Tactile Illusion Reveals Central Neural Basis for Touch Pleasantness

Although the general outlines of pleasant touch perception and C low threshold mechanoreceptors (C-LTMRs) have been sketched out, the current extent of knowledge still pales in comparison to our understanding of other neural pathways and somatosensory modalities, e.g., pain and nociceptive C-fibres.

This project explored the peripheral and central mechanisms in affective touch, through comparing gentle stroking with apparent motion – an illusory perception of movement produced by successively presented touches on the skin. We examined whether previously established velocity tuning of true lateral motion is also observed in apparent motion, when the local information provided to peripheral nerve afferents is held constant. If similar patterns were observed, then central modulation may govern the velocity dependence of the perception of pleasant touch.

To investigate this relationship, pleasantness-ratings were collected across an array of velocities (1 - 300mm/s; N=23).

Linear and quadratic regression analyses were performed on group- and individual-level. For both conditions, the addition of a quadratic term improved the overall model fit, reaching significance (p < 0.001). The quadratic term coefficients were negative for both conditions, displaying an inverted-U shape. Additionally, a multilevel mixed-effects model revealed that motion-condition did not alter the relationship between stimulus velocity and pleasantness, neither for the linear nor the quadratic velocity-term.

In conclusion, similar patterns in velocity-dependent pleasantness were seen in both apparent motion and the control condition, brushing-like motion. These findings suggest that the velocity tuning of pleasantness in apparent motion cannot be attributed to velocity tuning of individual C-LTMRs.