Setup and Procedure for the Experiment with Human Participants.

A: Participants experience the task as deforming a black ellipse to track the shape of a white ellipse by moving their left index (LI), right index (RI) and right middle (RM) fingers up and down (left). Internally, finger positions are mapped to the joint angles q1, q2 and q3 of the 3 DoF planar arm (bottom right, Method Sect Task). The size and elongation of the ellipses represents the position of target and reach-endpoint in goal space (top right). B: When a participant keeps the three fingers LI, RI and MI level (bottom left), this corresponds to taking the Baseline Posture q* (top left), which is in the middle of the two home postures (top middle and top right) in motor space. To take one of the home postures, the fingers have to be moved away from the midline in a manner that depends on the random mapping between fingers and joints (example mapping: (-2, 3, -1)). C: Each session consisted of 24 training and test blocks (right). A training block (top left) was started by taking the home posture. The deforming white target ellipse had to be tracked with the black ellipse (reach-endpoint) continuously for the next 80 s. In test blocks, only the white target ellipse was displayed and participants moved their finger till they reached a configuration they thought corresponds to the target and then submitted their response by pressing a foot pedal.