Physical Factors Influencing Pleasant Touch during Tactile Exploration

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.

Specification of Individuality in Causal Relationships among Texture-related Attributes, Emotions, and Preferences

“…All three groups tested in this study included the same types of nodes in the bottom-or psychophysical-layer, indicating that the variance across people at this basic sensory level is minimal. Nonetheless, as suggested in earlier studies [1,16,22], individuality should not be neglected even in this layer. Compared with the bottom layer, the contents of the middle and higher layers varied significantly between groups A and B.…”

“…2, with example values. For participant k, the causality rating from adjective dyad i to another adjective dyad j was defined as (1) and was assigned a decimal value from 0 to 5 according to the participant's answer. To acquire the direct effect matrix Z (k) , the matrix X (k) of x ij (k) was normalized for each participant by the formula…”

“…In order to effectively design surface textures of products, it is crucial to understand the causal relationships among these different types of texture-related experiences. Especially because emotional and personal experiences significantly influence the value of a product, many studies have investigated experiences of pleasantness, comfort, and preference for product design purposes [1][2][3][4][5].…”

A Brief Overview of the Human Somatosensory System

“…This was also documented when fingers are dry since dry skin is more slippery [2]. As another example, recent studies in hedonic touch have established a link between the sensation of pleasantness and the skin's tribological properties that in turn influence the physics of contact [47].…”

Experimental studies of the quality of embossed characters of the Braille alphabet

“…Over the last years, as new methods of making convex printouts were introduced, the blind started to pay more attention to an important feature of convex writing, namely the roughness of the surface that is to be touched. The issue of tactile sensing is quite individual and depends on the distribution of receptors over the finger [14], and to determine whether something is pleasant while touched is very subjective, although a defined scale is proposed [15]. Surface roughness is not the most important factor in deciding whether a text is readable, but the blind are often afraid that surfaces are too rough and that it can cause hardening of their finger tips decreasing their tactile sensitivity.…”