Improving the Perception of Mid-Air Tactile Shapes With Spatio-Temporally-Modulated Tactile Pointers

Ultrasound mid-air haptic (UMH) devices can remotely render vibrotactile shapes on the skin of unequipped users, e.g., to draw haptic icons or render virtual object shapes. Spatio-temporal modulation (STM), the state-of-the-art UMH shape rendering method, provides large freedom in shape design and produces the strongest possible stimuli for this technology. Yet, STM shapes are often reported to be blurry, complicating shape identification. Dynamic tactile pointers (DTP) were recently introduced as a technique to overcome this issue. By tracing a contour with an amplitude-modulated focal point, they significantly improve shape identification accuracy over STM, but at the cost of much lower stimulus intensity. Building upon this, we propose Spatio-temporally-modulated Tactile Pointers (STP), a novel method for rendering clearer and sharper UMH shapes while at the same time producing strong vibrotactile sensations. We ran two human participant experiments, which show that STP shapes are perceived as significantly stronger than DTP shapes, while shape identification accuracy is significantly improved over STM and on par with that obtained with DTP. Our work has implications for effective shape rendering with UMH, and provides insights which could inform future psychophysical investigation into vibrotactile shape perception in UMH.