Mid-Air Action Contributes to Pseudo-Haptic Stiffness Effects.

Pseudo-haptic feedback takes advantage of a cross-modal integration between vision and haptics. Previous studies have shown that object stiffness can be rendered with pseudo-haptic feedback with external haptic inputs. This article explored whether the pseudo-haptic feedback was feasible with a mid-air action wherein no external haptic input was given. On each trial of the experiments, participants conduced a mid-air action to laterally move their hands as if they horizontally stretched an object in the display. In synchronized with the hands' motion, the object horizontally deformed. The magnitude of the object deformation varied with the horizontal distance between participants' hands (i.e., a hand distance). The ratio of deformation magnitudes to the hand distance (i.e., a deformation-to-distance ratio) was controlled; With a larger ratio, a smaller hand distance produced the maximum level of object deformation. The Poisson's ratio was also controlled; a higher Poisson's ratio produced a larger magnitude of vertical deformation. The participants were asked to report the stiffness of the objects with a five-point rating scale. Consequently, the stiffness rating decreased with the deformation-distance ratio and with the Poisson's ratio. The results indicate that pseudo-haptic stiffness can be rendered with mid-air action by manipulating the deformation-distance ratio and Poisson's ratio.