Heterogeneous Layered Structures Can Modulate Human Softness Perception

TL;DR

This study investigates how layered heterogeneity in objects influences human softness perception, revealing that outer layers and subsurface layers significantly impact perceived softness in haptic interactions.

Contribution

It demonstrates that perceived softness is affected by the outermost layer and the distribution of compliance within layered structures, advancing understanding of haptic perception.

Findings

Perceived softness correlates with displacement under load.

Outermost layer stiffness has the greatest impact on softness perception.

More soft subsurface layers increase perceived softness, especially near the surface.

Abstract

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.