Initial contact shapes the perception of friction

Abstract

Humans efficiently estimate the grip force necessary to lift a variety of objects, including slippery ones. The regulation of grip force starts with the initial contact and takes into account the surface properties, such as friction. This estimation of the frictional strength has been shown to depend critically on cutaneous information. However, the physical and perceptual mechanism that provides such early tactile information remains elusive. In this study, we developed a friction-modulation apparatus to elucidate the effects of the frictional properties of objects during initial contact. We found a correlation between participants’ conscious perception of friction and radial strain patterns of skin deformation. The results provide insights into the tactile cues made available by contact mechanics to the sensorimotor regulation of grip, as well as to the conscious perception of the frictional properties of an object.

Rapid change of friction causes the illusion of touching a receding surface

“…The skin of the fingertip exhibits a lateral displacement during compression (M = 15.6 μm, s.d. = 17.1 μm), in line with previously reported findings [2].…”

“…The deformation stimulates mechanoreceptors embedded in the tissues that signal to the brain material properties such as compliance, texture and how far the object is from sliding away. Particularly, our sense of touch can discriminate between the slipperiness of a wet soap or the grip provided by a rough surface at the first instant of touch [ 1 , 2 ]. The tactile sensations reinforce the internal model of our surroundings and are fundamental to properly tune our motor commands [ 3 ].…”

“…We previously demonstrated that during the initial compression of the skin on a rigid surface, the friction constrains the lateral expansion of the skin [ 2 ]. In parallel, we also showed that rapid friction changes during compression lead to a clear feeling of pressing a button [ 9 , 10 ].…”

Contact evolution of dry and hydrated fingertips at initial touch

“…This deformation is influenced by the finger’s geometry (i.e., size, shape, dimensions of fingerprint ridges, tissue layer thicknesses) as well as its material properties (i.e., elastic modulus, Poisson’s ratio) [ 4 , 5 ]. Characteristics of both the surface (e.g., roughness and hardness) [ 6 ] and the skin-surface interface (e.g., static and kinetic friction coefficients) [ 7 , 8 ] also affect finger deformation during contact.…”

“…The complexity of touch mechanics largely stems from the interdependence between the elastic properties of the skin and the frictional forces applied to the finger by the contact surface. A change in the elastic moduli alters the total friction force due to the alteration of the real contact area [ 9 ], while a change in the friction coefficient results in a different elastic deformation due to the changed tangential forces that resist stretching [ 8 ]. In addition, these two variables are both influenced by moisture, an important external factor.…”