Figure 1.

Assigning responsibility for motor errors.

(A) Bimanual actions have redundancy because either or both arms can contribute to the action. So an error — missing the ball — could have been caused by a mistake from either arm. It might also be due to external events, such as a gust of wind. The ellipses indicate unequal certainty about the state of each arm. For right handers, the right arm is more reliable, less uncertain (red ellipse). So the mistake is more likely caused by the more uncertain left arm (blue). (B) Experimental design. The forward movement of a single cursor (centre) towards a target (yellow) is controlled by both unseen arms, but is rotated clockwise about its origin. The two hands share the correction (C_L and C_R). On the subsequent trial, the two hands also adjust their initial direction to better control the rotated cursor. (Panel B adapted from [7].)