Coupled pairs do not necessarily interact

Previous studies that examined paired sensorimotor interaction suggested that rigidly coupled partners negotiate roles through the coupling force [1-3]. As a result, several human-robot interaction strategies have been developed with such explicit role distribution [4-6]. However, the evidence for role formation in human pairs is missing; to understand how rigidly coupled pairs negotiate roles through the coupling, we systematically examined rigidly coupled pairs who made point-to-point reaching movements. Our results reveal the consistency of the coupling force during the movement, from the very beginning of interaction. Do partners somehow negotiate the roles prior to interaction? A more likely explanation is that the coupling force is a by-product of two people who independently planned their reaching movements. We developed a computational model of two independent motion planners, which explains inter-pair coupling force variability. We demonstrate that the coupling force alone is an unreliable measure of interaction, and that coupled reaching is not a suitable task to examine sensorimotor interaction between humans. [1] Reed KB, Peshkin M (2008), IEEE Trans Haptics 1: 108-20. [2] Stefanov N, Peer A, Buss M (2009), Proc Worldhaptics 51-6. [3] van der Wel RPRD, Knoblich G & Sebanz N (2011), J Exp Psychol 37: 1420-31. [4] Evrard P, Kheddar A (2009), Proc Worldhaptics 45-50. [5] Oguz S, Kucukyilmaz A, Sezgin T, Basdogan C (2010), Proc Worldhaptics 371-8. [6] Mörtl A, Lawitzky M, Kucukyilmaz A, Sezgin M, Basdogan C, Kirche S (2012), Int J of Robotics Research 31(13): 1656-74.