Motoric and Perceptual Kinesthetic Symmetry in Bi-Manual Interactions

Bi-manual (two-handed) actions have shown notable success in rehabilitative and therapeutic applications from the point of motor symmetry. Recent studies have shown that symmetry in actions is attributed to sensorimotor perception than mere co-activation of homologous muscles. In this paper, we present a study of symmetric and asymmetric haptic (specifically force) feedback on human perception and motor action during bi-manual spatial tasks. To the best of our knowledge, ours is the first procedure to specifically test the perceptual aspect of bi-manual actions in contrast to other works that typically characterize the physical/bio-mechanical aspects. Thereby in our experiment, healthy individuals were tasked with stretching a virtual spring using two symmetrically located haptics devices that provide an equal amount of resistive force on each hand while pulling the spring. In this experiment, we implement four kinesthetic conditions, namely (1) feedback on both hands, (2) feedback only on dominant hand, (3) feedback only on non-dominant hand, and (4) no feedback as our control. Our first goal was to determine if there exists a range of spring stiffness in which the individual incorrectly perceives bi-manual forces when the feedback is deactivated on one hand. Subsequently, we also wanted to investigate what range of spring stiffness would lead to such perceptual illusions. Our analysis shows that not only does such a range exist, it is wide enough so as to be potentially utilized in future rehabilitative applications.