The three DoF planar arm reaching task.

A: Both humans and simulated agents learned to move a three DoF planar arm with joints q1, q2 and q3 (three-dimensional motor space) to reach to targets in an x and y plane (two-dimensional goal space). B: Top: The home postures H1 and H2 are different but reach to the same point in goal space. Note that in home posture H1, the q2 joint is folded backwards (q2 = π) so the reach-endpoint is located on top of the q2 joint. Bottom: The colour map depicts the redundancy, that is the relative number of joint configurations (q1, q2, q3) that reach to a given point in the goal space (lighter colours imply more joint configurations to reach this point). The task is redundant, especially for targets close to the origin of the goal space. Participants were trained and tested on targets in the top quadrant of the goal space (green disks).