An elasticity-curvature illusion decouples cutaneous and proprioceptive cues in active exploration of soft objects

Abstract

Our sense of touch helps us encounter the richness of our natural world. Across a myriad of contexts and repetitions, we have learned to deploy certain exploratory movements in order to elicit perceptual cues that are salient and efficient. The task of identifying optimal exploration strategies and somatosensory cues that underlie our softness perception remains relevant and incomplete. Leveraging psychophysical evaluations combined with computational finite element modeling of skin contact mechanics, we investigate an illusion phenomenon in exploring softness; where small-compliant and large-stiff spheres are indiscriminable. By modulating contact interactions at the finger pad, we find this elasticity-curvature illusion is observable in passive touch, when the finger is constrained to be stationary and only cutaneous responses from mechanosensitive afferents are perceptible. However, these spheres become readily discriminable when explored volitionally with musculoskeletal proprioception available. We subsequently exploit this phenomenon to dissociate relative contributions from cutaneous and proprioceptive signals in encoding our percept of material softness. Our findings shed light on how we volitionally explore soft objects, i.e., by controlling surface contact force to optimally elicit and integrate proprioceptive inputs amidst indiscriminable cutaneous contact cues. Moreover, in passive touch, e.g., for touch-enabled displays grounded to the finger, we find those spheres are discriminable when rates of change in cutaneous contact are varied between the stimuli, to supplant proprioceptive feedback.

3D Visual Tracking to Quantify Physical Contact Interactions in Human-to-Human Touch

“…From the perspective of neurophysiology, changes in the skin’s mechanics caused by physical contact could elicit different responses of peripheral afferents ( Johnson, 2001 ; Yao and Wang, 2019 ; Xu et al, 2021a ). For example, the firing frequency of C-tactile afferents is associated with the stroking velocity in an inverted-U shape relationship ( Löken et al, 2009 ; Ackerley et al, 2014a ; Liljencrantz and Olausson, 2014 ).…”

“…The second experiment was designed to validate the effectiveness of the 3D visual tracking system in measuring controlled human movements against those from independent devices, including an electromagnetic motion tracker, sensorized pressure mat, and laser displacement sensor. These techniques are used commonly in haptics studies ( Silvera-Tawil et al, 2014 ; Jung et al, 2015 ; Hauser et al, 2019a ; Xu et al, 2020 ; Lo et al, 2021 ; Xu et al, 2021a ). In this experiment, the observed contact attributes were compared within controlled touch conditions, e.g., stroking in different directions at preset velocities, pressing with different parts of the hand varying in contact area, and tapping at different depth magnitudes.…”

An individual's skin stiffness predicts their tactile discrimination of compliance

“…This was performed because, when stimulus velocity is controlled in passive touch, what differs between the stimulus pair is the change in force rate. The rationale for this approach is that it that affords perceptual judgments based on the most minimal levels of skin deformation, especially with stimuli less compliant than skin (Bergmann Tiest & Kappers, 2009;Hauser & Gerling, 2018a;Kaim & Drewing, 2011;Lamotte, 2000), which conserves energy, and is tied with human volitional touch strategies (Xu & Gerling, 2020;Xu et al, 2021aXu et al, , 2021b.…”

Softness presentation by combining electro-tactile stimulation and force feedback

“…In preparation for reproducing realistic contact areas using the apparatus, the contact areas of two elastic samples were measured as a function of the amount of indentation applied to the samples by pressing them with a finger. Regarding the method of contact area measurement, we utilized an ink transfer method on the actual object to achieve precise rendering of the contact area on the real sample, which was based on previous studies that measured contact area on elastic objects (Hauser and Gerling, 2016;Hauser and Gerling, 2018;Xu, et al, 2020;Xu, et al, 2021).…”