Aβ-induced inference of velocity contributes to pleasant touch in interaction with visual cues

The pleasantness of touch is commonly associated with C-tactile (CT) afferents, as evidenced by multiple studies showing similar inverted-U curves for pleasantness and CT activity as a function of velocity. Recent research suggests a contribution of Aβ-afferents and cognitive processes, potentially via velocity inference. To investigate the contribution of Aβ-afferents, our study compared vibrotactile apparent motion, which recruits almost exclusively Aβ-afferents, with brush stroking, which additionally recruits CT afferents and provides richer tactile cues. The first experiment investigated whether stroke contrast affects the pleasantness of 3 cm/s strokes by interleaving them with either pleasant (1 and 10 cm/s) or less pleasant (0.5 and 30 cm/s) ones. A second experiment, conducted in a virtual reality setting, examined the impact of visual velocity modulation on perceived pleasantness by testing the effect of congruence between visual and tactile velocities. The results showed that the pleasantness of strokes targeting Aβ-afferents followed an inverted U-curve, and that 3 cm/s reference strokes remained unaffected by the speed of neighbouring strokes regardless of the type of stimulation. Discrepant visual velocities influenced the pleasantness only for 1 cm/s vibrotactile apparent strokes, yielding increased pleasantness ratings for faster velocities. Overall, sequential vibrotactile stimulation can convey pleasantness but seems to be more influenced by multisensory inference, as only vibrotactile apparent strokes were prone to visual modulation. Moreover, since none of the tested discrepancies were clearly detected, larger discrepancies could be used in future studies to further probe their impact on pleasantness inference. KEY POINTS: Pleasantness of touch is commonly associated with C-tactile (CT) afferents, given the strong correlation observed between CT firing rates and perceived ratings. However, there is growing evidence on the contribution of Aβ-afferents to perceived pleasantness, and the cognitive processes underlying the inference of stroking velocity remain to be elucidated. Vibrotactile apparent motions targeting Aβ-afferents were directly compared to classical brush strokes, which activate both Aβ- and CT-afferents, in two experiments: one purely tactile and one combining tactile strokes with congruent or incongruent strokes made by a brush in virtual reality. Vibrotactile apparent strokes elicit an inverted U-shaped curve of pleasantness ratings, confirming that pleasant touch is not exclusively dependent on CT afferents. The pleasantness of 3 cm/s strokes remains stable and robust, irrespective of pleasant or unpleasant accompanying stimuli. Only the pleasantness of slow apparent vibrotactile strokes is affected by the discrepant visual velocity, showing increased susceptibility to visual illusions.