Tactile Texture Perception Research Articles

Tactile roughness perception in the presence of olfactory and trigeminal stimulants.

Previous research has shown that odorants consistently evoke associations with textures and their tactile properties like smoothness and roughness. Also, it has been observed that olfaction can modulate tactile perception. We therefore hypothesized that tactile roughness perception may be biased towards the somatosensory connotation of an ambient odorant. We performed two experiments to test this hypothesis. In the first experiment, we investigated the influence of ambient chemosensory stimuli with different roughness connotations on tactile roughness perception. In addition to a pleasant odor with a connotation of softness (PEA), we also included a trigeminal stimulant with a rough, sharp or prickly connotation (Ethanol). We expected that—compared to a No-odorant control condition—tactile texture perception would be biased towards smoothness in the presence of PEA and towards roughness in the presence of Ethanol. However, our results show no significant interaction between chemosensory stimulation and perceived tactile surface roughness. It could be argued that ambient odors may be less effective in stimulating crossmodal associations, since they are by definition extraneous to the tactile stimuli. In an attempt to optimize the conditions for sensory integration, we therefore performed a second experiment in which the olfactory and tactile stimuli were presented in synchrony and in close spatial proximity. In addition, we included pleasant (Lemon) and unpleasant (Indole) odorants that are known to have the ability to affect tactile perception. We expected that tactile stimuli would be perceived as less rough when simultaneously presented with Lemon or PEA (both associated with softness) than when presented with Ethanol or Indole (odors that can be associated with roughness). Again, we found no significant main effect of chemosensory condition on perceived tactile roughness. We discuss the limitations of this study and we present suggestions for future research.

Natural scenes in tactile texture

Sensory systems are designed to extract behaviorally relevant information from the environment. In seeking to understand a sensory system, it is important to understand the environment within which it operates. In the present study, we seek to characterize the natural scenes of tactile texture perception. During tactile exploration complex high-frequency vibrations are elicited in the fingertip skin, and these vibrations are thought to carry information about the surface texture of manipulated objects. How these texture-elicited vibrations depend on surface microgeometry and on the biomechanical properties of the fingertip skin itself remains to be elucidated. Here we record skin vibrations, using a laser-Doppler vibrometer, as various textured surfaces are scanned across the finger. We find that the frequency composition of elicited vibrations is texture specific and highly repeatable. In fact, textures can be classified with high accuracy on the basis of the vibrations they elicit in the skin. As might be expected, some aspects of surface microgeometry are directly reflected in the skin vibrations. However, texture vibrations are also determined in part by fingerprint geometry. This mechanism enhances textural features that are too small to be resolved spatially, given the limited spatial resolution of the neural signal. We conclude that it is impossible to understand the neural basis of texture perception without first characterizing the skin vibrations that drive neural responses, given the complex dependence of skin vibrations on both surface microgeometry and fingertip biomechanics.

Spatial and temporal codes mediate the tactile perception of natural textures

When we run our fingers over the surface of an object, we acquire information about its microgeometry and material properties. Texture information is widely believed to be conveyed in spatial patterns of activation evoked across one of three populations of cutaneous mechanoreceptive afferents that innervate the fingertips. Here, we record the responses evoked in individual cutaneous afferents in Rhesus macaques as we scan a diverse set of natural textures across their fingertips using a custom-made rotating drum stimulator. We show that a spatial mechanism can only account for the processing of coarse textures. Information about most natural textures, however, is conveyed through precise temporal spiking patterns in afferent responses, driven by high-frequency skin vibrations elicited during scanning. Furthermore, these texture-specific spiking patterns predictably dilate or contract in time with changes in scanning speed; the systematic effect of speed on neuronal activity suggests that it can be reversed to achieve perceptual constancy across speeds. The proposed temporal coding mechanism involves converting the fine spatial structure of the surface into a temporal spiking pattern, shaped in part by the mechanical properties of the skin, and ascribes an additional function to vibration-sensitive mechanoreceptive afferents. This temporal mechanism complements the spatial one and greatly extends the range of tangible textures. We show that a combination of spatial and temporal mechanisms, mediated by all three populations of afferents, accounts for perceptual judgments of texture.

A Method for Extraction of Visual Expectation Effect on Surface Texture (A Case of Visual Expectation Effect on Tactile Roughness Perception of Plastic Texture)

To perceive a quality of a surface texture, people often switch different sensory modalities through an interaction with a product, e.g., we see and then touch a product surface. In such modality transitions, people expect tactile quality of a product from its appearance before touching. Present authors previously found that such visual expectation affects its tactile perception, and called the effect visual expectation effect. The authors demonstrated the existence of the effect from a results of a sensory evaluation experiment with combinations of visual and tactile texture samples that are synthesized using a half-mirror. However, the method used in the previous work had problems in qualification of the effect. To design texture quality with the visual expectation effect, engineers need to know the extent of the effect based on a physical value. In this paper, we proposed a quantification method for the visual expectation effect. The method maps tactile perception with a visual prediction onto a tactile scale without the visual expectation. In the method, evaluators compare samples synthesized using a half-mirror with a set of real samples used as a tactile scale, and select real samples that corresponds to the synthesized samples. With the method, we demonstrated a degree of visual expectation effect on tactile roughness perception of plastic textures.

Psychophysical dimensions of tactile perception of textures.

This paper reviews studies on the tactile dimensionality of physical properties of materials in order to determine a common structure for these dimensions. Based on the commonality found in a number of studies and known mechanisms for the perception of physical properties of textures, we conclude that tactile textures are composed of three prominent psychophysical dimensions that are perceived as roughness/smoothness, hardness/softness, and coldness/warmness. The roughness dimension may be divided into two dimensions: macro and fine roughness. Furthermore, it is reasonable to consider that a friction dimension that is related to the perception of moistness/dryness and stickiness/slipperiness exists. Thus, the five potential dimensions of tactile perception are macro and fine roughness, warmness/coldness, hardness/softness, and friction (moistness/dryness, stickiness/slipperiness). We also summarize methods such as psychological experiments and mathematical approaches for structuring tactile dimensions and their limitations.

Millisecond Precision Spike Timing Shapes Tactile Perception

In primates, the sense of touch has traditionally been considered to be a spatial modality, drawing an analogy to the visual system. In this view, stimuli are encoded in spatial patterns of activity over the sheet of receptors embedded in the skin. We propose that the spatial processing mode is complemented by a temporal one. Indeed, the transduction and processing of complex, high-frequency skin vibrations have been shown to play an important role in tactile texture perception, and the frequency composition of vibrations shapes the evoked percept. Mechanoreceptive afferents innervating the glabrous skin exhibit temporal patterning in their responses, but the importance and behavioral relevance of spike timing, particularly for naturalistic stimuli, remains to be elucidated. Based on neurophysiological recordings from Rhesus macaques, we show that spike timing conveys information about the frequency composition of skin vibrations, both for individual afferents and for afferent populations, and that the temporal fidelity varies across afferent class. Furthermore, the perception of skin vibrations, measured in human subjects, is better predicted when spike timing is taken into account, and the resolution that predicts perception best matches the optimal resolution of the respective afferent classes. In light of these results, the peripheral representation of complex skin vibrations draws a powerful analogy with the auditory and vibrissal systems.

Reproducibility of Tactile Assessments for Children with Unilateral Cerebral Palsy

ABSTRACTA systematic review identified tactile assessments used in children with cerebral palsy (CP), but their reproducibility is unknown. Sixteen children with unilateral CP and 31 typically developing children (TDC) were assessed 2–4 weeks apart. Test–retest percent agreements within one point for children with unilateral CP (and TDC) were Semmes-Weinstein monofilaments: 75% (90%); single-point localization: 69% (97%); static two-point discrimination: 93% (97%); and moving two-point discrimination: 87% (97%). Test–retest reliability for registration and unilateral spatial tactile perception tests was high in children with CP (intraclass correlation coefficient [ICC] = 0.79–0.96). Two tests demonstrated a learning effect for children with CP, double simultaneous and tactile texture perception. Stereognosis had a ceiling effect for TDC (ICC = 0) and variability for children with CP (% exact agreement = 47%–50%). The Semmes-Weinstein monofilaments, single-point localization, and both static and moving two-point discrimination are recommended for use in practice and research. Although recommended to provide a comprehensive assessment, the measures of double simultaneous, stereognosis, and tactile texture perception may not be responsive to change over time in children with unilateral CP.

Measurement of sound, vibration and friction between soft materials under light loads

Tactile perception of materials and surface texture involves friction under light normal loads and is fundamental to further advancing areas such as tactile sensing, haptic systems used in robotic gripping of sensitive objects, and characterization of products that range from fabrics to personal care products, such as lotions, on skin. This paper describes a new apparatus to measure friction simultaneously with dynamic quantities such as accelerations, forces, and sound pressures resulting from light sliding contact over a soft material, much like a finger lightly touching a soft material. The paper also introduces a novel friction and adhesion measurement method that can be particularly useful for soft materials and light normal loads.

Encoding electric signals by Gymnotus omarorum: Heuristic modeling of tuberous electroreceptor organs

The role of different substructures of electroreceptor organs in signal encoding was explored using a heuristic computational model. This model consists of four modules representing the pre-receptor structures, the transducer cells, the synapses and the afferent fiber, respectively. Simulations reproduced previously obtained experimental data. We showed that different electroreceptor types described in the literature can be qualitative modeled with the same set of equations by changing only two parameters, one affecting the filtering properties of the pre-receptor, and the other affecting the transducer module. We studied the responses of different electroreceptor types to natural stimuli using simulations derived from an experimentally-obtained database in which the fish were exposed to resistive or capacitive objects. Our results indicate that phase and frequency spectra are differentially encoded by different subpopulations of tuberous electroreceptors. A different type of receptor responses to the same input is a necessary condition for encoding a multidimensional space of stimuli as in the waveform of the EOD. Our simulation analysis suggested that the electroreceptive mosaic may perform a waveform analysis of electrosensory signals. As in color vision or tactile texture perception, a secondary attribute, “electric color” may be encoded as a parallel activity of various electroreceptor types.This article is part of a Special Issue entitled Neural Coding.

Effect of fingerprints orientation on skin vibrations during tactile exploration of textured surfaces

In humans, the tactile perception of fine textures is mediated by skin vibrations when scanning the surface with the fingertip. These vibrations are encoded by specific mechanoreceptors, Pacinian corpuscules (PCs), located about 2mm below the skin surface. In a recent article, we performed experiments using a biomimetic sensor which suggest that fingerprints (epidermal ridges) may play an important role in shaping the subcutaneous stress vibrations in a way which facilitates their processing by the PC channel. Here we further test this hypothesis by directly recording the modulations of the fingerpad/substrate friction force induced by scanning an actual fingertip across a textured surface. When the fingerprints are oriented perpendicular to the scanning direction, the spectrum of these modulations shows a pronounced maximum around the frequency v/λ, where v is the scanning velocity and λ the fingerprints period. This simple biomechanical result confirms the relevance of our previous finding for human touch.

The Role of Fingerprints in the Coding of Tactile Information Probed with a Biomimetic Sensor

In humans, the tactile perception of fine textures (spatial scale <200 micrometers) is mediated by skin vibrations generated as the finger scans the surface. To establish the relationship between texture characteristics and subcutaneous vibrations, a biomimetic tactile sensor has been designed whose dimensions match those of the fingertip. When the sensor surface is patterned with parallel ridges mimicking the fingerprints, the spectrum of vibrations elicited by randomly textured substrates is dominated by one frequency set by the ratio of the scanning speed to the interridge distance. For human touch, this frequency falls within the optimal range of sensitivity of Pacinian afferents, which mediate the coding of fine textures. Thus, fingerprints may perform spectral selection and amplification of tactile information that facilitate its processing by specific mechanoreceptors.

Factors Involved in Tactile Texture Perception Through Probes

An understanding of texture perception by robotic systems can be gained by examining human texture perception through a probe. As with texture perception in direct touch with the finger, texture perception by indirect touch by means of a probe is multidimensional, comprising rough, hard and sticky texture continua. In this study we describe variability among individual subjects in probe-mediated texture perception, and compare similarities and differences of texture perception between direct touch and indirect touch. The results show variability among subjects, as individual subjects may choose to rely on different texture dimensions, and examine the texture surface using different scanning velocities and contact forces. Despite the fact that the subjects use different scanning velocities and forces, they discriminate textures reliably and make subjective magnitude estimates consistently along a texture continuum when indirectly exploring texture surfaces with a probe. These data will be potentially useful in designing robotic sensory systems and understanding of sensory feedback, which is essential to teleoperations.

Vibrotactile adaptation impairs discrimination of fine, but not coarse, textures

The effect of vibrotactile adaptation on the ability to discriminate textured surfaces was examined in three experiments. The surfaces were rectilinear arrays of pyramids produced by etching of silicon wafers. Adaptation to 100-Hz vibration severely hampered discrimination of surfaces with spatial periods below 100 w m (Experiment 1), but had little effect on the discrimination of coarser textures (Experiment 2). To determine which vibrotactile channel—Rapidly Adapting or Pacinian—plays the larger role in mediating the discrimination of fine textures, widely separated adapting frequencies (10 and 250 Hz) were used in Experiment 3. The fact that high- but not low-frequency adaptation interfered with discrimination suggests that the Pacinian system contributes importantly to this ability. Taken as a whole, the results of this study strongly support the duplex theory of tactile texture perception, according to which different mechanisms—spatial and vibrotactile—mediate the perception of coarse and fine textures, respectively.

Imposed vibration influences perceived tactile smoothness.

According to the duplex theory of tactile texture perception, detection of cutaneous vibrations produced when the exploring finger moves across a surface contributes importantly to the perception of fine textures. If this is true, a vibrating surface should feel different from a stationary one. To test this prediction, experiments were conducted in which subjects examined two identical surfaces, one of which was surreptitiously made to vibrate, and judged which of the two was smoother. In experiment 1, the vibrating surface was less and less often judged smoother as the amplitude of (150 Hz) vibration increased. The effect was comparable in subjects who realized the surface was vibrating and those who did not. Experiment 2 showed that different frequencies (150-400 Hz) were equally effective in eliciting the effect when equated in sensation level (dB SL). The results suggest that vibrotaction contributes to texture perception, and that, at least within the Pacinian channel, it does so by means of an intensity code.

Evidence for the duplex theory of tactile texture perception.

Three experiments are reported bearing on Katz's hypothesis that tactile texture perception is mediated by vibrational cues in the case of fine textures and by spatial cues in the case of coarse textures. Psychophysical responses when abrasive surfaces moved across the skin were compared with those obtained during static touch, which does not provide vibrational cues. Experiment 1 used two-interval forced-choice procedures to measure discrimination of surfaces. Fine surfaces that were readily discriminated when moved across the skin became indistinguishable in the absence of movement; coarse surfaces, however, were equally discriminable in moving and stationary conditions. This was shown not to result from any inherently greater difficulty of fine-texture discrimination. Experiments 2 and 3 used free magnitude estimation to obtain a more comprehensive picture of the effect of movement on texture (roughness) perception. Without movement, perception was seriously degraded (the psychophysical magnitude function was flattened) for textures with element sizes below 100 microns; above this point, however, the elimination of movement produced an overall decrease in roughness, but not in the slope of the magnitude function. Thus, two components of stimulation (presumably vibrational and spatial) contribute to texture perception, as Katz maintained; mechanisms for responding to the latter appear to be engaged at texture element sizes down to 100 microns, a surprisingly small value.

The role of spatially selective attention in the tactile perception of texture

During manual sensing of object attributes like surface texture, the tactile receptor sheet is conjointly activated at numerous loci. What is the role of spatially selective attention in perception under these conditions? We show that the requirement for spatial focus of attention is minimal in the detection of an abrupt change in texture. In contrast, spatial attention may help in a variety of other tasks of comparable difficulty. These include detecting the absence of a texture change at one of many loci, distinguishing its direction, and discriminating between different textures. Attentional demand appears to vary along a continuum in these tasks, rather than being all-or-none.