Figure 2.

Behavioral task and time course of experiment 1

(A) Participants performed planar reaching movements with their dominant arm. Color-coded timing feedback was provided at the end of each trial.

(B) The experiment consisted of baseline, exposure, and washout phases. During the exposure phase, random, time-varying disturbances displaced the hand lateral to the goal target. Vertical gray lines denote constant step-torque perturbations (mechanical probes) that were used to probe the control policy (i.e., feedback gains) on randomly selected trials. Positive torques cause flexion motion of the elbow joint and displace the hand leftward of the target. The mechanical probes were applied at movement onset, at the instant the participant left the start target, and ramped down after participants held the goal target position.

(C) Time course of a representative trial. The random disturbances were sampled every 25 ms from a normal distribution (μ = 0, σ = 1 Nm).

(D) Distribution of random disturbances encountered by an exemplar participant across trials in the exposure phase.