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

Abstract

Research has identified an inverted u-shaped relationship between the pleasantness of arm stroking and stroking velocity. 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 and 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 stroking trajectories. 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 trajectory 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 trajectory 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 physical parameters including touch duration and spatial pattern. Public Significance StatementPsychophysical 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.

Characterizing affiliative touch in humans and its role in advancing haptic design

“…Unsurprisingly, another contextual feature known to modulate the pleasantness of affiliative touch is the identity of the toucher. At a basic level, strokes at velocities rated as more human are consistently perceived as more pleasant 29 . Touch from people perceived to be more attractive is also experienced as more pleasant 79 .…”

“…They interact with fibers carrying discriminatory touch information 21,24 and project directly or indirectly to multiple nodes of affective evaluation in the central nervous system 25–27 . Importantly, stimuli that evoke higher CT firing rates also typically evoke higher ratings of subjective pleasantness, 28 although the relationship between velocity and pleasantness is also modulated by the duration and spatial pattern of contact with the skin 29 . CT receptors may also be only one of a putative family of receptors, each sensitive to distinct qualities of affiliative social touch 21,30 .…”

“…higher CT firing rates also typically evoke higher ratings of subjective pleasantness, 28 although the relationship between velocity and pleasantness is also modulated by the duration and spatial pattern of contact with the skin. 29 CT receptors may also be only one of a putative family of receptors, each sensitive to distinct qualities of affiliative social touch. 21,30 There is also growing evidence that myelinated A fibers contribute to experienced pleasantness of soft stroking and pressure, 29,31,32 although the precise mechanisms by which this occurs-and whether it is a function of centrally modulated associative processes-is still not known.…”

“…29 CT receptors may also be only one of a putative family of receptors, each sensitive to distinct qualities of affiliative social touch. 21,30 There is also growing evidence that myelinated A fibers contribute to experienced pleasantness of soft stroking and pressure, 29,31,32 although the precise mechanisms by which this occurs-and whether it is a function of centrally modulated associative processes-is still not known. 33 The findings summarized above first provided the foundation for the affective 34 and social 12,35 touch hypotheses.…”

“…At a basic level, strokes at velocities rated as more human are consistently perceived as more pleasant. 29 Touch from people perceived to be more attractive is also experienced as more pleasant. 79 Consistent with this finding, heterosexual men showed greater activation in S1 in response to sensual touch when they thought the toucher was an attractive woman versus a man.…”