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This study investigates how heterogeneous layered stiffness affects human haptic softness perception using 3D-printed lattice structures with varying upper layer stiffness. Psychophysical experiments revealed that perceived softness is primarily driven by displacement under load, but also significantly modulated by the stiffness of the outermost layer and the distribution of softer subsurface layers up to a depth of three layers. The results demonstrate that human softness perception integrates depth-dependent compliance, challenging the assumption that it relies solely on overall stiffness.
Human perception of softness isn't just about overall stiffness; it's surprisingly sensitive to the depth-dependent arrangement of soft layers within a material.
Human softness perception in haptics has mainly been studied using mechanically homogeneous objects, despite the fact that many real-world objects exhibit heterogeneous layered structures with nonuniform stiffness. This study examined how layered heterogeneity modulates haptic softness perception. Sixteen lattice-structured stimuli were fabricated by 3D printing, with the stiffness of the upper four layers systematically varied while the bottom two layers remained fixed. Twenty-two participants evaluated the softness of the stimuli in a psychophysical task, and compression tests were conducted to quantify their mechanical properties. Perceived softness was significantly predicted by displacement under load, however, perceptual ranking did not fully coincide with the physical ranking. Linear mixed-effects analyses showed that the softness of the outermost layer had the greatest impact on the perceived softness. Perceived softness also increased as the number of soft subsurface layers increased, although this contribution decreased with depth. Layers 2 and 3 showed significant effects, whereas Layer 4 did not. These findings suggest that haptic softness perception depends not only on the overall stiffness but also on the depth-dependent distribution of compliance within layered structures.