Vicarious ratings of social touch reflect the anatomical distribution & velocity tuning of C-tactile afferents: A hedonic homunculus?

Abstract

A subclass of C-fibres, C-tactile afferents (CTs), have been discovered which respond preferentially to low force/velocity stroking touch, that is typically perceived as pleasant. Molecular genetic visualization of these low-threshold mechanosensitive C-fibres (CLTMs) in mice revealed a denser distribution in dorsal than ventral thoracic sites, scattered distal limb innervation and a complete absence from glabrous paw skin (Liu et al., 2007). Here we used third-party ratings to examine whether affective responses to social touch reflect the anatomical distribution and velocity tuning of CTs. Participants viewed and rated a sequence of video clips depicting one individual being touched by another at different skin sites and at 3 different velocities (static, 3cm/s, 30cm/s). Immediately after viewing each clip participants were asked to rate how pleasant they perceived the touch to be. Vicarious preferences matched the previously reported anatomical innervation density of rodent CLTMs, with touch on the back being rated significantly more pleasant than any other location. Furthermore, in contrast to all other skin sites, CT optimal (3cm/s) touch on the palm of the hand was not preferred to static touch, consistent with the anatomical absence of CTs in glabrous skin. Our findings demonstrate that humans recognise the specific rewarding value of CT optimal caressing touch and their preferences reflect the hypothesised anatomical distribution of CTs.

Effects of MDMA on attention to positive social cues and pleasantness of affective touch

“…In a second task, we assessed how participants responded to the mere observation of others being touched, both at CT-optimal (3cm/s) or non-optimal (fast, 30 cm/s; static, 0 cm/s) velocities. Several previous studies have reported that vicarious ratings of moving touch show the same relationship to stroking velocity as directly felt touch (Björnsdotter et al, 2011;Morrison et al, 2011;Walker et al, 2017b). In both tasks, we hypothesized that MDMA, but not MA, would increase perceived pleasantness of touch at the CT-optimal velocity.…”

The multidimensional representational space of observed socio-affective touch experiences

“…This topic has been under investigation before, but with important limitations. Most importantly, previous studies have almost exclusively focused on one type of simple touch such as brush/hand stroking of the skin ( Blakemore et al., 2005 ; Ebisch et al., 2008 , 2011 ; Morrison et al., 2011 ; Schaefer et al., 2012 ; Walker et al., 2016 ). While these studies probably underestimate the complexity of processes involved in more complex touch interactions, they already showed some very interesting results.…”

Socio-affective touch expression database

“…Over the past decade, psychologists and neuroscientists have increasingly studied the role of touch in social context, how the emotions were conveyed through touch, and its neural basis [ 4 , 7 ]. Despite of the importance and high frequency of touch communication and increasing interest in this topic, however, stimuli created for these studies poorly represent the actual phenomenon of socio-affective touch, and are mostly limited to video clips showing simple events such as a hand being stroked, slapped or brushed [ 8 – 15 ]. These studies therefore only address a very small part of "affective touch" since humans use much more complex and varied interactions to express affective intentions (both positive and negative) by touch.…”