On the Role of Lateral Force in Texture-Induced Motion Bias During Reaching Tasks

In previous work, we reported that tactile information (tactile slip) during finger sliding and reaching actions over a textured surface contributes to the control of the hand movement. More specifically, we observed a significant bias in the motion trajectories, which was explained by the tactile estimate accounted by the tactile flow model - i.e. a perceived motion direction always perpendicular to the ridge orientation, and its integration with the muscular-skeletal proprioceptive cues. However, to which extent this observed behavior also depends on the reaction force exerted by the surface ridges on the finger pad during the dynamic interaction still represents a largely unexplored research question. If not properly addressed, this point could rise the alternative explanation that the systematic bias is determined by the insufficient compensation of the reaction force by participants. In this work, we investigate the role of the lateral component of the reaction force on the surface plane (lateral force) in texture-related motion bias. We asked participants to slide their finger straight on a lubricated ridged plate towards a target goal displayed in a virtual reality environment. They exerted two different levels of normal force, which produced two different levels of lateral force during the finger interaction with the ridges. The effect of ridge orientation was found to be larger for the high compared to the low force level. However, also in the latter case, we still observed the same biased trajectories reported in our previous work, despite the negligible value of the lateral force. This supports our hypothesis that the motor bias arises from the integration of the tactile motion estimate, biased by the texture, and the other proprioceptive cues.