Tactile Texture Perception Research Articles

Processing the fine-grained features of tactile textures involves the primary somatosensory cortex

Abstract Dynamic tactile perception and discrimination of textures require the ability to encode and differentiate complex vibration patterns elicited at the level of the skin when sliding against a surface. Whether the primary somatosensory cortex (S1) can encode the fine-grained spectrotemporal features distinguishing textures remains debated. To address this question, EEG frequency-tagging was used to characterize responses to vibrotactile oddball contrasts between two textures. In a first session designed to identify the topographical distribution of responses originating from the hand and foot representations in S1, standard and deviant stimuli were pure sinusoidal vibrations differing in frequency and intensity. In a second session, standard and deviant stimuli were two different snippets of bandpass-filtered white noise matched in terms of intensity and average frequency content, but differing in terms of their complex spectrotemporal content. Using the S1 functional localizer, we showed that oddball responses to a spectrotemporal contrast follow the somatotopical organization of S1. Our results suggest that the encoding of fine-grained spectrotemporal features associated with different vibration patterns involves S1.

Visual effects on tactile texture perception

How does vision affect active touch in judgments of surface roughness? We contrasted direct (combination of visual with tactile sensory information) and indirect (vision alters the processes of active touch) effects of vision on touch. Participants judged which of 2 surfaces was rougher using their index finger to make static contact with gratings of spatial period 1580 and 1620 μm. Simultaneously, they viewed the stimulus under one of five visual conditions: No vision, Filtered vision + touch, Veridical vision + touch (where vision alone yielded roughness discrimination at chance), Congruent vision + touch, Incongruent vision + touch. Results from 32 participants showed roughness discrimination for touch with vision was better than touch alone. The visual benefit for touch was strongest in a filtered (spatially non-informative) vision condition, thus results are interpreted in terms of indirect integration. An indirect effect of vision was further indicated by a finding of visual benefit in some but not all visuo-tactile congruency conditions.

Tactile and visual perception of plastic textures for car interiors: Psychophysical and affective dimensions

Surface textures play an important role in the perception of consumer products. In car interiors, they are a major contributor to the perceived quality of a vehicle. However, limited attention has been paid to the affective responses elicited by touch and observation of a textured surface. Here we report an investigation in which Kansei Engineering was used to explore the perception of texturized plastic surfaces, both in its psychophysical and affective dimensions, as well as the influence of the textures’ physical parameters. In a controlled study, 21 subjects were asked to touch 18 samples of two types of plastics, commonly used in car interiors, Polypropylene (PP) and Acrylonitrile Butadiene Styrene (ABS), under tactile (only touch) and visual-tactile (touch and vision) conditions. Participants were asked to rate the samples according to 8 descriptors of psychophysical and affective nature. Results show a relation between perceived roughness and the positive/negative valence of the affective response. Importantly, this relation is moderated by the visual appearance of the textures, when these are visible. Both psychophysical and affective dimensions were modelled through height parameters of the texture elements and a variable describing the type of material. The present findings are relevant for the understanding of visual-tactile perception of textures and have implications in the design of textured components for automotive car interiors.

Feeling Illusory Textures Through a Hole: Rotating Frame At Skin-Object Interface Modifies Perceived Tactile Texture.

Modulating tactile texture perception for the surface of real objects is a promising way to artificially present various tactile textures. Here, we propose a simple method of modulating tactile textures for various materials, which is named the rotating-frame method. In the method, one touches an arbitrary material's surface through a hole in a cardboard frame. When the frame is rotated between the hand and material, the tactile texture of the material is perceived as if it has turned into another material. We investigated the qualitative and quantitative characteristics of the illusory modulation created by the method in a series of psychophysical experiments. We found that the method altered the tactile textures of the surfaces of touched materials such as glass and carpet to seem softer, smoother, slipperier, and warmer than they actually are. The illusory texture change occurred robustly when the method was applied with different categories of materials. Our method paves the way for the development of simple techniques for texture augmentation that can be applied to a wide range of materials and do not disrupt stable direct contact between the hand and the materials.

The roles of lower- and higher-order surface statistics in tactile texture perception.

Humans can haptically discriminate surface textures when there is a significant difference in the statistics of the surface profile. Previous studies on tactile texture discrimination have emphasized the perceptual effects of lower-order statistical features such as carving depth, inter-ridge distance, and anisotropy, which can be characterized by local amplitude spectra or spatial-frequency/orientation subband histograms. However, the real-world surfaces we encounter in everyday life also differ in the higher-order statistics, such as statistics about correlations of nearby spatial-frequencies/orientations. For another modality, vision, the human brain has the ability to use the textural differences in both higher- and lower-order image statistics. In this work, we examined whether the haptic texture perception can use higher-order surface statistics as visual texture perception does, by three-dimensional (3-D)-printing textured surfaces transcribed from different "photos" of natural scenes such as stones and leaves. Even though the maximum carving depth was well above the haptic detection threshold, some texture pairs were hard to discriminate. Specifically, those texture pairs with similar amplitude spectra were difficult to discriminate, which suggests that the lower-order statistics have the dominant effect on tactile texture discrimination. To directly test the poor sensitivity of the tactile texture perception to higher-order surface statistics, we matched the lower-order statistics across different textures using a texture synthesis algorithm and found that haptic discrimination of the matched textures was nearly impossible unless the stimuli contained salient local features. We found no evidence for the ability of the human tactile system to use higher-order surface statistics for texture discrimination.NEW & NOTEWORTHY Humans can discriminate subtle spatial patterns differences in the surrounding world through their hands, but the underlying computation remains poorly understood. Here, we 3-D-printed textured surfaces and analyzed the tactile discrimination performance regarding the sensitivity to surface statistics. The results suggest that observers have sensitivity to lower-order statistics whereas not to higher-order statistics. That is, touch differs from vision not only in spatiotemporal resolution but also in (in)sensitivity to high-level surface statistics.

Describing the Sensation of the 'Velvet Hand Illusion' in Terms of Common Materials.

When sandwiching two moving parallel metallic wires between both hands, one often experiences an unexpected tactile sensation known as the "velvet hand illusion" (VHI). Researchers have revealed the optimal conditions for inducing VHI, while the subjective nature of VHI remains obscure. In this article, we conducted a psychophysical experiment to investigate the quality and magnitude of the illusory sensation felt during VHI. Participants were asked to evaluate the tactile sensation of moving wires by giving tactile adjective and intensity ratings of the illusory sensation. In the same experiment, for the sake of comparison, participants also rated the sensation for various common materials one may encounter in daily life. We found that, as the intensity of the illusory sensation increased, the tactile sensation became softer, wetter, warmer, and more favorable. We also found that, when a strong illusion was reported, the sensation was similar to those for leather and fabrics rather than metallic wire, which suggests that the illusion indeed changes the perceived material category. These findings provide a better characterization of VHI as well as a better understanding of tactile texture perception.

Tactile perception by mouth: Perceiving properties of objects when vision is impaired

The lips and tongue demonstrate similar or greater spatial acuity than the fingertips. Indeed, infants use the mouth to perceive properties of objects such as hardness, texture, and shape. In normal development, it is assumed that mouthing decreases in favour of increasingly advanced hand exploration patterns. However, anecdotal reports suggest that mouthing continues to serve a perceptual function when a person’s vision is abnormal. This study explored blind or visually impaired (BVI) adults’ self-reported use of mouthing to perceive properties of objects. We conducted semi-structured interviews with 20 BVI adults with visual acuities ranging from no light perception to 20/40. Data were analysed using content analysis to identify specific properties perceived by the mouth. Despite social norms that discourage mouthing, some BVI adults use oral tactile perception of texture, shape, temperature, and taste to better characterize objects. These findings suggest that compensatory behaviours using the mouth can support the rehabilitation of individuals with abnormal vision.

Improving the tactile perception of image textures based on adjustable amplitude, frequency, and waveform

In this paper, we present a tactile rendering algorithm applied to an electrostatic tactile display that adjusts three parameters of the driving signal (amplitude, frequency, and waveform) to modulate the tangential friction force between a user’s finger and a touch screen. The aim of this work is to find an effective electrostatic tactile rendering algorithm to improve the tactile perception of image textures. The key idea is to jointly adjust the three parameters of the driving signal to increase the perceptual difference interval between image textures. We first explore the tactile representation characteristics of amplitude, frequency and waveform through subjective perception experiments. Based on these characteristics, we establish tactile mapping models between the three parameters of the driving signal and image textures. Finally, we use subjective evaluation experiments to verify the effectiveness of the proposed rendering method. The results show that the proposed rendering method can achieve a better tactile experience compared with rendering methods that realize tactile representation by only varying amplitude.

From finger friction to brain activation: Tactile perception of the roughness of gratings

The formation of tactile perception is related to skin receptors and the cerebral cortex. In order to systematically study the tactile perception from finger friction to the brain response, a 32-channel Brain Products system and two tri-axial force sensors were used to obtain electroencephalograph (EEG) and friction signals during fingers exploring grating surfaces. A finite element finger model was established to analyze the stress changes of the skin receptors during tactile perception. Samples with different grating widths and spaces were chosen. The results indicated that different gratings induced different stress concentrations within skin that stimulated Meissner and Merkel receptors. Skin friction was affected by gratings during the tactile perception. It was also found that P300 evoked by gratings was related with the skin deformation, contact area, friction force, and stress around cutaneous mechanoreceptors. The wider grating width generated larger skin deformation, friction force, and stress, which induced stronger tactile stimulation. The smaller grating spacing generated higher vibration frequency, inducing stronger tactile stimulation. The latency of the P300 peak was related to the difference between the textured target stimulus and the smooth non-target stimulus. This study proofed that there was a relationship between the activation in brain regions, surface friction, and contact conditions of skin during the tactile perception. It contributes to understanding the formation process and cognitive mechanism of tactile perception of different surface textures.

Tactile Perception of Virtual Edges and Gratings Displayed by Friction Modulation via Ultrasonic Actuation.

Tactile discrimination and roughness perception of real textures are extensively studied and underlying perceptual mechanisms are relatively well-established. However, tactile perception of virtual textures rendered by friction modulation techniques on touch surfaces has not been investigated in detail yet. In this article, we investigated our ability to discriminate two consecutive step changes in friction (called edges), followed by discrimination and roughness perception of multiple edges (called periodic gratings). The results showed that discrimination of two consecutive edges was significantly influenced by edge sequence: a step fall in friction ( FF) followed by a step rise in friction ( RF) was discriminated more easily than the reverse order. On the other hand, periodic gratings displayed by consecutive sequences of FF followed by RF were perceived with the same acuity as compared to vice versa. Independent of the edge sequence, we found that a relative difference of 14% in spatial period was required to discriminate two periodic gratings. Moreover, the roughness perception of periodic gratings decreased with increasing spatial period for the range that we have investigated (spatial period 2mm), despite the lack of spatial cues on grating height. We also observed that rate of change in friction coefficient was better correlated with the roughness perception than the friction coefficient itself. These results will further help to understand and design virtual textures for touch surfaces.

High-dimensional representation of texture in somatosensory cortex of primates

In the somatosensory nerves, the tactile perception of texture is driven by spatial and temporal patterns of activation distributed across three populations of afferents. These disparate streams of information must then be integrated centrally to achieve a unified percept of texture. To investigate the representation of texture in somatosensory cortex, we scanned a wide range of natural textures across the fingertips of rhesus macaques and recorded the responses evoked in Brodmann's areas 3b, 1, and 2. We found that texture identity is reliably encoded in the idiosyncratic responses of populations of cortical neurons, giving rise to a high-dimensional representation of texture. Cortical neurons fall along a continuum in their sensitivity to fine vs. coarse texture, and neurons at the extrema of this continuum seem to receive their major input from different afferent populations. Finally, we show that cortical responses can account for several aspects of texture perception in humans.

Phase difference between normal and shear forces applied to fingertip can be discriminated in tactile perception of textures

Phase difference between normal and shear forces applied to fingertip can be discriminated in tactile perception of textures

Complexity, rate, and scale in sliding friction dynamics between a finger and textured surface

Sliding friction between the skin and a touched surface is highly complex, but lies at the heart of our ability to discriminate surface texture through touch. Prior research has elucidated neural mechanisms of tactile texture perception, but our understanding of the nonlinear dynamics of frictional sliding between the finger and textured surfaces, with which the neural signals that encode texture originate, is incomplete. To address this, we compared measurements from human fingertips sliding against textured counter surfaces with predictions of numerical simulations of a model finger that resembled a real finger, with similar geometry, tissue heterogeneity, hyperelasticity, and interfacial adhesion. Modeled and measured forces exhibited similar complex, nonlinear sliding friction dynamics, force fluctuations, and prominent regularities related to the surface geometry. We comparatively analysed measured and simulated forces patterns in matched conditions using linear and nonlinear methods, including recurrence analysis. The model had greatest predictive power for faster sliding and for surface textures with length scales greater than about one millimeter. This could be attributed to the the tendency of sliding at slower speeds, or on finer surfaces, to complexly engage fine features of skin or surface, such as fingerprints or surface asperities. The results elucidate the dynamical forces felt during tactile exploration and highlight the challenges involved in the biological perception of surface texture via touch.

Brain networks underlying conscious tactile perception of textures as revealed using the velvet hand illusion.

Humans are adept at perceiving textures through touch. Previous neuroimaging studies have identified a distributed network of brain regions involved in the tactile perception of texture. However, it remains unclear how nodes in this network contribute to the tactile awareness of texture. To examine the hypothesis that such awareness involves the interaction of the primary somatosensory cortex with higher order cortices, we conducted a functional magnetic resonance imaging (fMRI) study utilizing the velvet hand illusion, in which an illusory velvet-like surface is perceived between the hands. Healthy participants were subjected to a strong illusion, a weak illusion, and tactile perception of real velvet. The strong illusion induced greater activation in the primary somatosensory cortex (S1) than the weak illusion, and increases in such activation were positively correlated with the strength of the illusion. Furthermore, both actual and illusory perception of velvet induced common activation in S1. Psychophysiological interaction (PPI) analysis revealed that the strength of the illusion modulated the functional connectivity of S1 with each of the following regions: the parietal operculum, superior parietal lobule, precentral gyrus, insula, and cerebellum. The present results indicate that S1 is associated with the conscious tactile perception of textures, which may be achieved via interactions with higher order somatosensory areas.

Correlation between friction-induced vibrations and tactile perception during exploration tasks of isotropic and periodic textures

Abstract The sliding contact between the finger and the surface of an object generates vibrations that propagate into the skin and activate the mechanoreceptors. The characteristics of the vibrating signals give information about the texture features, affecting the tactile perception. Previous studies have focused on the relation between the physical characteristics of the surfaces and the detected signals. This study investigates the direct correlation between contact-induced vibrations and tactile perception of textures. Two sets of samples were investigated: periodic samples and isotropic samples. Within the same set, each sample exhibits a clearly distinct surface roughness relative to one another. Sensory tests were conducted with blind people to evaluate the tactile discrimination of the samples as a function of roughness. The analysis of the Root Mean Square of the acceleration and the acceleration spectra obtained for the different samples, allowed explaining the mismatch between the actual roughness of the samples and the roughness perceived by the subjects, which is directly related to the induced vibrations.

Speed invariance of tactile texture perception.

The nervous system achieves stable perceptual representations of objects despite large variations in the activity patterns of sensory receptors. Here, we explore perceptual constancy in the sense of touch. Specifically, we investigate the invariance of tactile texture perception across changes in scanning speed. Texture signals in the nerve have been shown to be highly dependent on speed: temporal spiking patterns in nerve fibers that encode fine textural features contract or dilate systematically with increases or decreases in scanning speed, respectively, resulting in concomitant changes in response rate. Nevertheless, texture perception has been shown, albeit with restricted stimulus sets and limited perceptual assays, to be independent of scanning speed. Indeed, previous studies investigated the effect of scanning speed on perceived roughness, only one aspect of texture, often with impoverished stimuli, namely gratings and embossed dot patterns. To fill this gap, we probe the perceptual constancy of a wide range of textures using two different paradigms: one that probes texture perception along well-established sensory dimensions independently and one that probes texture perception as a whole. We find that texture perception is highly stable across scanning speeds, irrespective of the texture or the perceptual assay. Any speed-related effects are dwarfed by differences in percepts evoked by different textures. This remarkable speed invariance of texture perception stands in stark contrast to the strong dependence of the texture responses of nerve fibers on scanning speed. Our results imply neural mechanisms that compensate for scanning speed to achieve stable representations of surface texture.NEW & NOTEWORTHY Our brain forms stable representations of objects regardless of viewpoint, a phenomenon known as invariance that has been described in several sensory modalities. Here, we explore invariance in the sense of touch and show that the tactile perception of texture does not depend on scanning speed. This perceptual constancy implies neural mechanisms that extract information about texture from the response of nerve fibers such that the resulting neural representation is stable across speeds.

Investigation of the role of contact-induced vibrations in tactile discrimination of textures

Tactile perception of texture is meaningful for numerous industrial and social applications. While the sense of touch is provided by several contributing factors, each related to different sensory cutaneous units, textures with non-low spatial gradients can be appreciated only by the relative motion between the finger and the object. This work focuses on investigating the role that the vibrations induced during sliding contact play in the origin of tactile perception and surface texture discrimination. We shall focus in particular on the effect of the different spectral contents of the induced vibrations. To this end, first the vibration signals produced by touching different sample surfaces are recovered and analysed under controlled boundary conditions. Then, a collection of data from different subjects is recorded and reproduced by a dynamic exciter. Finally, the reproduced signals are randomly submitted to a panel of subjects in order to test their ability to distinguish the different surfaces only through their perception of the reproduced vibrations. The results, while highlighting the key role played by the spectral distribution of friction-induced vibrations, simultaneously reveal the limits of taking only vibrational signals into account if we are to achieve satisfactory recognition of the whole panel of tested textures.

Experimental research on the tactile perception from fingertip skin friction

Skin friction is a main perception channel for humankind to recognize object surface properties. However, the relationship between tactile perception and skin friction is a typical bio-tribology interdisciplinary problem, which is rarely studied both in biology and tribology fields. In order to study the brain stimulation response to the fingertips sliding friction, cognitive related fingertip friction testing was carried out, and the tactile perception evoked by the friction and vibration of various surface textures was investigated in this paper, human subjective tactile experiments was carried out through twenty volunteers testing for the tactile perception of surface properties, focused on three fabric samples (cotton, linen and silk) and three paper samples (photo paper, kraft paper and normal paper). The friction coefficient and frictional vibration signals were measured, and the brain response signals stimulated by the fingertip friction were simultaneously recorded and analyzed. The research works in this paper are beneficial for the deep understanding of tactile perception mechanism by fingertips, and building up a biomimetic model for smart robotic fingers.The research results indicate that the cerebral cortex responds sensitively to the variations in fingertips frictional force and vibration amplitudes. The human tactile perception is very sensitive to the fabric texture properties, which are described by the dimensions such as rough-smooth, coarse-fine and complex-uniform. The higher values of the fabric texture dimensions, the lower values in friction coefficient, and the more sensitive of tactile perception for the surface textures. Furthermore, it is noticed that the amplitude and distribution of brain signals response, i.e. ERPs (event related potentials) evoked by fingertip friction, is affected by the surface texture or friction coefficient. It is found that the response sensibility shown on the beginning peaks of ERP waveform has strong relation to the frictional stimulus on fingertip friction. The parameter of P3 latency (i.e. the third positive peak of ERP) is the distinct identifying information for the tactile perception of surface textures.

A contact mechanics interpretation of the duplex theory of tactile texture perception

The tactile perception of a surface texture originates from scanning a finger on the surface. This kind of sliding contact activates the mechanoreceptors located into the skin, allowing the brain to identify the object and to perceive information about the scanned surface. Perception is collected by mechanoreceptors either by sensing pressure or by sensing vibration: the first mechanism is typical of large spaced surface textures, while the second is necessary to perceive finer textures. These different behaviors are well known in the literature as the duplex perception mechanism. In the present paper a numerical model describing finger-surface scanning is introduced in order to investigate the relationship between contact induced vibrations and scanning conditions. The model has been validated by experimental comparisons in a previous work. The perception model is used to develop a parametric analysis of the vibration induced from the finger-surface scanning as a function of surface geometry, scanning speed and contact force. The proposed parametric analysis points out the minimum number of parameters needed to describe the tactile perception of a periodic texture, and it shows the tribological reasons for which duplex perception mechanism is an effective biological evolution towards optimal tactile perception.

EEG frequency tagging to explore the cortical activity related to the tactile exploration of natural textures.

When sliding our fingertip against a textured surface, complex vibrations are produced in the skin. It is increasingly recognised that the neural transduction and processing of these vibrations plays an important role in the dynamic tactile perception of textures. The aim of the present study was to develop a novel means to tag the cortical activity related to the processing of these vibrations, by periodically modulating the amplitude of texture exploration-induced vibrations such as to record a steady-state evoked potential (SS-EP). The EEG was recorded while the right index fingertip was scanned against four different textures using a constant exploration velocity. Amplitude modulation of the elicited vibrations was achieved by periodically modulating the force applied against the finger. Frequency analysis of the recorded EEG signals showed that modulation of the vibrations induced by the fingertip-texture interactions elicited an SS-EP at the frequency of modulation (3 Hz) as well as its second harmonic (6 Hz), maximal over parietal regions contralateral to the stimulated side. Textures generating stronger vibrations also generated SS-EPs of greater magnitude. Our results suggest that frequency tagging using SS-EPs can be used to isolate and explore the brain activity related to the tactile exploration of natural textures.