Reduction of complex dynamic touch information to a single stable perceptual feature

Dynamic touch requires the perceptual system to extract stable material properties from complex, evolving signals. Here, we examine potential material property features for a constrained dynamic-touch regime: brief taps, in which contact-induced vibratory transients are prominent and sustained deformation cues are reduced. Using a spectral-energy compensation method, we conducted five psychophysical experiments in two degraded-feedback scenarios: soft finger interfaces, in which fingertip stiffness was reduced by an inflatable silicone bubble, and compliant-surface interactions, in which participants tapped foam surfaces. Participants made explicit hardness judgments in two-alternative forced-choice tasks and identified rendered materials in multi-alternative matching and classification tasks. Under these tapping-based conditions, judgments scaled systematically with total spectral energy and were largely independent of waveform family. When spectral energy and dominant-frequency cues were placed in conflict, classifications predominantly followed spectral energy. These results identify total spectral energy as a robust cue for hardness judgments and material identification from brief taps, rather than as a complete account of hardness perception across all exploratory procedures. The findings provide a compact rendering principle for haptic interfaces in which contact-triggered vibrotactile feedback is available.