Tactile afferents encode grip safety before slip for different frictions

Abstract

Adjustments to frictional forces are crucial to maintain a safe grip during precision object handling in both humans and robotic manipulators. The aim of this work was to investigate whether a population of human tactile afferents can provide information about the current tangential/normal force ratio expressed as the percentage of the critical load capacity - the tangential/normal force ratio at which the object would slip. A smooth stimulation surface was tested on the fingertip under three frictional conditions, with a 4 N normal force and a tangential force generated by motion in the ulnar or distal direction at a fixed speed. During stimulation, the responses of 29 afferents (12 SA-I, 2 SA-II, 12 FA-I, 3 FA-II) were recorded. A multiple regression model was trained and tested using cross-validation to estimate the percentage of the critical load capacity in real-time as the tangential force increased. The features for the model were the number of spikes from each afferent in windows of fixed length (50, 100 or 200 ms) around points spanning the range from 50% to 100% of the critical load capacity, in 5% increments. The mean regression estimate error was less than 1% of the critical load capacity with a standard deviation between 5% and 10%. A larger number of afferents is expected to improve the estimate error. This work is important for understanding human dexterous manipulation and inspiring improvements in robotic grippers and prostheses.

The biology of skin wetness perception and its implications in manual function and for reproducing complex somatosensory signals in neuroprosthetics

“…Contact with slippery objects excites FA1 afferents more strongly than contact with a less slippery surface (Johansson and Flanagan 2009;Khamis et al 2014b). Accordingly, it has been proposed that FA1 mechanoreceptors could be exquisitely sensitive to mechanical deformations induced by microslips occurring at the papillary surface of the fingers (Delhaye et al 2012; Johansson and Flanagan 2009; Khamis et al 2014a). In this respect, the recent "slip hypothesis" proposes that cutaneous sensory coding of microslips could be indeed responsible for the force adjustments observed during lifting of slippery objects (Schwarz 2016).…”

Initial contact shapes the perception of friction

“…Despite its crucial importance, the mechanical de-formation that underpins the encoding of the frictional strength on initial contact remains unclear. It is well known that the timing of the impulses of tactile afferents encodes the information related to force direction (10), local curvature (11), edges (12), shapes (13) and also contains information about the frictional strength (14, 15). One hypothesis suggests that, at the mechanical level, micro-slip events at the finger-object interface induce vibrations of the skin (16, 17).…”

High-resolution imaging of skin deformation shows that afferents from human fingertips signal slip onset

“…A previous study showed that information about grip safety, i.e. the increment of tangential force needed to reach full slip, is present in the response of local tactile afferents at the level of a population of afferents, irrespectively of the level of friction (Khamis et al, 2014). Given that all afferents respond to progressing slips, and that the timing of each FA-I afferent's burst is dependent on how far from the center of contact the afferent is, it is not surprising that a linear combination of multiple FA-I units can provide a good estimate of the grip safety.…”