The influence of force on the encoding and perception of affective touch

Stroking touch has been shown to be most pleasant at intermediate velocities of 1-10 cm s^-1, which relates well to the activity of C-low-threshold mechanoreceptors (C-LTMRs), also called C-tactile afferents in humans, that have been implicated in encoding positive affective touch. This well-established finding has been demonstrated at gentle stroking forces (typically, 0.4 N peak normal force), yet few studies have investigated the effect of force on the perception of stroking touch or on the activity of C-LTMRs. We investigated the pleasantness and intensity of stroking touch (0.3, 1, 3, 10 and 30 cm s^-1) at different forces (0.2, 0.4, 0.8, 1.2, 1.6 and 2.0 N) on hairy forearm skin and compared this with responses from C-LTMRs during stroking touch at the same velocities, but over fewer forces (0.05, 0.4 and 1.5 N). We found significant effects of stroking velocity and force for both tactile pleasantness and intensity ratings. Pleasantness showed the typical inverted U-shaped relationship over stroking velocities, but this was modified by force; higher forces significantly decreased pleasantness at faster velocities. Conversely, tactile intensity increased linearly with both increasing velocity and force. Recordings from seven C-LTMRs showed that their firing frequency changed with stroking velocity, but that they were strongly modulated by force. Overall, we demonstrate the profound effect that force has on the perception of stroking touch, whereby tactile pleasantness and intensity appear to be a multi-faceted construct that is related to C-LTMR and Aβ-LTMR activity, respectively, but that the firing in individual mechanoreceptor populations cannot account fully for percepts. KEY POINTS: The skin is highly sensitive to small mechanical deformations, including dynamic touch and different forces. We obtained perceptual ratings for touch pleasantness and intensity over different stroking velocities and forces applied to the forearm. This was also compared with microneurography recordings from C-low-threshold mechanoreceptors (C-LTMRs). Building on previous work, we find that it is not only stroking velocity that affects pleasantness and intensity perception, but also stroking force. Pleasantness ratings decreased more at higher forces when the stroking was faster. C-LTMR firing showed that differences in force were readily encoded, although the same force-velocity interaction was not seen. This work expands the literature on affective touch to encompass the effects of force, which clearly modulates pleasantness. C-LTMR force and velocity information is likely to combine with information from all types of mechanoreceptors, including those from the fascia, to produce the resulting percept.