The Perception and Evaluation of Pleasant Touch and the Development of "Satiety" for Hedonic Tactile Experiences

Sammanfattning: Pleasant touch is a hedonic experience and one of the most powerful means for social bonding and for communicating affection. Touch similar to a human caress is detected by the “C tactile (CT) afferents”, a class of mechanoreceptors which respond preferentially to stroking velocities between 1 and 10 cm/s. Study I investigated whether brush strokes manually delivered or delivered by a mechanical source are comparable in terms of perceived pleasantness, and whether hedonic touch is influenced by the awareness of the source of the stimulation. A within-subjects design was administered to the participants, who received brush strokes on the forearm either handheld or delivered by a robot, with three different velocities (0.3 cm/s, 3 cm/s and 30 cm/s). In two conditions, the participants were aware about the source of the stimulation, while in the other two conditions they were not. The participants rated pleasantness and intensity. Pleasantness ratings were similar regardless the awareness of the source. The CT optimized stroking velocity (3 cm/s) was perceived as the most pleasant. Intensity ratings for the two slower velocities only (0.3 cm/s and 3 cm/s) were higher in the handheld condition. These findings highlight the high comparability of human and robot brush stroking, as well as the absent influence of the awareness of the source on the pleasantness evaluation. Moreover, they validate the method of administering tactile stimulation to be used in Study II. Study II investigated whether the perceived tactile pleasantness changes with repeated exposure, leading to “satiety”, and whether this varies with different velocities. The reward components of “liking” (pleasantness) and “wanting” (wish to be further exposed to the same stimulus) for touch were investigated. Brush strokes were delivered to the forearm for about 50 minutes and the participants rated liking and wanting. In experiment 1 (within-subjects), three different velocities (0.3 cm/s, 3 cm/s and 30 cm/s) were administered to the participants. In experiment 2 (between-subjects), the participants received brush strokes at only one velocity (either 3 cm/s or 30 cm/s). Experiment 1 showed a slight decrease in both liking and wanting ratings for the CT optimized stroking velocity (3 cm/s) only, but the stimulation didn´t become unpleasant. Experiment 2 showed a decrease in liking and wanting for both velocities, with a steeper decrease for stroking at 3 cm/s. These findings indicate that “satiety” for touch occurs particularly for the caress-like CT optimized stroking velocity; however it takes time (more than 50 minutes). In conclusion, the two studies demonstrate that the experienced pleasantness of touch is robust against the knowledge of the source of the tactile stimulation and that its rewarding value persists for a rather long time. Such findings may have implications in the field of social interactions and in circumstances where touch is applied for a long time, as touch therapies and massages. Moreover, the two studies provide a contribution to the investigation of how bottom up processes (conveyed by the CT afferents) and top down mechanisms are related to each other in the evaluation of pleasant tactile stimuli.