right/bottom-right (dashed line) directions. The dotted line shows the force during no-jump trials. All of the data are aligned on the jump onset, and the cursor jump profile is shown as a green dashed line (out of vertical scale). N = 48 for each curve. B, The magenta line shows the difference between horizontal forces after cursor jumps in the top-left/bottom-left and in the top-right/bottom-right directions (i.e., between solid and dashed lines in A). Difference was computed for each subject and probing batch, and then averaged over batches and subjects (N = 64 for each curve). The cyan line shows the same, but after the subjects were instructed to push in the direction of the cursor jump. An arrow at 265 ms depicts the voluntary reaction time. A tick at 185 ms marks the beginning of the reflex response (p < 0.05 difference from the baseline, one-sided t test). C, Difference between horizontal forces after cursor jumps in the top-left/bottom-left and in the top-right/bottom-right directions (calculated for each subject and probing batch and averaged over subjects in each group; N_HOR = 64 and N_VERT = 56). The horizontal group is shown in blue, and the vertical group, in red. The thick black lines show the time interval of significant difference between groups (*p < 0.05; **p < 0.01; ***p < 0.001). D–F, The same as A–C for the vertical force.

Force field experiment. A, Subjects were making fast straight movements between two targets, this time experiencing different horizontal or vertical force fields instead of visuomotor rotations. Force was proportional to the velocity along the z-axis, and the gain changed between ±10 N · s/m in blocks of five trials. The red and blue dots on the plane of gains show the gains used for the vertical and horizontal structures. The crosses mark the gains used for probing. B, Gains used by the subjects on the last trial of every block during the structure learning period on the third day (estimated with error-clamp trials) (see Results). The small open ellipses show SEM for group averages, and the large filled ellipses show SD, in blue for the horizontal and in red for the vertical group (N_HOR = 18 and N_VERT = 12 for each ellipse). Note that, on this panel, the axes do not go until 10 N · s/m, and the average adaptation is only ∼50%.