Stroking and tapping the skin: behavioral and electrodermal effects.

Being caressed represents one of the most typical expressions of affection conveyed by touch. Converging evidence suggests that the pleasant perception of gentle and slow stroking delivered to the hairy skin is mediated by C-Tactile afferents (CTs). While behavioral and neural responses to CT-optimal touch have been moderately explored so far, less is known about the autonomic reaction to different kinds of touch (both CT-optimal and not). Here, we investigated whether physiological arousal varies as a function of the specific tactile stimulation provided. Stroking (slow: 3 cm/s 'CT-optimal'; or fast: 30 cm/s) and tapping (random or fixed spatial order) stimulations were delivered to the participants' forearm with a brush, for durations of 9 or 60 s. Participants' skin conductance response (SCR) and level (SCL), as well as subjective evaluations, were recorded. The results revealed that being stroked (at both the velocities) induced higher SCR and SCL than being tapped. Moreover, while higher SCR was elicited by CT-suboptimal stroking compared to CT-optimal stroking, SCL was not affected differently by CT-optimal touch. No differences were found between the effects of 9 and 60 s stimulations. Slow stroking was evaluated as the most pleasant, relaxing and 'social' type of touch compared to the other tactile stimulations. Taken together, these findings shed light on the psychophysiological responses to stroking (including CT-optimal touch) and tapping, and contribute to elucidate the mechanisms underlying hedonic tactile perception.

Limb-specific autonomic dysfunction in complex regional pain syndrome modulated by wearing prism glasses.

In unilateral upper-limb complex regional pain syndrome (CRPS), the temperature of the hands is modulated by where the arms are located relative to the body midline. We hypothesized that this effect depends on the perceived location of the hands, not on their actual location, nor on their anatomical alignment. In 2 separate cross-sectional randomized experiments, 10 (6 female) unilateral CRPS patients wore prism glasses that laterally shifted the visual field by 20°. Skin temperature was measured before and after 9-minute periods in which the position of one hand was changed. Placing the affected hand on the healthy side of the body midline increased its temperature (Δ°C=+0.47 ± 0.14°C), but not if prism glasses made the hand appear to be on the body midline (Δ°C=+0.07 ± 0.06°C). Similarly, when prism glasses made the affected hand appear to be on the healthy side of the body midline, even though it was not, the affected hand warmed up (Δ°C=+0.28 ± 0.14°C). When prism glasses made the healthy hand appear to be on the affected side of the body midline, even though it was not, the healthy hand cooled down (Δ°C=-0.30 ± 0.15°C). Friedman's analysis of variance and Wilcoxon pairs tests upheld the results (P<0.01 for all). We conclude that, in CRPS, cortical mechanisms responsible for encoding the perceived location of the limbs in space modulate the temperature of the hands.