Cue integration of texture and elasticity induces roughness metamers in touch

Roughness perception is a fundamental dimension of touch that guides object recognition and manipulation. While perceived roughness is typically attributed to surface texture, realworld materials rarely vary in texture alone–they also differ in material properties such as elasticity. Whether material properties contribute to roughness perception, and how they might interact with surface cues, remains poorly understood. Here, we investigated how texture and elasticity jointly influence perceived roughness by parametrically varying both features within a Bayesian optimization discrimination task. Participants compared pairs of stimuli differing in stochastic surface roughness and material elasticity, under both direct and tool-mediated touch. This approach enabled us to estimate two-dimensional perceptual functions and identify haptic roughness metamers–physically distinct stimuli perceived as equally rough. These perceptual equivalences were mirrored in confidence ratings and varied systematically with the relative stiffness between the stimulus and the probing tool or finger, implicating contact-induced vibrations as a mediating factor. Our findings reveal how texture and elasticity cues jointly constrain roughness perception, demonstrating that perceived roughness emerges from the integration of multiple stimulus dimensions rather than surface properties alone. These findings offer practical implications for the design of haptic interfaces and prosthetics, where equivalent percepts may be achieved through different combinations of material and texture, and contribute to a broader understanding of cue integration in haptic perception.