The Influence of Surface Commensurability on Roughness Perception with the Bare Finger François Martinot LIFL
POLYTECH'LILLE-INRIA Futurs
ABSTRACT Perception mechanisms of fine gratings still remain very obscure. Among their determinants, the knowledge of the fingerprint role could be a central issue: epidermal ridges are already known to be responsible for detection of small features on a surface, slippage and anisotropies in the perception of gaps or gratings. It is the reason why it appears essential to better characterize their “psychotribologic” role in the dynamic contact of lateral touch. The first experiment shows that roughness perception depends on the commensurability and conformance between ridges of the epidermis and gratings. Then, a vibroacoustic experiment is proposed. It is splitted into three phases to better understand mechanisms of vibration emergence. Contrary to previous approach on the topic that are mainly centred on the vibration on the finger, the attention is focused on the consequences of the excitation of the finger on the texture. Spectrograms and scalograms illustrate that deformable bodies at contact generate inertial force on each other. Systematic relationships between the intensive or frequency parameters of the vibration at contact and perception of gratings were absent. However, the influence of the relative orientation between ridges of the texture and fingerprint on the vibratory mechanisms showed the importance of spatial determinants. The more textures fit together at the scale of the epidermal ridge, the more the average intensity in shear force, part of friction, and resulting vibration at contact increases. This suggests that a roughness code could be initiated spatially, even at small scales. The reliability of a non spatial roughness code with only intensive and temporal cues is finally checked. Vibration recorded with a vibrometer and played back on the skin with an electromagnetic shaker were barely sensed, and at best, as a flutter of negligible amplitude when amplified and applied tangentially to the skin. 1 Keywords: commensurability, roughness, vibration. 1 BACKGROUND Previous research in neurophysiology suggests that three kinds of specialized neurons with different sensitivities mediate the perception of roughness. Among these mechanoreceptors, the Merkel’s disks (connected to a slowly adapting or SAI channel) account for the perception of spatial shapes. Pacinian corpuscules (PC channel) are sensitive to high-frequency vibration at contact. As for the Meissner’s corpuscules (rapidly adapting or RA channel), they allow the detection of localized relative motion between skin and a surface form, when surface details are too small to activate the SAI afferents effectively [1]. Within this e-mail:
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framework, recent findings reveal that the RA channel seems most suited to transduce tangential forces since the Ruffini endings (SAII) as other mechanoreceptors, are placed at the base of the finger nails and few Pacinian corpuscles (PC) are located in the deep dermis [2]. However, the coding mechanisms of the stress field imposed on the fingerpulp, leading to perception of fine textures, remain widely unclear. For instance, it was recently suggested that the relative timing in which afferent initially discharge spatially [3] could, in addition with possible intensive, spatial and temporal coding mechanisms, determine the perception of complex mechanical events occurring at the fingertip. Other shortcomings for the understanding of tactual perception are the kinematics of finger, the rheology of the living tissues and the tribology of the skin that all filter or “window” the inputs of our biological sensors. It is the reason why it matters to pay attention to the “texture of the skin”. The epidermal ridge serves a dual function in primate, frictional and tactile [4], that aids the perception and prevention of slippage. Its detailed contribution to tactual perception in human has been particularly studied so far. Using raised dots of different height positioned on a smooth surface, LaMotte and Whitehouse [5] observed that an impact of a small asperity (2µm in height, 550µm in diameter) against a papillary ridge (fingerprint line) triggers an action potential for the RA nerve fibers. Thus, the awareness of slippage [6] could rely on stick-slip induced vibration resulting from stiction of a dot or a texture, and subsequent spatiotemporal responses of the RA, or intensive discharges in the PC afferent fibers. Maeno, Kobayashi and Yamazaki [7] built a finite element model incorporating the geometry of the epidermal ridges. They showed, according with the lever mechanism proposed by Cauna [8], that skin ridges could increase the intensity of the tactile sensation by concentrating the stress around the Merkel’s disks. In static conditions, Gerling and Thomas [9] specified that a gap axis parallel to the lines of the epidermal ridges elicits a more pronounced pressure around the intermediate ridges and Merkel cells complexes than a gap axis perpendicular to ridge lines. The grating or gap orientation (GO) or smooth-grooved (SG) discrimination are psychophysical experiments considered as valid measures of spatial or, more generally, tactile acuity. These methods have strongly contributed to support the spatial mechanisms of the tactual perception and the dominance of the SAI system, confirmed by psychophysical experiments with ridges or grooves of gatings varying in width [10-12]. Conversely, they also allow to demonstrate that tactual performance is impaired without dynamic motion for very fine gratings. Thanks to an experiment of static detection (GO), Phillips and Johnson [15] provided evidence that no information is available to the decision process concerning the orientation of gratings with spatial wavelenghts below 1mm. In a dynamic SG task, their participants performed better than chance to detect a gap with a groove width (GW) of 0.2mm. They also found that the relative orientation between edges of deep gratings and fingerprint ridges had a major effect on the responses of the SAI fibers. Moreover,
they found an increase in sensitivity with the adjacence between skin ridges [16]. In a GO task, Essock et al. [17,18] found a better sensitivity to the orientation of striated stimuli presented in the proximal-distal orientation on their fingers. They suggested that orientation-selective neurons may explain this directional sensitivity. With measurements taken 15mm proximal to the fingertip and a contact force of 100gf, Craig [19] found that there was no evidence for directional acuity in a GO task. He also noted that there was no relationship between the thresholds of angle and the density of the primary fibers. In a SG task using gloves and JVP (Johnson, Van Boven, Phillips) domes, Gibson and Craig [20] suggested the importance of both spatial and intensive factors. Using gaps which width ranged from 0.75mm to 12mm, they recently [21] found that subjects were 33% more sensitive when the edge of the gap was parallel to the fingerprint ridges at the fingertip. No difference was found for the fingerpad. In a SG task, Stevens and Patterson [22] also reported aboundant taskdependant orientation effects in tactile discrimination. Using a grating with a ridge width of 1.5 mm and a groove width of 0.75 mm, Wheat and Goodwin [23] found a strong orientation effect. They proposed a partial explanation: the “beam”-like structure of the ridges may cause a greater stiffness parallelly to the ridges. Vega-Bermundez and Johnson [24] found that the increased compliance along the skin ridges of young subjects improves their tactual performance. 2 MOTIVATION AND ORGANIZATION OF THE ARTICLE The optimality in the voluntary movement of lateral touch and/or the dynamics of contact have motivated several studies [25-30] to find arguments for the involvement of temporal cues in the discrimination process of finely textured surfaces (generally called duplex model of roughness perception [27]). Strong correlations were reported between perception and data issued from measurement of the temporal variation of the tangential force [25], or the weighted power of the texture-induced vibration on the skin [28]. Within this theoretical framework, the contribution of the epidermal ridges to a complex tribologic interaction between two textured surfaces, skin included, remains overlooked. Consequently, It is interesting to study the effect on roughness perception of the conformance of epidermal ridges to commensurable fine textures. In this work, the effects of penetration of texture ridges into epidermal grooves on perceptual responses and mechanical energy dissipated on the touched surface are investigated. Materials with similar average roughness (tribologic definition) are used to constrain the stretched skin to a same baseline resting position at the spatial scale of the fingerpad tissues. Their surfaces differ with their rate of change, or density of lines (spatial changes in RW and GW
