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

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 optimal and efficient. Such cues help us assess an object’s roughness, or stickiness, or as in this case, its softness. Leveraging empirical experiments combined with computational modeling of skin deformation, we develop a perceptual illusion for softness, or compliance, where small-compliant and large-stiff spheres are indiscriminable. The elasticity-curvature illusion, however, becomes readily discriminable when explored volitionally. This tactile illusion is unique because it naturally decouples proprioceptive cues from those involving identical, cutaneous contact attributes. Furthermore, the illusion sheds light into exactly how we explore soft objects, i.e., by volitionally controlling force, to optimally elicit and integrate proprioceptive cues amidst illusory cutaneous contact.