A MEMS-based tactile sensor to study human digital touch: mechanical transduction of the tactile information and role of ﬁngerprints

J Scheibert,G Debregeas,A Prevost

doi:10.1051/epjconf/20100621006

Abstract

We present recent results showing that human epidermal ridges (fingerprints) could play a central role in fine texture discrimination tasks by spatially modulating the contact stress field between the fingertip and the substrate. Using an original biomimetic finger whose surface is patterned with parallel ridges, we demonstrate that the subsurface stress signals elicited by continuous rubbing of randomly textured substrates is dominated by fluctuations at a frequency defined by the inter-ridge distance divided by the rubbing velocity. In natural exploratory conditions, this frequency matches the best frequency of one type of mechanoreceptors, namely the Pacinian corpuscles, which are specifically involved in the tactile coding of fine textures. The use of white-noise patterned stimuli has alloowed us to extract, using a reverse-correlation analysis, the stimulus-signal response function associated with roughness modality. Its shape could provides spectral, spatial and directional selectivity to the digital tactile system. It offers a physiological basis for the recently proposed hypothesis of a dual-coding (spatio-temporal and vibratory) of tactile information.

Highlights

The hand is an essential tool for human interaction with its surroundings [1,2,3]
A legitimate question is whether the vibrations evidenced with this idealized device are relevant to an actual fingertip
One may expect that differences in texture-induced subcutaneous vibrations may show up in the global friction force acting on the finger during tactile exploration

Summary

Introduction

The hand is an essential tool for human interaction with its surroundings [1,2,3]. The high tactile sensitivity of the fingertips allows for many complex tasks, including precise grasping and manipulation of objects, detection of individual defects on smooth surfaces and texture discrimination. The digital skin exhibits characteristic epidermal ridges (fingerprints) that have long been suspected to improve tactile perception, the mechanism at play remains poorly understood [4]. They are believed to enhance friction and adhesion of the finger pads improving the ability to securely grasp objects or supports [5]. It is clear that fingerprints modifiy significantly the subcutaneous stress [6,7,8,9] In this respect, individual ridges may enhance tactile sensitivity by magnifying subsurface strains [10, 11]. It has been shown that two independent neural coding mechanisms are involved in this dynamic exploratory mode, which might be associated with separate coding channels [16,17,18]: for textural features of lateral extension greater than about 200 micrometers, a spatio-temporal coding involving slowly adapting mechanoreceptors (sensitive to a constant stimulus) is used [19] whereas finer features are vibratory encoded principally through (rapidly adapting) Pacinian fibers [18, 20,21,22,23]

Methods

Results

Conclusion