Cutaneous Wave Propagation Shapes Tactile Motion: Evidence from Air-Coupled Ultrasound

Tactile stimulation of the skin excites cutaneous waves that travel tens of centimeters, but the implications for haptic engineering and perception are not well understood. We present evidence from optical vibrometry that tactile motion cues delivered via air-coupled ultrasound excite complex spatiotemporal wave fields in the hand. We distinguished two physical regimes based on the ratio of the motion speed to the cutaneous wave speed. At low speeds (1-4 m/s), waves generated by a moving stimulus propagated to similar distances in all directions. At high speeds (4-15 m/s), waves in the direction of motion were compressed. We also studied tactile motion perception at these speeds, which were faster than those used in prior studies. Motion sensitivity was impaired when waves were inhibited in front of the moving stimulus. This occurred for motion at high speeds and across disconnected skin areas. Together, our findings suggest that tactile motion perception is aided by waves propagating in the skin. This paper presents the first time-resolved observations of cutaneous responses to focused ultrasound, and contributes practical knowledge for the use of tactile motion and mid-air haptic feedback.