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. Presently, we investigated the pleasantness and intensity of stroking touch (0.3, 1, 3, 10, 30 cm.s-1) at different forces (0.2, 0.4, 0.8, 1.2, 1.6, 2.0 N) on hairy forearm skin and compared this to responses from C-LTMRs during stroking touch at the same velocities, but over fewer forces (0.05, 0.4, 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, where tactile pleasantness and intensity appear to be a multi-faceted construct that are related to C-LTMR and A{beta}-LTMR activity, respectively, but that the firing in individual mechanoreceptor populations cannot fully account for percepts.