![]() The OFC is associated with assigning reward to hedonic experience (Kringelbach 2005). Pleasant touch is processed by a network that involves the orbitofrontal cortex (OFC) (Francis et al. Thus, the CT fibers are proposed to have a specialized role for the processing of hedonic, emotional, and plausibly social aspects of touch (Ellingsen 2010). Activation is triggered by soft touch with a stroking velocity range of 1–10 cms −1 with a peak at 3 cms −1, which is congruent with subjective touch pleasantness ratings showing an inverted-U pattern (Löken et al. CT fibers are activated by low-threshold unmyelinated mechanoreceptors that are only present in hairy skin, and project to the posterior insula, a region involved in processing of emotion and internal bodily signals (Craig 2002). The arousal reducing effects of touch may be caused by activating a specific type of afferents, the C-tactile afferents (CT fibers) that have been proposed to play a unique role in processing the hedonic valence of touch (Olausson et al. For example, skin-to-skin innocuous touch has been found to evoke decelerations in heart rate in humans (Gray et al. Although studies investigating the effect of innocuous touch on pupil size are scarce, other psychophysical measures of arousal have been investigated in relation to this type of touch. 2008), and electrical stimulation (Chapman et al. 2005), algometer pressure stimulation (Ellermeier and Westphal 1995 Höfle et al. These studies have shown that the pupil dilates after various forms of painful stimulation, such as cold pressure stimulation (Tassorelli et al. The relation between pupil size and touch has been predominantly described in pain studies. 2006), one would expect to see a reflection of these effects in the pupil. Given that interpersonal touch is able to evoke strong emotions in the touched person depending on the identity of the toucher and the interpersonal relationship (Hertenstein et al. Although the emotional and psychological effects of touch are well established (Gallace and Spence 2010), it is currently unknown how pupil size responds to innocuous touch. One domain that is currently underexplored is somatosensory processing. Pupil size is generally seen as a measure of arousal and attention and is, therefore, frequently used to index perception, language processing, memory, decision making, emotion, and even cognitive development (Mathôt and Van der Stigchel 2015 Sirois and Brisson 2014). These results suggest that pupil-size dilation does respond to tactile input, but that this response is related to arousal caused by changes in stimulus intensity (e.g., stroking velocity) rather than specific C-tactile stimulation. In addition, skin conductance recordings showed a pattern of response to different stroking velocities similar to pupil dilation. ![]() Furthermore, the palm of the hand was used as a control site for tactile stimulation, for which similar findings were obtained as for the dorsal side of the hand. This relation was confirmed in a second experiment. Results, however, revealed no specific pupil dilation for the 3 cms −1 condition instead, a logarithmic relation was found between pupil-size dilation and stroking velocity. Results revealed that stroking velocity had a significant effect on TPT-item scores, showing higher that affective touch was rated as more pleasant compared to non-affective touch, thereby replicating the previous studies. In Experiment 1, affective (3 cms −1) and non-affective (0.3 and 30 cms −1) stroking was applied to the dorsal side of the right hand. In addition, an adjusted version of the Touch Perception Task (TPT) was used to assess subjective touch pleasantness ratings. Pupil-size data were obtained to investigate differences between stroking conditions. The current study investigated in two experiments how pupil size reacts to both affective and non-affective stroking. However, it is unclear whether pupil dilation can be modulated by hedonic touch. For other sensory modalities, hedonic processing has been associated with pupil dilation. This system been proposed to play a unique role in hedonic valence and emotion of touch. An important system for interpersonal touch is the C-tactile (CT) system, which is activated by a soft stroke on hairy skin with a velocity of 1–10 cms −1. Interpersonal touch is known to influence human communication and emotion.
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