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.

Submillimeter Lateral Displacement Enables Friction Sensing and Awareness of Surface Slipperiness

“…In that study subjects actively contacted the surface with a finger movement constrained in the orthogonal direction relative to the contact surface. Findings in the current study and by Willemet et al [3] both agree and demonstrate that exploratory sliding movement and full slip are not required to perceive surface slipperiness. Thus, sensory conditions under which we perceive surface slipperiness are very similar to friction sensing used for grip force control during object manipulation, indicating that frictional information might be shared between motor and perceptual systems and rely on the same sensory input.…”

“…The differences between Willemet et al [3] and the current study is that under our experimental conditions here the skin divergence pattern when making contact with a surface orthogonally wasn't enough to make judgements about slipperiness, and additional submillimeter range lateral movement was required. This could be explained by the fact that in Willemet et al [3] subjects made active movements and, regardless of lateral movement constraint, were able to exploit the kinematics of contact by optimizing movement to enhance sensing of the primary cue available-the skin radial divergence pattern.…”

“…The differences between Willemet et al [3] and the current study is that under our experimental conditions here the skin divergence pattern when making contact with a surface orthogonally wasn't enough to make judgements about slipperiness, and additional submillimeter range lateral movement was required. This could be explained by the fact that in Willemet et al [3] subjects made active movements and, regardless of lateral movement constraint, were able to exploit the kinematics of contact by optimizing movement to enhance sensing of the primary cue available-the skin radial divergence pattern. In that study we noticed that, on average, subjects chose to use much higher normal forces (5.5 N), faster force rate (3.6 N/s), and movement was not constrained normal to the surface, so the development of net tangential forces is unavoidable due to mechanical anisotropies of the finger tissue and anatomical asymmetry.…”

“…It shows that displacements that do not induce a full slip at the interface can be used to discriminate between frictional properties of two surfaces. Importantly, the finger was immobilized, so no movement related proprioceptive information could contribute, nor was active movement execution required to trigger perception [3]. The experiment was conducted using a smooth flat surface, thus no information about the displacement of surface texture elements was available, as it would be with a textured surface [1].…”

“…This is an important factor enabling friction sensing in the context of object manipulation. In fact, frictional differences could be sensed, and consciously perceived, when no lateral movement is made, based only on radial skin divergence patterns [3] . In that study subjects actively contacted the surface with a finger movement constrained in the orthogonal direction relative to the contact surface.…”