CT-Optimal Stimulation Modulates Somatosensory Processing.

Touch has an affective dimension, conveyed through low-threshold mechanoreceptors known as C-tactile (CT) afferents, which are activated by gentle, caress-like contact. While there is evidence that these fibers modulate nociceptive input, their influence on the processing of other somatosensory afferent activity remains largely unknown. In this study, we explored how slow brushing (CT-optimal stimulation) modulates somatosensory evoked potentials (SEPs) elicited by electrical stimulation of the median nerve (occurring at 0.7 to 3.7 s after stimulus onset), compared to vibration (at 200 Hz) and no touch, in 30 healthy participants. CT-targeted stimulation was delivered using a robotic arm developed in-house equipped with a cosmetic brush, which applied slow brushing movements at CT-optimal speeds (~3 cm/s) over the dorsal forearm. Vibrotactile stimulation, targeting A-beta fibers, was delivered using vibration motors adjacent to the brushed area, with intensity calibrated to match the perceived strength of brushing. SEPs were recorded under these three conditions. Our results showed no differences between slow brushing, vibration, and no touch conditions in the amplitude of early SEPs recorded over the somatosensory cortex (N20, P25, N30, and P45), which may indicate that CT stimulation does not affect early cortical processing of somatosensory information. However, a reduced frontocentral P150 SEP component was observed during slow brushing compared to the other conditions. This finding suggests that CT stimulation may reduce somatosensory input transmitted through the lemniscal system, possibly engaging brain areas involved in attentional and cognitive processing.