Abstract
BackgroundWhen scanning surfaces, humans perceive some of their physical attributes. These percepts are frequently accompanied by a sensation of (un)pleasantness. We therefore hypothesized that aspects of the mechanical activity induced by scanning surfaces with fingertips could be objectively associated with a pleasantness sensation. Previously, we developed a unidimensional measure of pleasantness, the Pleasant Touch Scale, quantifying the pleasantness level of 37 different materials. Findings of this study suggested that the sensation of pleasantness was influenced by the average magnitude of the frictional forces brought about by sliding the finger on the surface, and by the surface topography. In the present study, we correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness.ResultsEight blindfolded participants tactually explored twelve materials of the Pleasant Touch Scale through lateral sliding movements of their index fingertip. During exploration, the normal and tangential interaction force components, fN and fT, as well as the fingertip trajectory were measured. The effect of the frictional force on pleasantness sensation was investigated through the analysis of the ratio fT to fN, i.e. the net coefficient of kinetic friction, μ. The influence of the surface topographies was investigated through analysis of rapid fT fluctuations in the spatial frequency domain. Results showed that high values of μ were anticorrelated with pleasantness. Furthermore, surfaces associated with fluctuations of fT having higher amplitudes in the low frequency range than in the high one were judged to be less pleasant than the surfaces yielding evenly distributed amplitudes throughout the whole spatial frequency domain.ConclusionCharacteristics of the frictional force fluctuations and of the net friction taking place during scanning can reliably be correlated with the pleasantness sensation of surfaces.
Highlights
Most tactile sensations arise from contact with surfaces
Humans perceive some of its physical attributes, such as fine topography, hardness and softness through complex, nonlinear mechanics taking place during sliding and pushing
The results of this study showed that materials having an irregular surface topography or eliciting high friction during exploration, had a lower pleasantness level than materials having a more regular surface topography or materials being more slippery
Summary
Humans perceive some of its physical attributes, such as fine topography, hardness and softness through complex, nonlinear mechanics taking place during sliding and pushing. These mechanics typically result in rapid, but specific oscillations [1,2,3,4,5]. It is believed that a subtype of afferents, described as slowly conducting unmyelinated C-fibers, play a fundamental role in determining the pleasantness character of a stimulus [6,7,8,9] These fibers have been identified as C-Tactile (CT) afferents. We correlated (i) characteristics of the fluctuations of frictional forces resulting from the interaction between the finger and the surface asperities as well as (ii) the average friction with the sensation of pleasantness
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