Aβ-CT affective touch: Touch pleasantness ratings for gentle stroking and deep pressure exhibit dependence on A-fibers

Abstract

Gentle stroking of the skin is a common social touch behavior with positive affective consequences. A preference for slow versus fast stroking of hairy skin has been closely linked to the firing of unmyelinated C-tactile (CT) somatosensory afferents. Because the firing of CT afferents strongly correlates with touch pleasantness, the CT pathway has been considered a social-affective sensory pathway. Recently, ablation of the spinothalamic pathway- thought to convey all C-fiber sensations- in patients with cancer pain impaired pain, temperature, and itch, but not ratings of pleasant touch. This suggested integration of A and CT fiber input in the spinal cord, or A-fiber contributions to computations of touch pleasantness in the brain. However, the causal contribution of A-fibers to touch pleasantness- in humans without pain- remains unknown. In the current, single-blinded study we performed two types of peripheral nerve blocks in healthy adults to temporarily eliminate the contribution of A-fibers to touch perception. Our findings show that when A-fiber function is greatly diminished, the perceived intensity and pleasantness of both gentle stroking and deep pressure are nearly abolished. These findings demonstrate that explicit perception of the pleasantness of CT-targeted brushing and pressure both critically depend on A-fibers.

Chronic pain mediated changes in the appetitive value of affective gentle touch in mice

“…This indicates that plasticity in skin-to-brain circuits for affective touch produces a hedonic gradient ranging from appetitive to neutral to aversive. This is especially salient considering recent preclinical and clinical findings that have shown that pathways once thought of as dedicated labelled lines for "gentle touch" are not as strict in their coding 18,22 . Redundancies in pathways for a sensation as critical as pleasurable touch and plasticity in each neuron's ability to transmit broad modalities of cutaneous information is likely to be evolutionarily beneficial and indicates a high degree of combinatorial coding.…”

“…10,16,17 Classic microneurography studies in humans had identified C-tactile afferents as key players in the transmission of pleasant slow stroking touch. 10,[18][19][20] A comparative class of low threshold C-fibers has also been identified in rodents. These afferents can be distinguished through their expression of a G-protein coupled receptor, MrgprB4.…”

Characterizing affiliative touch in humans and its role in advancing haptic design

“…CT receptors may also be only one of a putative family of receptors, each sensitive to distinct qualities of affiliative social touch 21,30 . There is also growing evidence that myelinated A fibers contribute to experienced pleasantness of soft stroking and pressure, 29,31,32 although the precise mechanisms by which this occurs—and whether it is a function of centrally modulated associative processes—is still not known 33 …”