Physical Factors Influencing Pleasant Touch during Passive Fingertip Stimulation

OBJECTIVE: Tactile explorations with the fingertips provide information regarding the physical properties of surfaces and their relative pleasantness. Previously, we performed an investigation in the active touch domain and linked several surface properties (i.e. frictional force fluctuations and net friction) with their pleasantness levels. The aim of the present study was to investigate physical factors being important for pleasantness perception during passive fingertip stimulation. Specifically we were interested to see whether factors, such as surfaces' topographies or their frictional characteristics could influence pleasantness. Furthermore, we ascertained how the stimulus pleasantness level was impacted by (i) the normal force of stimulus application (F(N)) and (ii) the stimulus temperature (T(S)). METHODS AND RESULTS: The right index fingertips of 22 blindfolded participants were stimulated using 27 different stimuli, which varied in average roughness (Ra) and T(S). A 4-axis robot moved the stimuli horizontally under participants' fingertips with three levels of F(N). The robot was equipped with force sensors, which recorded the F(N) and friction force (F(T)) during stimulation. Participants rated each stimulus according to a three-level pleasantness scale, as very pleasant (scored 0), pleasant (scored 1), or unpleasant (scored 2). These ordinal pleasantness ratings were logarithmically transformed into linear and unidimensional pleasantness measures with the Rasch model. Statistical analyses were conducted to investigate a possible link between the stimulus properties (i.e. Ra, F(N), F(T), and T(S)) and their respective pleasantness levels. Only the mean Ra and F(T) values were negatively correlated with pleasantness. No significant correlation was detected between F(N) or T(S) and pleasantness. CONCLUSION: Pleasantness perception, resulting from passive fingertip stimulation, seems to be influenced by the surfaces' average roughness levels and average F(T) occurring during fingertip stimulation.