Pleasant touch mediated by Aβ-afferents remains primarily dependent on tactile velocity
Pleasantness of touch is classically attributed to C-tactile (CT) afferents, based on multiple evidence showing identical inverted-U curves as a function of velocities for pleasantness and CT activity. Recent research suggests that pleasant touch is not solely mediated by CT activation, pointing to a potential contribution of A{beta}-afferents and cognitive processes, potentially through the mechanism of velocity inference. To investigate the contributions of A{beta}-afferents, our study compared the illusion of vibrotactile apparent motion, which recruits almost exclusively A{beta}-afferents, and brush stroking, which additionally recruits CT afferents. A first experiment investigated the impact of stroke contrast on the pleasantness of 3 cm/s strokes by mingling them with pleasant (1 and 10 cm/s) or less pleasant (0.5 and 30 cm/s) ones. A second experiment in virtual reality probed velocity inference from visual cue by testing how congruence between visual and tactile velocity affects perceived pleasantness. The results showed that the pleasantness of strokes targeting A{beta}-afferents also follows an inverted U-curve and that 3 cm/s reference strokes remained unaffected by the speed of mixed strokes regardless of the condition. Even though differences in incongruence were not clearly perceived, discrepant visual velocities influenced the pleasantness of vibrotactile 1 cm/s strokes, for which the pleasantness ratings followed the visual rather than the tactile velocity. Overall, vibrotactile pleasantness can be mediated by A{beta}-afferents, but most probably through velocity inference since only vibrotactile strokes were prone to visual illusion. Because not clearly detected, visual inferences could have played a stronger role through larger speed discrepancies.