Miller, J. O., & Franz, E. A. (2004) Dissociation of bimanual responses with the Simon effect: On the nonunitization of bimanual responses. Journal of Motor Behavior, 37, 146-156.

We examined whether responses of the two hands are completely unitized when participants produce bimanual responses to lateralized targets in a Simon-type paradigm. A primary aim was to investigate whether lateralized stimuli differentially influence the response dynamics of the two hands. Simon effects were obtained in reaction time and force, with faster and more forceful components of the bimanual response with the hand on the same side as the lateralized stimulus as compared with the other hand. Also, Simon effects were larger when the lateralized target appeared alone than when it was accompanied by a distractor on the other side of the display. Finally, responses of the two hands were correlated most strongly when stimulus displays were symmetrical. We conclude that bimanual responses are strongly coupled but not perfectly so.