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Strong unilateral contractions are accompanied by excitatory effects to the ipsilateral cortex. This activity can even result in overt contractions of muscles in the contralateral limb. We used this inadvertent, associated activity to study whether the cortical presentation of movements is organized in a directional-related or a muscle-related reference frame. We assessed the contralateral activation for the left index finger during a sustained maximal abduction of the right index finger. In the first experiment, both hands were held vertically in a symmetrical orientation, and in the second experiment the hands were in an asymmetrical orientation (left hand, palm downward; right hand, vertical). In both experiments, the direction of the contralateral associated contraction was upward, i.e., in the symmetrical hand orientation the contralateral force increased mainly in abduction direction, whereas in the asymmetrical hand orientation the contralateral force increased in the extension direction. Thus, the contralateral contractions reflected the direction of the target movement rather than simply the activity of the muscles activated on the target side. These observations provide strong evidence that motor commands are organized in an extrinsic, direction-related reference frame, as opposed to an internal muscle-related reference frame.

Inadvertent Contralateral Activity during a Sustained Unilateral Contraction Reflects the Direction of Target Movement