Modeling affective touch_Suppl_Analysis.pdf
Creators: name: Chancel, Marie
name: Crucianelli, Laura
Publisher: name: figshare
Year: 2023
Subjects: classificationCode: 520401
subject: Cognition
classificationCode: 520406
subject: Sensory processes, perception and performance

description: Touch is perceived as most pleasant when delivered at velocities known to optimally activate the C-Tactile afferent system. At the group level, pleasantness ratings of touch delivered at velocities in the range between 0.3 and 30 cm/s follow an inverted-U shape curve, with maximum pleasantness between 1 and 10 cm/s. However, the prevalence, reliability, and stability of this function at the individual level and across skin types based on hair density remains unknown.In a previously published work (Crucianelli, Chancel &amp; Ehrsson, 2022), we tested a range of seven velocities (0.3, 1, 3, 6, 9, 18, 27 cm/s) delivered with a soft brush, on both hairy (forearm and dorsal hand) and non-hairy skin (palm) in 123 participants. The results presented in this paper suggested that the relationship between pleasantness and velocity of touch is significantly best described by a negative quadratic model at the individual level in most participants both on hairy (67.1%) and non-hairy (62.6%) skin, a larger extent than previously reported. Higher interoceptive accuracy and self-reported depression were related to a better fit of the quadratic model and to the steepness of the curve, respectively. The prevalence of the quadratic model at the individual level was stable across body sites (62.6%, Experiment 1), across two experimental sessions (73-78%, Experiment 2), and regardless of the number of repetitions of each velocity (Experiment 3). Thus, the individual perception of tactile pleasantness appeared to follow a characteristic velocity-dependent function across skin types and shows trait characteristics.In affective touch literature, in the fitting procedure to capture the relationship between touch velocity and perceived velocity, it is custom to consider the tested velocity steps as evenly spaced, either by using velocities as a categorical factor or by choosing velocities that are evenly spaced in a logarithmic scale (Löken et al., 2009; Ackerley et al., 2014). In our study, we used the first approach as explained in the article (the full details of our procedure can be found in Crucianelli, Chancel &amp; Ehrsson, 2022). After discussing with other experts in the field, we thought it could be valuable for the scientific community also to present the results of our analyses obtained by following the latter approach. Thus, in the present open-access archive document, we present the results of the individual level fit analyses related to the seven velocities using log-transformed of these velocities instead of seven categorical levels (Table 1). Please note that the methods and analysis strategy are identical to the ones described in Crucianelli, Chancel &amp; Ehrsson, 2022 except for the way velocities are used in the individual fitting procedure.Furthermore, since this is a post-hoc analysis, the velocities we initially tested in the experiment are not evenly spaced in the log-scale. This limitation made us expect slight changes from the original results and must be kept in mind when comparing the present results to other work. Moreover, we hope that this supplementary analysis will be considered as an effort towards full transparency and data sharing best practice among scientists, not as an erratum of our initial study that we fully stand by. Our aim is simply to provide additional data analyses that could be useful for future studies investigating the relationship between velocity of touch and perceived pleasantness and to provide novel insight into the difference between using touch velocity as a logarithmic or as a categorical variable.