Gentle stroking elicits somatosensory ERP that differentiates between hairy and glabrous skin.

Here we asked whether, similar to visual and auditory event-related potentials (ERPs), somatosensory ERPs reflect affect. Participants were stroked on hairy or glabrous skin at five stroking velocities (0.5, 1, 3, 10 and 20 cm/s). For stroking of hairy skin, pleasantness ratings related to velocity in an inverted u-shaped manner. ERPs showed a negativity at 400 ms following touch onset over somatosensory cortex contra-lateral to the stimulation site. This negativity, referred to as sN400, was larger for intermediate than for faster and slower velocities and positively predicted pleasantness ratings. For stroking of glabrous skin, pleasantness showed again an inverted u-shaped relation with velocity and, additionally, increased linearly with faster stroking. The sN400 revealed no quadratic effect and instead was larger for faster velocities. Its amplitude failed to significantly predict pleasantness. In sum, as was reported for other senses, a touch's affective value modulates the somatosensory ERP. Notably, however, this ERP and associated subjective pleasantness dissociate between hairy and glabrous skin underscoring functional differences between the skin with which we typically receive touch and the skin with which we typically reach out to touch.

The human "feel" of touch contributes to its perceived pleasantness.

This study explored whether a human-like feel of touch biases perceived pleasantness and whether such a bias depends on top-down cognitive and/or bottom-up sensory processes. In 2 experiments, 11 materials were stroked across the forearm at different velocities (bottom-up) and participants rated tactile pleasantness and humanness. Additionally, in Experiment 1, participants identified the materials (top-down), whereas in Experiment 2, they rated each material with respect to its somatosensory properties (bottom-up). Stroking felt most pleasant at velocities optimal for the stimulation of CT-afferents, a mechanosensory nerve hypothesized to underpin affective touch. A corresponding effect on perceived humanness was significant in Experiment 1 and marginal in Experiment 2. Whereas material identification was unrelated to both pleasantness and humanness, we observed a robust relation with the somatosensory properties. Materials perceived as smooth, slippery, and soft were also pleasant. A corresponding effect on perceived humanness was significant for the first somatosensory property only. Humanness positively predicted pleasantness and neither top-down nor bottom-up factors altered this relationship. Thus, perceiving gentle touch as human appears to promote pleasure possibly because this serves to reinforce interpersonal contact as a means for creating and maintaining social bonds. (PsycINFO Database Record (c) 2020 APA, all rights reserved).