Touch perceptions across skin sites: differences between sensitivity, direction discrimination and pleasantness

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.

Keywords: C-tactile; affective touch; body map; human; psychophysics; sensory afferent; somatosensory; tactile.

Figure 1

Diagrammatic representations of the stroking stimuli and the tactile direction discrimination probe. (A) The rotary tactile stimulator, where a soft brush was stroked at precise velocities across each skin site and psychophysical ratings of pleasantness were gained. (B) The probe used for the tactile direction discrimination task, which was moved across the skin over specified distances.

Figure 2

The mean scores for pleasantness ratings at different stroking velocities over the skin sites. A significant effect of stroking velocity overall the skin sites. The slowest (0.3 cm s⁻¹) and fastest (30 cm s⁻¹) were rated as less pleasant compared to the middle velocities (1–10 cm s⁻¹) for the (A) forehead, (B) arm, (C) palm, (D) thigh, and (E) shin. The ratings scale was from −10 to +10. Error bars correspond to ±S.E.M.

Figure 3

The effect of skin site for pleasantness ratings at higher stroking velocities. There was a significant main effect of skin site at (A) 10 cm s⁻¹ and (B) 30 cm s⁻¹ stroking velocities; for comparison of different skin sites at these separate velocities, the other skin sites were compared to the palm (* indicates significant differences, p < 0.05 to the palm ratings). The ratings scale was from −10 to +10. The upper and lower edges of the boxes relate to the ± 1 S.E.M. around the mean, and the lines are the upper and lower bound 95% confidence intervals.

Figure 4

The median values for and tactile sensitivity and discrimination over the skin. Medians are shown for (A) force indentation threshold using monofilaments, and (B) tactile direction discrimination level for each of the five skin sites from the main experiment. Significant effects of skin site were found for both tests; see Results for individual differences between sites. Error bars indicate inter-quartile confidence intervals for the ordinal data; note that there are no inter-quartile confidence bars for the results in Figure 4A forehead, arm and palm and in Figure 4B forehead, due to there being no variation in the ranked middle 50% of ordinal scores.