Individual stability of pleasantness ratings for stroking touch
Résumé
Many studies have investigated the perception of tactile pleasantness on the skin over a range of stroking velocities. On average, pleasantness is low at very slow (e.g. 0.3 cm/s) and very fast (e.g. 30 cm/s) velocities, but is highest at velocities between 1-10 cm/s. This results in a pleasantness ratings curve in an inverted-U shape that can be represented by a negative quadratic equation. We conducted a meta-analysis into the stability of the inverted-U shape pleasantness curve to investigate whether it can be found at the level of the individual, not just at a group level, which would make it a useful diagnostic tool. We chose five studies with similar design and a total of 127 participants. In all of these, the dorsal forearm was stroked with five different velocities ranging from 0.3 to30 cm/s and each velocity was repeated three times. Participants rated the pleasantness of the stimulation. In addition to touch velocity, other factors were varied in these studies (e.g. temperature of the stroke, stroking in the presence of an odour). For each study and condition and for each individual and condition, we calculated a quadratic regression between velocity and pleasantness ratings and determined the significance of the fit. On the group level and in each study, the rated pleasantness had a significant quadratic relation with velocity, following an inverted-U shaped curve. On the level of the individual, however, only ~ 40% of the participants showed a significant quadratic curve fit, irrespective of condition. Furthermore, the steepness of the inverted-U correlated only moderately across conditions. This has important implications for future work, where differences in the pleasantness curve cannot be used to predict or diagnose issues at an individual level.