New visuomotor maps are immediately available to the opposite limb.

Humans can learn to make accurate movements when the required map between vision and motor commands changes, but can visuomotor maps obtained through experience with one limb benefit the other? Complete transfer would require new maps to be both fully compatible and accessible between limbs. However, when this question is addressed by providing subjects with rotated visual feedback during reaching, transfer is rarely apparent in the first few trials with the unpracticed limb and is sometimes absent altogether. Partial transfer might be explained by limited accessibility to remapped brain circuits, since critical visuomotor transformations mediating unilateral movements appear to be lateralized. Alternatively, if adaptation involves movement representations associated with both extrinsic (i.e., direction of motion in space) and intrinsic (i.e., joint or muscle based) frames of reference, new visuomotor maps might be incompatible with opposite limb use when visual distortions have opposite effects for the two limbs in intrinsic coordinates. Here we addressed this issue when subjects performed an isometric aiming task with the index finger. We manipulated the alignment of visuomotor distortion for the two hands in different reference frames by altering body posture relative to the orientation of the finger and the visual display. There was strong, immediate transfer of adaptation between limbs only when visuomotor distortion had identical effects in eye- and joint-based coordinates bilaterally. This implies that new visuomotor maps are encoded in neural circuits associated with both intrinsic and extrinsic movement representations and are available to both limbs.