title: Physical factors influencing pleasant touch during passive fingertip stimulation.
abstract: 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 (FN) and (ii) the stimulus temperature (TS).
  
  
  RESULTS
  The right index fingertips of 22 blindfolded participants were stimulated using 27 different stimuli, which varied in average roughness (Ra) and TS. A 4-axis robot moved the stimuli horizontally under participants' fingertips with three levels of FN. The robot was equipped with force sensors, which recorded the FN and friction force (FT) 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, FN, FT, and TS) and their respective pleasantness levels. Only the mean Ra and FT values were negatively correlated with pleasantness. No significant correlation was detected between FN or TS and pleasantness.
  
  
  CONCLUSIONS
  Pleasantness perception, resulting from passive fingertip stimulation, seems to be influenced by the surfaces' average roughness levels and average FT occurring during fingertip stimulation.
authors.affiliation: Institute of Neuroscience, Université catholique de Louvain, Brussels, Belgium.
authors.forename: Anne
authors.initials: A
authors.lastname: Klöcker
authors.affiliation: The BioRobotics Institute, Scuola Superiore Sant'Anna, Pisa, Italy.
authors.forename: Calogero Maria
authors.initials: CM
authors.lastname: Oddo
authors.affiliation: The BioRobotics Institute, Scuola Superiore Sant'Anna, Pisa, Italy.
authors.forename: Domenico
authors.initials: D
authors.lastname: Camboni
authors.affiliation: Institute of Neuroscience, Université catholique de Louvain, Brussels, Belgium.
authors.forename: Massimo
authors.initials: M
authors.lastname: Penta
authors.affiliation: Institute of Neuroscience, Université catholique de Louvain, Brussels, Belgium; Cliniques Universitaires Saint-Luc, Physical and Rehabilitation Medicine Department, Université catholique de Louvain, Brussels, Belgium.
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Thonnard
mesh_terms: D000328:Adult; D005260:Female; D005385:Fingers / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D055585:Physical Phenomena; D010812:Physical Stimulation; D057181:Pleasure / Q000502:physiology*; D012044:Regression Analysis; D013499:Surface Properties; D013696:Temperature; D055698:Touch Perception; D055815:Young Adult