Abstract
Human skin is innervated with different tactile afferents, which are found at varying densities over the body. We investigate how the relationships between tactile pleasantness, sensitivity and discrimination differ across the skin. Tactile pleasantness was assessed by stroking a soft brush over the skin, using five velocities (0.3, 1, 3, 10, 30 cm s−1), known to differentiate hedonic touch, and pleasantness ratings were gained. The ratings velocity-profile is known to correlate with firing in unmyelinated C-tactile (CT) afferents. Tactile sensitivity thresholds were determined using monofilament force detection and the tactile discrimination level was obtained in the direction discrimination of a moving probe; both tasks readily activate myelinated touch receptors. Perceptions were measured over five skin sites: forehead, arm, palm, thigh and shin. The assessment of tactile pleasantness over the skin resulted in a preference for the middle velocities (1–10 cm s−1), where higher ratings were gained compared to the slowest and fastest velocities. This preference in tactile pleasantness was found across all the skin sites, apart from at the palm, where no decrease in pleasantness for the faster stroking velocities was seen. We find that tactile sensitivity and discrimination vary across the skin, where the forehead and palm show increased acuity. Tactile sensitivity and discrimination levels also correlated significantly, although the tactile acuity did not relate to the perceived pleasantness of touch. Tactile pleasantness varied in a subtle way across skin sites, where the middle velocities were always rated as the most pleasant, but the ratings at hairy skin sites were more receptive to changes in stroking velocity. We postulate that although the mechanoreceptive afferent physiology may be different over the skin, the perception of pleasant touch can be interpreted using all of the available incoming somatosensory information in combination with central processing.
Highlights
The experience of touch gives rise to sensations concerning both sensory and emotional aspects (McGlone et al, 2007)
The scores from the other tactile tests were entered as covariates, but neither of these showed significant main effects with respect to the tactile pleasantness rating
The present results showed that the perception of touch varies across the skin according to tactile pleasantness, sensitivity and direction discrimination
Summary
The experience of touch gives rise to sensations concerning both sensory (e.g., tactile sensitivity and discrimination) and emotional (e.g., pleasant, painful) aspects (McGlone et al, 2007). Differences in tactile sensitivity can be related to the underlying neurophysiology of the skin. Both the sensory afferent type and density affect the level of sensitivity. The glabrous skin contains four types of myelinated mechanoreceptive afferents, namely: rapidly-adapting type 1 (Meissner), rapidlyadapting type 2 (Pacinian), slowly-adapting type 1 (Merkel), slowly-adapting type 2 (Ruffini) afferents (Vallbo and Johansson, 1984; Johnson, 2001). Hairy skin does not contain Meissner afferents, but instead contains the myelinated, rapidly-adapting hair and field mechanoreceptive afferents, and the unmyelinated Ctactile (CT) afferents (Vallbo et al, 1993, 1995, 1999). The hairy skin of the body has both fast-conducting (myelinated Aβ fiber) and slowly-conducting (unmyelinated CT fiber) tactile systems
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