Pleasantness of touch in human glabrous and hairy skin: order effects on affective ratings

The tactile sense comprises pathways for both discriminative and affective touch. Low threshold unmyelinated mechanoafferents (C tactile, CT) in the human hairy skin have recently been linked to pleasant touch sensation. Here, we investigated how perception of the hedonic aspect of tactile stimulation differs between the hairy skin of the arm, and the glabrous skin of the palm, which is not innervated by CT afferents. Three groups of naïve, healthy subjects (total n=28) rated pleasantness on a visual analogue scale (VAS) when we stroked with a soft brush with speeds from 0.1 to 30cm/s on the palm or forearm. We used two different experimental approaches: in experiments 1 and 2, stimuli were delivered successively on the palm and arm (or arm and palm) in temporally separate sequential blocks. In experiment 3, stimuli were delivered alternately on arm and palm. We found that the order of stimulus presentation, palm/arm or arm/palm, has an effect on pleasantness ratings of gentle brush stroking with varying velocity. Notably, the perception of pleasantness for palm stimulation was affected by previous stimulation of the arm, but not vice versa. Thus, assessment of valence of touch may be influenced by affective reactions elicited by activation of the CT afferent pathway.