Ultra-late EEG potential evoked by preferential activation of unmyelinated tactile afferents in human hairy skin.

Human tactile sensibility in hairy skin is mediated not only by fast conducting myelinated (Aβ) afferents, but also by a system of slow conducting, unmyelinated afferents that respond preferentially to light touch, C-tactile (CT) afferents. This system has previously been shown to correlate with the pleasantness of tactile stimuli, where a soft brush moving at 1-3cm/s activates CT afferents strongly. Functional magnetic resonance imaging (fMRI) studies have shown that preferential CT fiber stimulation activates the posterior insula cortex. The present study aims to assess brain activity evoked by the activation of CT afferents using electroencephalography (EEG). We present evidence for a late cortical potential over frontal electrodes, evoked from slow, gentle brush strokes at 3cm/s. We relate this to the CT afferent input based on the conduction velocity of the CT fibers and the force feedback from the brush; the potential started 0.7s after the brush contacted the skin and continued throughout the brush stimulation. Furthermore, results from brushing at lower and higher speeds showed that the CT potential was modulated by this stimulation. We conclude that the late potential is consistent with activity in a frontal cortical network following hairy skin peripheral stimulation. This provides an important tool for further studies of the CT fiber system and for clinical examination of peripheral unmyelinated afferents.