Affective Touch Dimensions: From Sensitivity to Metacognition

Abstract Touch can give rise to different sensations including sensory, emotional and social aspects. Tactile pleasure typically associated with caress-like skin stroking of slow velocities (1-10 cm/s) has been hypothesised to relate to an unmyelinated, slow-conducting C-tactile afferent system (CT system), developed to distinguish affective touch from the ‘‘noise’’ of other tactile information on hairy skin (the so-called ‘social touch hypothesis’). However, to date, there is no psychometric examination of the discriminative and metacognitive processes that contribute to accurate awareness of pleasant touch stimuli. Over two studies (total N= 194), we combined for the first time CT stimulation with signal detection theory and metacognitive measurements to assess the social touch hypothesis on the role of the CT system in affective touch discrimination. Participants’ ability to accurately discriminate pleasantness of tactile stimuli of different velocities, as well as their response bias, was assessed using a force-choice task (high versus low pleasantness response) on two different skin sites: forearm (CT-skin) and palm (non-CT skin). We also examined whether such detection accuracy was related to the confidence in their decision (metacognitive sensitivity). Consistently with the social touch hypothesis, we found higher sensitivity d’ on the forearm versus the palm, indicating that people are better at discriminating between stimuli of high and low tactile pleasantness on a skin site that contains CT afferents. Strikingly, we also found more negative response bias on the forearm versus the palm, indicating a tendency to experience all stimuli on CT-skin as ‘high-pleasant’, with such effects depending on order, likely to be explained by prior touch exposure. Finally, we found that people have greater confidence in their ability to discriminate between affective touch stimuli on CT innervated skin than on non-CT skin, possibly relating to the domain specificity of CT touch hence suggesting a domain-specific, metacognitive hypothesis that can be explored in future studies as an extension of the social touch hypothesis. Highlights Touch mediated by C-tactile (CT) afferents on hairy skin elicits pleasant sensations We combine for the first time CT stimulation with signal detection theory Better accuracy to detect pleasantness of tactile stimuli at CT optimal speeds on CT skin Higher confidence in ability to accurately distinguish affective touch on CT skin