Tactile detection of slip: surface microgeometry and peripheral neural codes

Abstract

1. The role of the microgeometry of planar surfaces in the detection of sliding of the surfaces on human and monkey fingerpads was investigated. By the use of a servo-controlled tactile stimulator to press and stroke glass plates on passive fingerpads of human subjects, the ability of humans to discriminate the direction of skin stretch caused by friction and to detect the sliding motion (slip) of the plates with or without micrometer-sized surface features was determined. To identify the associated peripheral neural codes, evoked responses to the same stimuli were recorded from single, low-threshold mechanoreceptive afferent fibers innervating the fingerpads of anesthetized macaque monkeys. 2. Humans could not detect the slip of a smooth glass plate on the fingerpad. However, the direction of skin stretch was perceived based on the information conveyed by the slowly adapting afferents that respond differentially to the stretch directions. Whereas the direction of skin stretch signaled the direction of impending slip, the perception of relative motion between the plate and the finger required the existence of detectable surface features. 3. Barely detectable micrometer-sized protrusions on smooth surfaces led to the detection of slip of these surfaces, because of the exclusive activation of rapidly adapting fibers of either the Meissner (RA) or the Pacinian (PC) type to specific geometries of the microfeatures. The motion of a smooth plate with a very small single raised dot (4 microns high, 550 microns diam) caused the sequential activation of neighboring RAs along the dot path, thus providing a reliable spatiotemporal code. The stroking of the plate with a fine homogeneous texture composed of a matrix of dots (1 microns high, 50 microns diam, and spaced at 100 microns center-to-center) induced vibrations in the fingerpad that activated only the PCs and resulted in an intensive code. 4. The results show that surprisingly small features on smooth surfaces are detected by humans and lead to the detection of slip of these surfaces, with the geometry of the microfeatures governing the associated neural codes. When the surface features are of sizes greater than the response thresholds of all the receptors, redundant spatiotemporal and intensive information is available for the detection of slip.

Visual and Somatosensory Information about Object Shape Control Manipulative Fingertip Forces

“…These afferents offer many types of information that may be of relevance for the control of fingertip forces in manipulation, for instance, information related to frictional slips and creep Srinivasan et al, 1990;Milner et al, 1991), the shape of the contact surface (Goodwin et al, 1995), and contact angle (Goodwin and Morley, 1987), as well as distribution within the contact area of normal and tangential forces Srinivasan et al, 1990;. Thus, information related to object shape in the present study should have been readily available from signals in populations of tactile afferents.…”

Evidence for the duplex theory of tactile texture perception

“…LaMotte and Srinivasan (1991; see also Srinivasan, Whitehouse, & LaMotte, 1990) have found that an array of dots or bars that are on the order of 0.1,um in height can be detected by scanning, but not by static, touch. Subjects reported that their detection ofthese ultrafine stimuli (to which only Pacinian afferents were found to respond) was based on vibratory sensations.…”

The Role of Compliant Fingerpads in Grasping and Manipulation: Identification and Control

“…In order for the identification and control procedures to be robust, continuous monitoring of contact conditions with tactile sensors is absolutely necessary. In humans, specialized tactile mechanoreceptors enable detection of slipping of objects on the skin [9,12] and analogous robotic sensors have also been developed [18].…”

“…Suppose fc falls below the lower bound, detection of slip is of paramount importance, and can be accomplished by specialized tactile slip detectors. In fact, it has been demonstrated that primates use tactile sensors to detect slip [9]. In human studies, it has been shown that subjects apply an external force which is about 30% more than the lower bound needed to overcome slip [12].…”

“…The skin, joints, tendons, and muscles are richly innervated by a wide variety of mechanosensitive receptors that convey sensory information to the brain through associated nerve fibers [8]. This tactual sensory information can be divided into two categories: (1) Tactile, which refers to direct information about the contact conditions at the interface, such as normal and shear forces, existence and direction of slip [9], etc; (2) kinesthetic, which refers to the positions and motions of, and the forces acting on, the bones, conveyed not only by the receptors in the skin around the joints, within joints, tendons, and muscles, but also by information derived from motor commands for intended movements. When the fingerpads come in contact with an object, their compliance generally ensures that the contact region has a finite area, and hence rich information contained in the spatio-temporal variation of the mechanical loads is conveyed to the receptors and subsequently to the brain.…”