Research Project

An elasticity-curvature illusion decouples cutaneous and proprioceptive cues in softness exploration

A tactile illusion for softness perception

April 14, 2021

Chang Xu, Yuxiang Wang, Steven C. Hauser, and Gregory J. Gerling

How do we differentiate soft objects by touch, as we do in judging the ripeness of fruit? Our understanding of how material softness is perceptually encoded remains incomplete. This study investigates an illusion phenomenon that occurs in discriminating material compliances. We find that small-compliant and large-stiff spheres are naturally indistinguishable when pressed into a stationary finger, but readily discriminable when pressed upon. This phenomenon illuminates an interplay within our somatosensory system, in particular, between cutaneous responses from skin receptors and proprioceptive feedback traditionally tied to joint movements. It also reveals how our movements optimally evoke these cues to inform our percept of softness. Understanding how softness is encoded at skin contact is key to designing touch-enabled displays.

Pressing an object onto the finger does not reveal its softness, but pressing actively does, therefore our perception of softness is a product of both sensation and volition, and depends upon both afferents in skin and proprioception

Computational modeling of contact mechanics with compliant spheres

Spatial distributions of stress are simulated at a load of 2 N for contact with spheres of (A)10 kPa-4 mm, (B) 90 kPa-6 mm, (C) 90 kPa-8 mm, and (D) 10 kPa-8 mm respectively. The epidermal-dermal interface was indicated in (B) and was consistently modeled for all simulation conditions. Although the deformation of the spherical stimuli differs greatly from (A) to (C), the resultant stress distributions and surface deflection at the finger pad are nearly identical.

Experimental setup and biomechanical analysis of finger-stimulus contact

(A) For passive touch, the compliant stimulus is indented into the fixed finger pad by the motion stage. (B) For active touch, the designated stimulus is fixed and volitionally contacted by the index finger. (C) Contacted fingerprints are stamped and digitized for analysis. (D) The contact region is identified and color-thresholded. (E) Contact area is calculated based on the exterior outline and scaled pixels. (F) Non-distinct relationships of touch force and contact area are indeed observed in both passive and active touch between the illusion case spheres.

An active, force-control movement strategy is optimal for softness perception

(A) Psychophysical evaluations of illusion case spheres under different experimental conditions with all participants aggregated. The detection threshold is set as 75% for the same-different procedure. Points denote individual results. (B) Non-distinct force-rate cues are behaviorally applied for each illusion case sphere inactive exploration of compliances. (C)Significantly higher fingertip displacement is applied for the small-compliant sphere, as opposed to the harder spheres.


  • Xu, C., Wang, Y., and Gerling, G.J., An elasticity-curvature illusion decouples cutaneous and proprioceptive cues in active exploration of soft objects [DOI]. PLoS Computational Biology, 2021.
  • Xu, C., Wang, Y., Hauser, S.C., and Gerling, G.J., In the tactile discrimination of compliance, perceptual cues in addition to contact area are required [DOI]. Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 2018.
  • Wang, Y. and Gerling, G.J., Computational modeling reinforces that proprioceptive cues may augment compliance discrimination when elasticity is decoupled from radius of curvature [DOI]. EuroHaptics 2014: Haptics: Neuroscience, Devices, Modeling, and Applications, 2014.
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