Fig. 1.

Motor task. At each trial, 1 of 3 target force profiles was shown during the “ready” period. The specific force profile was selected according to a predetermined schedule (random or blocked). Two seconds later, a “GO” signal appeared and the participant was instructed to reach and grasp the apparatus and modulate the power grip force to approximate the target profile that lasted 2 s. The computer screen then became blank for 4 s, and feedback was shown for 4 s. Feedback included the actual force profile superimposed with the desired profile, the root mean squared error (RMSE) between the 2 profiles, as a well as a “best score,” which reflected the smallest error so far and was included for motivational purposes.