Figure 1.

Psychophysics of motor learning. Data are shown from a representative experimental session. (A) Trajectories in real space. The trajectories are roughly straight when the movements are not perturbed (baseline). When a counterclockwise (CCW) force field is turned on, trajectories are deviated at first (early force). After adaptation has occurred, however, trajectories become straight (late force). After the perturbing force is turned off, the first movements show an aftereffect, inasmuch as they are deviated in the clockwise direction (early washout). Within a few trials, however, the monkey readapts to the unperturbed condition, and trajectories become straight again (late washout). (B) Correlation coefficient. The correlation coefficient (Cc) quantifies the quality of the movements by comparison with an ideal trajectory. The coefficient is close to 1 for straight and smooth movements. In the baseline condition, the Cc is high (≈0.95). As the force field is introduced, the Cc drops, returning to high values (≈0.9) as the monkey adapts to the external force. When the force field is turned off, the Cc temporarily drops (aftereffect) and returns to its original values as the monkey readapts. The data shown in Fig. 1 were recorded on an early day of training.