Human brain processing of gentle naturalistic touch

International audienceThe sensation of innocuous touch to the hairy skin in humans is mediated by fast conducting myelinated (Aβ) afferents and slow conducting unmyelinated C-tactile (CT) afferents. Aβs signal discriminative properties of tactile input whereas CTs encode positive affective touch. CT afferents respond preferentially to caress-like stimulation, such as gentle brush strokes moving at 3 cm/s. Functional magnetic resonance imaging studies have shown that preferential CT fiber stimulation activates the posterior insula, among other brain areas. The timing of the activity in the insula to positive affective touch is however not known. The present study assessed brain activity evoked by gentle stroking touch via magnetoencephalography (MEG), using a custom-built, MEG-compatible brush robot providing precise and replicable brush strokes at 3 cm/s. Two conditions were performed in 15 healthy participants: first, brush strokes were delivered to the left upper arm, and second, to the left forearm. Based on previous electroencephalography work we expected that CT-related brain activity should have a long latency (>400 ms) due its slow peripheral conduction velocity. Source modeling, using dynamic statistical parametric maps, located responses in the primary somatosensory cortex, bilateral secondary somatosensory cortices and insula, Brodmann areas 6 and 8, and midcingulate cortex. Brain activity appeared within 100 ms in all these regions. The activity was sustained during the brush stimuli, but with variable time courses in different regions, which persisted after the end of stimulation. Activity in the insula peaked at 420–780 ms, but we could not confirm, nor rule out, that these long-latency peaks in the insular time series were driven by CT afferent activity. We conclude that the detailed information regarding time courses of brain activity provided by MEG provides a new window into the central processing of gentle, naturalistic tactile stimuli on the hairy skin in humans. However, further experiments are necessary to determine the relative contribution Aβ- vs. CT-afferent input to the brain