Human interaction with viscoelastic matter

Research on human perception of materials that are either strictly liquid or solid in nature is abundant. However, most real materials with which we interact are viscoelastic, meaning their behavior while probing depends on probing force and time itself, giving a complex, multi-dimensional material response. As rheological properties of viscoelastic materials change with interaction, it is not clear what strategies humans use to interrogate such materials when they need to learn about their properties. As a first step in answering this question, we performed a series of haptic discrimination tasks with real viscoelastic materials. We show that humans use vastly individualized interrogating strategies to judge firmness. A hierarchical clustering analysis of individual behavior patterns reveals a complex landscape that defies easy categorization, while analyzing simple force-time features with linear mixed-effects models suggests that behavioral heterogeneity reflects stable individual differences in haptic exploration strategies with consistent personal signatures in force production patterns. Such diverse probing yields an estimated nonlinear viscoelastic response that varies in a complex and unexpected manner across the materials. The large dimensionality of the material response, combined with diverse interrogation behavior suggest that any comprehensive model of haptic material perception of viscoelastic materials must link three complex and interconnected areas: the multi-dimensional nonlinear viscoelastic material response, the adaptive modulation of probing behavior, and individual variations in sensorimotor performance.