Tactile illusion reveals central neural basis for touch pleasantness

C-low threshold mechanoreceptors (C-LTMRs) have been highlighted in mediating the pleasantness of slow, caressing touch. This has given rise to the “Affective Touch Hypothesis,” which posits that velocity tuning of C-LTMRs underpins pleasantness perception. The known activation preferences of C-LTMRs have been used as a proxy for pleasant touch, yet recent findings have raised questions about the necessity of this peripheral mechanism.

This project explored the peripheral and central mechanisms in affective touch through comparing two motion-conditions: gentle brushing-like motion and apparent motion, an illusory perception of movement produced by successive touches along the skin. We used this illusion to examine whether previously established velocity tuning of true lateral motion is observed in apparent motion, when local information provided to individual peripheral afferents is held constant.

The sole dependence on peripheral modulation predicts that the characteristic inverted-U-shaped relationship between velocity and pleasantness, regularly associated with C-LTMRs, would only be observed for brushing-like motion. Central modulation would instead predict a more similar relationship between the motion-conditions. To investigate this, pleasantness-ratings and velocity-ratings were collected across different velocities (0.1-30cm/s, N=23) for both conditions.

Linear and quadratic regression analyses were performed and for both conditions adding 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. Further analyses revealed that motion-condition did not significantly alter the relationship between stimulus-velocity and pleasantness. These findings suggest that the velocity tuning of pleasantness cannot solely be attributed to velocity tuning of individual C-LTMRs.

Key points

• C-low threshold mechanoreceptors (C-LTMRs) have been shown to display a unique inverted-U-shaped relationship between firing frequencies and stroking velocity, which is not seen with any other cutaneous afferents

• An inverted-U-shaped relationship is also observed between perceived touch pleasantness and stroking velocity, giving rise to a hypothesis which posits that the velocity tuning of C-LTMRs underpins the perception of pleasant touch

• We compared traditional stroking (brushing-like motion) to an illusory perception of movement produced by successive touches along the skin (apparent motion) to examine if similar velocity-dependent pleasantness patterns would be observed when information provided to individual peripheral afferents is held constant

• In our experiment, the type of motion did not significantly alter the relationship between perceived pleasantness and stimulus-velocity

• These findings suggest that the velocity tuning of pleasantness cannot solely be attributed to velocity tuning of individual C-LTMRs