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  1. Abstract

    Individual differences in tactile acuity have been correlated with age, gender and finger size, whereas the role of the skin's stiffness has been underexplored. Using an approach to image the 3‐D deformation of the skin surface during contact with transparent elastic objects, we evaluate a cohort of 40 young participants, who present a diverse range of finger size, skin stiffness and fingerprint ridge breadth. The results indicate that skin stiffness generally correlates with finger size, although individuals with relatively softer skin can better discriminate compliant objects. Analysis of contact at the skin surface reveals that softer skin generates more prominent patterns of deformation, in particular greater rates of change in contact area, which correlate with higher rates of perceptual discrimination of compliance, regardless of finger size. Moreover, upon applying hyaluronic acid to soften individuals’ skin, we observe immediate, marked and systematic changes in skin deformation and consequent improvements in perceptual acuity in differentiating compliance. Together, the combination of 3‐D imaging of the skin surface, biomechanics measurements, multivariate regression and clustering, and psychophysical experiments show that subtle distinctions in skin stiffness modulate the mechanical signalling of touch and shape individual differences in perceptual acuity.Key points

    Although declines in tactile acuity with ageing are a function of multiple factors, for younger people, the current working hypothesis has been that smaller fingers are better at informing perceptual discrimination because of a higher density of neural afferents.

    To decouple relative impacts on tactile acuity of skin properties of finger size, skin stiffness, and fingerprint ridge breadth, we combined 3‐D imaging of skin surface deformation, biomechanical measurements, multivariate regression and clustering, and psychophysics.

    The results indicate that skin stiffness generally correlates with finger size, although it more robustly correlates with and predicts an individual's perceptual acuity.

    In particular, more elastic skin generates higher rates of deformation, which correlate with perceptual discrimination, shown most dramatically by softening each participant's skin with hyaluronic acid.

    In refining the current working hypothesis, we show the skin's stiffness strongly shapes the signalling of touch and modulates individual differences in perceptual acuity.

     
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    Free, publicly-accessible full text available November 9, 2024
  2. While often focused on our visual system, adding touch to VR/AR environments can help render more immersive, richer user experiences. One important touch percept to render is compliance, or ‘softness.’ Herein, we evaluate the perceptibility of soft, magnetorheological elastomers (MRE) in bare-finger interactions. Such materials can be reprogrammed to distinct states of compliance. We fabricated MRE samples over elastic moduli from 23–173 kPa and measured that small 0.25 T magnetic fields increased modulus by 10–60 kPa. MRE interfaces less and more compliant than finger skin were evaluated in discrimination experiments with and without a magnetic field. The results indicate changes in modulus of 11 kPa are required to reach a 75% threshold of discrimination, although greater differences are required when an MRE’s elasticity is about the same as skin. The perceptual results with these magnetically-induced materials are similar to those with non-actuated, solid silicone-elastomers that mimic naturalistic interactions.

     
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    Free, publicly-accessible full text available October 20, 2024
  3. About half the U.S. adult population suffers from chronic neuromusculoskeletal pain. While its evaluation and treatment are widely addressed by therapies using soft tissue manipulation (STM), their efficacy is based upon clinician judgment. Robust biomarkers are needed to quantify the effects of STM on patient outcomes. Among noninvasive methods to quantify the mechanics of myofascial tissue, most are limited to small (<10 mm2), localized regions of interest. In contrast, we develop an approach to optically simultaneously measure a larger (~100 cm2) field of deformation at the skin surface. Biomarkers based on skin lateral mobility are derived to infer distinctions in myofascial tissue stiffness. In specific, three cameras track ink speckles whose fields of deformation and stretch are resolved with digital image correlation. Their ability to differentiate bilateral distinctions of the cervicothoracic region is evaluated with four participants, as a licensed clinician performs STM. The results indicate that the optically derived surface biomarkers can differentiate bilateral differences in skin mobility, with trend directions within a participant similar to measurements with an instrumented force probe. These findings preliminarily suggest skin surface measurements are capable of inferring underlying myofascial tissue stiffness, although further confirmation will require a larger, more diverse group of participants 
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    Free, publicly-accessible full text available July 10, 2024
  4. Pleasant brush therapies may benefit those with autism, trauma, and anxiety. While studies monitor brushing velocity, hand-delivery of brush strokes introduces variability. Detailed measurements of human-delivered brushing physics may help under-stand such variability and subsequent impact on receivers’ perceived pleasantness. Herein, we instrument a brush with multi-axis force and displacement sensors to measure their physics as 12 participants pleasantly stroke a receiver’s forearm. Algorithmic procedures identify skin contact, and define four stages of arrival, stroke, departure, and airtime between strokes. Torque magnitude, rather than force, is evaluated as a metric to minimize inertial noise, as it registers brush bend and orientation. Overall, the results of the naturally delivered brushing experiments indicate force and velocity values in the range of 0.4 N and 3-10 cm/s, in alignment with prior work. However, we observe significant variance between brushers across velocity, force, torque, and brushstroke length. Upon further analysis, torque and force measures are correlated, yet torque provides distinct information from velocity. In evaluating the receiver’s response to individual differences between brushers of the preliminary case study, higher pleasantness is tied to lower mean torque, and lower instantaneous variance over the stroke duration. Torque magnitude appears to complement velocity’s influence on perceived pleasant-ness. 
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    Free, publicly-accessible full text available July 10, 2024
  5. Our daily observations tell us that the delivery of social sentiments and emotions differs between strangers and romantic partners. This work explores how relationship status influences our delivery and perception of social touches and emotions, by evaluating the physics of contact interactions. In a study with human participants, strangers and romantically involved touchers delivered emotional messages to receivers’ forearms. Physical contact interactions were measured using a customized 3D tracking system. The results indicate that strangers and romantic receivers recognize emotional messages with similar accuracy, but with higher levels of valence and arousal between romantic partners. Further investigation into the contact interactions which underlie the higher levels of valence and arousal reveals that a toucher tunes their strategy with their romantic partner. For example, when stroking, romantic touchers use velocities preferential to C-tactile afferents, and maintain contact for longer durations with larger contact areas. Notwithstanding, while we show that relationship intimacy influences the deployment of touch strategies, such impact is relatively subtle compared to distinctions between gestures, emotional messages, and individual preferences. 
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    Free, publicly-accessible full text available July 1, 2024
  6. To discriminate the compliance of soft objects, we rely upon spatiotemporal cues in the mechanical deformation of the skin. However, we have few direct observations of skin deformation over time, in particular how its response differs with indentation velocities and depths, and thereby helps inform our perceptual judgments. To help fill this gap, we develop a 3D stereo imaging method to observe contact of the skin’s surface with transparent, compliant stimuli. Experiments with human-subjects, in passive touch, are conducted with stimuli varying in compliance, indentation depth, velocity, and time duration. The results indicate that contact durations greater than 0.4 s are perceptually discriminable. Moreover, compliant pairs delivered at higher velocities are more difficult to discriminate because they induce smaller differences in deformation. In a detailed quantification of the skin’s surface deformation, we find that several, independent cues aid perception. In particular, the rate of change of gross contact area best correlates with discriminability, across indentation velocities and compliances. However, cues associated with skin surface curvature and bulk force are also predictive, for stimuli more and less compliant than skin, respectively. These findings and detailed measurements seek to inform the design of haptic interfaces. 
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    Free, publicly-accessible full text available April 1, 2024
  7. Across a plethora of social situations, we touch others in natural and intuitive ways to share thoughts and emotions, such as tapping to get one’s attention or caressing to soothe one’s anxiety. A deeper understanding of these human-to-human interactions will require, in part, the precise measurement of skin-to-skin physical contact. Among prior efforts, each measurement approach exhibits certain constraints, e.g., motion trackers do not capture the precise shape of skin surfaces, while pressure sensors impede skin-to-skin contact. In contrast, this work develops an interference-free 3D visual tracking system using a depth camera to measure the contact attributes between the bare hand of a toucher and the forearm of a receiver. The toucher’s hand is tracked as a posed and positioned mesh by fitting a hand model to detected 3D hand joints, whereas a receiver’s forearm is extracted as a 3D surface updated upon repeated skin contact. Based on a contact model involving point clouds, the spatiotemporal changes of hand-to-forearm contact are decomposed as six, high-resolution, time-series contact attributes, i.e., contact area, indentation depth, absolute velocity, and three orthogonal velocity components, together with contact duration. To examine the system’s capabilities and limitations, two types of experiments were performed. First, to evaluate its ability to discern human touches, one person delivered cued social messages, e.g., happiness, anger, sympathy, to another person using their preferred gestures. The results indicated that messages and gestures, as well as the identities of the touchers, were readily discerned from their contact attributes. Second, the system’s spatiotemporal accuracy was validated against measurements from independent devices, including an electromagnetic motion tracker, sensorized pressure mat, and laser displacement sensor. While validated here in the context of social communication, this system is extendable to human touch interactions such as maternal care of infants and massage therapy. 
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  8. Individual differences in tactile acuity are observed within and between age cohorts. Such differences in acuity may be attributed to various sources, including aspects of nervous system, skin mechanics, finger size, cognitive and behavioral factors, etc. This work considers individual differences, within a younger cohort of participants, in discriminating compliant surfaces. These participants exhibit a range of finger size and stiffness. Interestingly, both their finger size and stiffness well predict their discriminative performance, where softer/smaller fingers outperform stiffer/larger fingers. Stereo imaging captured biomechanical cues in the skin’s deformation, including contact area and penetration depth, and their change rates. In those individuals with stiffer/larger fingers, who perceptually performed worse, we observed less distinguishable contact areas and eccentricities, compared to softer/smaller fingers. These particular cues well predicted individual differences observed in perceptual discrimination. In comparison, with two other cues, curvature and penetration depth, the imaging readily distinguished the compliant surfaces irrespective of finger stiffness/size, not aligned with discrimination. In conclusion, in passive touch, we find that individuals with softer/smaller fingers were better at discriminating compliances, and that certain skin deformation cues predict individual differences in perception. 
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