Special nerves for pleasant social touch.

Neural correlates of pleasure have been studied mainly in our central nervous systems (brain and spinal cord). Löken et al., look at activity in our peripheral nervous system, specifically a small class of axons without myelin wrapping that send pressure information from skin to the central nervous system. They demonstrate a relationship between positive hedonic sensation and coding at the level of these peripheral afferent nerves, suggesting that C-tactile fibers contribute critically to pleasant touch. Soft brush stroking on hairy skin was perceived as most pleasant when it was delivered at velocities that were most effective at activating C-tactile afferents (1–10 cm s-1), with a linear correlation between C-tactile impulse frequency and pleasantness ratings. Here is the abstract:

Pleasant touch sensations may begin with neural coding in the periphery by specific afferents. We found that during soft brush stroking, low-threshold unmyelinated mechanoreceptors (C-tactile), but not myelinated afferents, responded most vigorously at intermediate brushing velocities (1-10 cm s^-1), which were perceived by subjects as being the most pleasant. Our results indicate that C-tactile afferents constitute a privileged peripheral pathway for pleasant tactile stimulation that is likely to signal affiliative social body contact.