Stroking trajectory shapes velocity effects on pleasantness and other touch percepts.

Research has identified an inverted u-shaped relationship between stroking velocity and perceived pleasantness. However, the generalizability of this relationship is questionable as much of the work relied on the rotary tactile stimulator (RTS), which strokes skin with force varying along an arc, but confounds stimulus velocity with duration. We explored how these parameters shape the subjective evaluation of touch. In Study 1, one group of participants was stroked by the RTS, while two other groups were stroked by a new robot capable of different types of skin stroking. Participants were stroked at five velocities and rated pleasantness, humanness, intensity, and roughness. In Study 2, participants were stroked by the new robot imitating the movement of the RTS exactly, imitating it while controlling stimulus duration, or moving linearly or ovally with both constant force and duration. Participants rated pleasantness and humanness. Although stroke velocity was related to both pleasantness and humanness in an inverted u-shaped manner, stimulus motion modulated this relationship and the association between velocity and the other ratings. Together, our results clearly link stroking velocity to the perception of touch, but highlight that this relationship is shaped by other parameters such as the duration and spatial trajectory of touch. Public Significance Statement Psychophysical research has identified an inverted u-shaped relationship between a touch's velocity and subjective pleasantness, which has guided current thinking about the processing and benefits of a gentle caress. Here, we show that this relationship depends on aspects of the tactile stimulus that, so far, have been overlooked, including the duration of skin contact and the trajectory of the touch. We find that stroking duration and trajectory shape how stroke velocity modulates subjective pleasantness, humanness, intensity, and roughness. Thus, we identify a need for research to go beyond velocity and to consider other motion features of touch, especially those that approximate human social touch outside the laboratory