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1. Individual Performance in Compliance Discrimination is Constrained by Skin Mechanics but Improved under Active Control

   https://doi.org/10.1109/WHC49131.2021.9517269  

   Xu, Chang; Wang, Yuxiang; Gerling, Gregory J. (July 2021, IEEE World Haptics Conference)

   Tactile acuity differs between individuals, likely a function of several interrelated factors. The extent of the impact of skin mechanics on individual differences is unclear. Herein, we investigate if differences in skin elasticity between individuals impact their ability to distinguish compliant spheres near limits of discriminability. After characterizing hyperelastic material properties of their skin in compression, the participants were asked to discriminate spheres varying in elasticity and curvature, which generate non-distinct cutaneous cues. Simultaneous biomechanical measurements were used to dissociate the relative contributions from skin mechanics and volitional movements in modulating individuals’ tactile sensitivity. The results indicate that, in passive touch, individuals with softer skin exhibit larger gross contact areas and higher perceptual acuity. In contrast, in active touch, where exploratory movements are behaviorally controlled, individuals with harder skin evoke relatively larger gross contact areas, which improve and compensate for deficits in their acuity as observed in passive touch. Indeed, these participants exhibit active control of their fingertip movements that improves their acuity, amidst the inherent constraints of their less elastic finger pad skin. 

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2. Faster Indentation Influences Skin Deformation To Reduce Tactile Discriminability of Compliant Objects

   https://doi.org/10.1109/TOH.2023.3253256  

   Li, Bingxu; Hauser, Steven C.; Gerling, Gregory J. (April 2023, IEEE Transactions on Haptics)

   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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3. Individual differences impacting skin deformation and tactile discrimination with compliant elastic surfaces

   https://doi.org/10.1109/WHC49131.2021.9517222  

   Li, Bingxu; Gerling, Gregory J. (July 2021, IEEE World Haptics Conference)

   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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4. Time-dependent Cues Encode the Minimum Exploration Time in Discriminating Naturalistic Compliances

   https://doi.org/10.1109/HAPTICS45997.2020.ras.HAP20.7.ec43f6a7  

   Xu, Chang; Gerling, Gregory J. (March 2020, 2020 IEEE Haptics Symposium (HAPTICS))

   Our perception of compliance is informed by multi-dimensional tactile cues. Compared with stationary cues at terminal contact, time-dependent cues may afford optimal efficiency, speed, and fidelity. In this work, we investigate strategies by which temporal cues may encode compliances by modulating our exploration time. Two potential perceptual strategies are considered, inspired by memory representations within and between explorations. For either strategy, we introduce a unique computational approach. First, a curve similarity analysis, of accumulating touch force between sequentially explored compliances, generates a minimum time for discrimination. Second, a Kalman filtering approach derives a recognition time from progressive integration of stiffness estimates over time within a single exploration. Human-subjects experiments are conducted for both single finger touch and pinch grasp. The results indicate that for either strategy, by employing a more natural pinch grasp, time-dependent cues afford greater efficiency by reducing the exploration time, especially for harder objects. Moreover, compared to single finger touch, pinch grasp improves discrimination rates in judging plum ripeness. The time-dependent strategies as defined here appear promising, and may tie with the time-scales over which we make perceptual judgments. 

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5. Perceptibility of programmable softness displays using magnetorheological elastomers

   https://doi.org/10.1177/21695067231193685  

   Li, Bingxu; Lee, Jaeyeon; Gerling, Gregory J. (October 2023, Proceedings of the Human Factors and Ergonomics Society Annual Meeting)

   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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