Coupling of postural sway to somatosensory drive: Effects of unilateral versus bilateral light touch

Introduction/Background Lightly touching a stable surface with the forefinger strongly reduces postural sway. Interestingly, bilateral light touch with both forefingers reinforces the postural stabilization. If the touched surface moves periodically, the Center-of-Pressure (CoP) oscillates in coherence with the finger movement. Based on this coupling, we presented in a previous study a closed loop control of the CoP position in the Anterior-Posterior (AP) direction. We were able to drive the CoP around an arbitrary predefined path only by using finger movements. The current objective was to study the influence on this coupling of reinforcing the light touch by asking the participants to put both forefingers on the platform. Material and method Participants stand on a force platform and touch lightly with the index a 1-degree-of-freedom robot, which performs linear displacements in the AP direction. Robot velocity, and thus the finger velocity, is driven proportionally to the error between an arbitrary predefined path and the current CoP position, measured by the force platform and fed back to the robot. The finger-movements/CoP coupling is assessed through the efficiency of the closed loop to drive the CoP. Fourteen participants repeated 3 times in a random way two closed-loop conditions: one with dominant hand forefinger and the other with both forefingers. The finger and the CoP positions are recorded and used to compute the tracking error between CoP and a predefined path. Results The efficiency of the closed loop is significantly increased (almost halved mean tracking error) with both forefingers compared to one. Conclusion This observation extends results with bilateral light touch on a stable surface i.e. bilateral contact increases light touch effect on postural control. Giving more weight to the light touch leads to a better integration of this reference and thus to a more consistent coupling between finger movements and CoP.