Contact of a Finger on Rigid Surfaces and Textiles: Friction Coefficient and Induced Vibrations

The tactile information about object surfaces is obtained through perceived contact stresses and frictioninduced vibrations generated by the relative motion between the fingertip and the touched object. The friction forces affect the skin stress-state distribution during surface scanning, while the sliding contact generates vibrations that propagate in the finger skin activating the receptors (mechanoreceptors) and allowing the brain to identify objects and perceive information about their properties. In this article, the friction coefficient between a real human finger and both rigid surfaces and fabrics is retrieved as a function of the contact parameters (load and scanning speed). Then, the analysis of the vibration spectra is carried out to investigate the features of the induced vibrations, measured on the fingernail, as a function of surface textures and contact parameters. While the friction coefficient measurements on rigid surfaces agree with empirical laws found in literature, the behaviour of the friction coefficient when touching a fabric is more complex, and is mainly the function of the textile constructional properties. Results show that frequency spectrum distribution, when touching a rigid surface, is mainly determined by the relative geometry of the two contact surfaces and by the contact parameters. On the contrary, when scanning a fabric, the structure and the deformation of the textile itself largely affect the spectrum of the induced vibration. Finally, some major features of the measured vibrations (frequency distribution and amplitude) are found to be representative of tactile perception compared to psychophysical and neurophysiologic works in literature

The interaction between motion and texture in the sense of touch

Besides providing information on elementary properties of objects, like texture, roughness, and softness, the sense of touch is also important in building a representation of object movement and the movement of our hands. Neural and behavioral studies shed light on the mechanisms and limits of our sense of touch in the perception of texture and motion, and of its role in the control of movement of our hands. The interplay between the geometrical and mechanical properties of the touched objects, such as shape and texture, the movement of the hand exploring the object, and the motion felt by touch, will be discussed in this article. Interestingly, the interaction between motion and textures can generate perceptual illusions in touch. For example, the orientation and the spacing of the texture elements on a static surface induces the illusion of surface motion when we move our hand on it or can elicit the perception of a curved trajectory during sliding, straight hand movements. In this work we present a multiperspective view that encompasses both the perceptual and the motor aspects, as well as the response of peripheral and central nerve structures, to analyze and better understand the complex mechanisms underpinning the tactile representation of texture and motion. Such a better understanding of the spatiotemporal features of the tactile stimulus can reveal novel transdisciplinary applications in neuroscience and haptics

Gamma Band Oscillation Response to Somatosensory Feedback Stimulation Schemes Constructed on Basis of Biphasic Neural Touch Representation

abstract: Prosthetic users abandon devices due to difficulties performing tasks without proper graded or interpretable feedback. The inability to adequately detect and correct error of the device leads to failure and frustration. In advanced prostheses, peripheral nerve stimulation can be used to deliver sensations, but standard schemes used in sensorized prosthetic systems induce percepts inconsistent with natural sensations, providing limited benefit. Recent uses of time varying stimulation strategies appear to produce more practical sensations, but without a clear path to pursue improvements. This dissertation examines the use of physiologically based stimulation strategies to elicit sensations that are more readily interpretable. A psychophysical experiment designed to investigate sensitivities to the discrimination of perturbation direction within precision grip suggests that perception is biomechanically referenced: increased sensitivities along the ulnar-radial axis align with potential anisotropic deformation of the finger pad, indicating somatosensation uses internal information rather than environmental. Contact-site and direction dependent deformation of the finger pad activates complimentary fast adapting and slow adapting mechanoreceptors, exhibiting parallel activity of the two associate temporal patterns: static and dynamic. The spectrum of temporal activity seen in somatosensory cortex can be explained by a combined representation of these distinct response dynamics, a phenomenon referred in this dissertation to “biphasic representation.” In a reach-to-precision-grasp task, neurons in somatosensory cortex were found to possess biphasic firing patterns in their responses to texture, orientation, and movement. Sensitivities seem to align with variable deformation and mechanoreceptor activity: movement and smooth texture responses align with potential fast adapting activation, non-movement and coarse texture responses align with potential increased slow adapting activation, and responses to orientation are conceptually consistent with coding of tangential load. Using evidence of biphasic representations’ association with perceptual priorities, gamma band phase locking is used to compare responses to peripheral nerve stimulation patterns and mechanical stimulation. Vibrotactile and punctate mechanical stimuli are used to represent the practical and impractical percepts commonly observed in peripheral nerve stimulation feedback. Standard patterns of constant parameters closely mimic impractical vibrotactile stimulation while biphasic patterns better mimic punctate stimulation and provide a platform to investigate intragrip dynamics representing contextual activation.Dissertation/ThesisDoctoral Dissertation Biomedical Engineering 201

Contact Force and Scanning Velocity during Active Roughness Perception

Haptic perception is bidirectionally related to exploratory movements, which means that exploration influences perception, but perception also influences exploration. We can optimize or change exploratory movements according to the perception and/or the task, consciously or unconsciously. This paper presents a psychophysical experiment on active roughness perception to investigate movement changes as the haptic task changes. Exerted normal force and scanning velocity are measured in different perceptual tasks (discrimination or identification) using rough and smooth stimuli. The results show that humans use a greater variation in contact force for the smooth stimuli than for the rough stimuli. Moreover, they use higher scanning velocities and shorter break times between stimuli in the discrimination task than in the identification task. Thus, in roughness perception humans spontaneously use different strategies that seem effective for the perceptual task and the stimuli. A control task, in which the participants just explore the stimuli without any perceptual objective, shows that humans use a smaller contact force and a lower scanning velocity for the rough stimuli than for the smooth stimuli. Possibly, these strategies are related to aversiveness while exploring stimuli

Limits on fine texture discrimination in humans and the role of friction

Science, Technology, Engineering and Mathematics (STEM) disciplines are challenging to blind and visually impaired (BVI) individuals. One of the possible reasons is the complexity in representing and understanding scientific content. Introducing tactile elements such as textures into existing Braille characters can potentially increase the information content of Braille and could likely simplify the complex notations. However, such a task requires a thorough understanding of the discrimination of textures through touch. The current dissertation focuses on: 1) Investigating the psychophysical factors involved in texture discrimination and, 2) Developing a testing system to assess friction induced skin damage from repetitive motion over textured surfaces. The tactile discrimination sensitivity for six fine textured non-patterned surfaces (fine-grit abrasive papers) was evaluated using a two-alternative forced choice technique. The surface roughness parameters and the coefficient of friction of the abrasive papers interacting with human skin were measured. Scanning electron microscopy images were used to observe the surface microstructure. The results suggest that differences in the mean spacing and the friction coefficients could be indicative of differentiability of fine textured samples. Three clearly differentiable textures identified from this study were used to investigate the effect of texture area on tactile discrimination sensitivity. A perception measurement experiment in combination with a friction measurement experiment was performed to understand the possible role of friction in touch-based texture discrimination. There was decrease in the discrimination ability with the decrease in the texture area. An elastomeric skin simulant with layered structure similar to that of human skin was constructed to replicate skin friction blisters. The relationship between applied normal load and number of cycles of reciprocating motion required for blistering was studied. Additionally, a crack-growth model was developed treating the skin simulant as an adhesive-bonded laminar composite. This study made it evident that complete profile of the tribological system is required to develop a skin simulant that can accurately predict skin friction damage. Based on the current literature, the role of surface topography and elastic properties of the human skin on friction was uncertain. Coefficient of friction of four probing surfaces, human index finger pad, silicone replicas of the finger with and without fingerprints, and a smooth silicone sphere, when sliding against fine grit abrasive papers were compared to identify these roles

An investigation of skin tribology phenomena involved in tactile communication through braille and its associated psychophysical response during task-based discrimination

Most individuals utilize all five senses, especially their sense of sight, to create a unique sensory experience depicting the surrounding environment. Unfortunately, individuals in the blind and visually impaired (BVI) community lack the sense of sight and rely primarily on tactile means to acquire valuable or potentially vital information, leading to the advent of tactile communication methods like braille. A key challenge in controlling the haptic experience of a surface is the lack of fundamental understanding of how various surface attributes, such as friction and texture, affect the tactile response. Oftentimes, braille users experience tactile confusion when scanning complex tactual codes such as tactile graphics or advanced mathematics commonly seen in the STEM fields, but coding standards and limitations in perceptive resolution reduce the opportunity for innovating or redesigning the language to aid the reader. This dissertation aims to address confusion in tactile information transfer by identifying, characterizing, and developing an understanding of the skin-surface contact interactions experienced during braille reading in order to promote innovations in surface engineering and material design that can improve existing tactile communication methods. The authors first propose a method to directly observe an individual’s cognitive response to tactile experiences through an “oddball paradigm” discrimination task using event-related potential (ERP) via electroencephalography (EEG), a technique that is common in visual and auditory psychological sensory studies. Results indicate that varying levels of friction and roughness from textured samples (i.e. sandpaper) elicit different magnitudes of cognitive activity, suggesting that this technique may prove to be a valuable tool in identifying and understanding the root causes of tactile confusion. The second aspect of the research seeks to characterize the fundamental frictional forces that occur during braille reading by investigating the loading interactions as the fingerpad slides over a single braille dot and then progressively increasing the complexity of the topographies (i.e. dot spacing, orientation, count). Derived from Greenwood and Tabor, the authors develop and propose a multi-term friction model that predicts the adhesion and deformation frictional effects of a single feature during skin-on-dot sliding, identifying deformation as the dominant friction mechanism when a soft body slides over a spherical geometry. Incorporating both computational modeling and large-scale tribological tests under displacement-controlled sliding further decomposes the frictional loading mechanisms showing that surface tension and compression are driven by the elastic material’s Poisson effect dependent on the bulk’s position with respect to the dot feature. Here, loads in the vertical direction are governed by bulk material deformation due to contact pressure and loads in the lateral direction are governed by bulk material deformation due to both contact pressure and frictional shear

Importance of spike timing in touch: an analogy with hearing?

Touch is often conceived as a spatial sense akin to vision. However, touch also involves the transduction and processing of signals that vary rapidly over time, inviting comparisons with hearing. In both sensory systems, first order afferents produce spiking responses that are temporally precise and the timing of their responses carries stimulus information. The precision and informativeness of spike timing in the two systems invites the possibility that both implement similar mechanisms to extract behaviorally relevant information from these precisely timed responses. Here, we explore the putative roles of spike timing in touch and hearing and discuss common mechanisms that may be involved in processing temporal spiking patterns

Global surface features contribute to human haptic roughness estimations

Previous studies have paid special attention to the relationship between local features (e.g., raised dots) and human roughness perception. However, the relationship between global features (e.g., curved surface) and haptic roughness perception is still unclear. In the present study, a series of roughness estimation experiments was performed to investigate how global features affect human roughness perception. In each experiment, participants were asked to estimate the roughness of a series of haptic stimuli that combined local features (raised dots) and global features (sinusoidal-like curves). Experiments were designed to reveal whether global features changed their haptic roughness estimation. Furthermore, the present study tested whether the exploration method (direct, indirect, and static) changed haptic roughness estimations and examined the contribution of global features to roughness estimations. The results showed that sinusoidal-like curved surfaces with small periods were perceived to be rougher than those with large periods, while the direction of finger movement and indirect exploration did not change this phenomenon. Furthermore, the influence of global features on roughness was modulated by local features, regardless of whether raised-dot surfaces or smooth surfaces were used. Taken together, these findings suggested that an object’s global features contribute to haptic roughness perceptions, while local features change the weight of the contribution that global features make to haptic roughness perceptions