Pleasant touch: Behavioural and hemodynamic responses to a protocol for systematic assessment of tactile stimulation

Pleasant touch is a form of tactile stimulation mediated by tactile C afferent fibres. It involves the encoding of the emotional value associated with tactile stimulation and subserves important social functions. Although pleasant touch has gathered increased interest in recent years, no protocol has been proposed to assess it with a robust and reliable method. In the present study we adopted a rigorous protocol for evaluating the pleasantness or unpleasantness of 9 tactile (pleasant, unpleasant, or neutral) stimuli delivered on eight body areas in healthy individuals. We recorded participants' ratings on pleasantness and intensity of the stimulus, as well as their activity in the prefrontal cortex (PFC) by functional near-infrared spectroscopy (fNIRS). A questionnaire evaluated participants' subjective experience of touch in everyday life. The behavioural results confirmed the effectiveness of the protocol as the stimuli selected to evoke pleasantness were perceived as significantly more pleasant than unpleasant and neutral ones, whereas unpleasant stimuli were perceived as more intense than all other stimuli. The participants reported the palm of the hand, particularly the left one, as the most sensitive area to tactile stimulation. Judgements of pleasantness were positively correlated with subjective experience of touch in everyday life. fNIRS data showed increased activity in the prefrontal cortex particularly during stimulation with pleasant and unpleasant stimuli, consistent with behavioural findings. Overall, this study contributes to understand the processing of pleasant touch and its neural correlates, while introducing a rigorous protocol for investigating tactile stimulation. This protocol holds promise for future utilisation in both healthy and clinical populations.