Tactile Perception of Virtual Edges and Gratings Displayed by Friction Modulation via Ultrasonic Actuation

Tactile discrimination and roughness perception of real textures are extensively studied and underlying perceptual mechanisms are relatively well-established. However, tactile perception of virtual textures rendered by friction modulation techniques on touch surfaces has not been investigated in detail yet. In this article, we investigated our ability to discriminate two consecutive step changes in friction (called edges), followed by discrimination and roughness perception of multiple edges (called periodic gratings). The results showed that discrimination of two consecutive edges was significantly influenced by edge sequence: a step fall in friction (<inline-formula><tex-math notation="LaTeX">$FF$</tex-math></inline-formula>) followed by a step rise in friction (<inline-formula><tex-math notation="LaTeX">$RF$</tex-math></inline-formula>) was discriminated more easily than the reverse order. On the other hand, periodic gratings displayed by consecutive sequences of <inline-formula><tex-math notation="LaTeX">$FF$</tex-math></inline-formula> followed by <inline-formula><tex-math notation="LaTeX">$RF$</tex-math></inline-formula> were perceived with the same acuity as compared to vice versa. Independent of the edge sequence, we found that a relative difference of 14% in spatial period was required to discriminate two periodic gratings. Moreover, the roughness perception of periodic gratings decreased with increasing spatial period for the range that we have investigated (spatial period <inline-formula><tex-math notation="LaTeX">$>$</tex-math></inline-formula> 2 mm), despite the lack of spatial cues on grating height. We also observed that rate of change in friction coefficient was better correlated with the roughness perception than the friction coefficient itself. These results will further help to understand and design virtual textures for touch surfaces.