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 afferent A and CT fiber input in the spinal cord, or mechanoreceptive A-fiber contributions to computations of touch pleasantness in the brain. However, contribution of mechanoreceptive 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 mechanoreceptive 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 afferent A-fibers.

Significance Statement

In the current study we performed two types of peripheral nerve blocks in healthy adults to temporarily eliminate the contribution of A-fiber afferents to touch perception. We show that when afferent A-fiber function is greatly diminished, the perceived intensity and pleasantness of gentle stroking are nearly abolished. These findings demonstrate for the first time that explicit perception of the pleasantness of C-tactile (CT)-targeted touch critically depends upon A-fiber afferents. In addition, we show the same outcome for deep pressure (similar to hugs and massage), another form of social-affective touch we have previously validated in the lab. Together these findings demonstrate that social touch is not conveyed solely by the CT pathway.