Surface roughness and moistening of the finger affect friction and detection threshold for a single microdot

Abstract

Abstract With sliding contact humans are able to perceive tactile features at the micron scale, such as a single dot raised only few microns when placed on a smooth surface. Frictional effects are important in determining the tactile cues available in sliding and depend on a variety of factors. In this study, we investigated how detection sensitivity to a single micro dot is affected by surface roughness and moistening of the index finger. These manipulations were chosen to alter the skin-surface interaction and the resulting forces acting on the skin. We found that detection threshold was 6-fold higher for the rough surfaces when compared to smooth surfaces. Moistening the finger with water or water and soap reduced the friction as well as the magnitude of tangential force variations when compared to the dry finger, regardless of the surface geometry. However, detection sensitivity improved for the ‘smooth’ surfaces but worsened for the ‘rough’ ones with moistening. We suggest that this is due to the different nature of neural noise generated when making contact with smooth or rough background surfaces, and the extent to which different fluid environments modulate friction and the forces acting on the skin with consequences for the neural response.