Yung-Hao Yang¹, Shin’ya Nishida¹; ¹Cognitive Informatics Lab, Graduate School of Informatics, Kyoto University

To detect 3D non-rigid deformations of moving objects, the visual system must separate image motion caused by shape change from that caused by posture change. Although changing an object's material alters its internal shading pattern and flow, recent work (van Zuijlen et al., 2025 JOV) suggests that deformation detection remains stable across diverse optical materials. Whether this robustness relies on material-dependent motion cues (internal shading flow) or material-independent cues (e.g., external contour dynamics) remains debated. In Experiment 1, we measured deformation-detection thresholds for a rotating knot object rendered with four materials (dotted matte, glossy, mirror, and transparent), and with a silhouette version lacking internal flow. Silhouette performance matched that of the first three material conditions when a simple material-invariant cue (e.g., object height) was available in a side-viewing condition, but significantly worse than the three when this cue was reduced in a diagonal-viewing condition. This suggests that internal flow cues can help disentangle deformation from posture change. To see which internal flow cues are used, Experiment 2 measured deformation-detection performance for four original materials, iso-luminance contours (simulating internal flow of the material stimuli without shading), rims (contour outlines), and silhouettes. Only the diagonal viewing condition was used. Removing shading had only minor effects, and silhouette and rim thresholds were almost as low as those in the three material conditions. Experiment 2 indicates no effect of internal flows, unlike Experiment 1. A possible interpretation of this dissociation is that observers in the latter experiment may have learned to use potentially ambiguous external contour cues more effectively during extensive viewing of diverse versions of the same stimulus set. Overall, the results we obtained so far suggest that deformation perception may rely on internal flow cues, but their contributions are evident only when external contour cues are less reliable.

Acknowledgements: This work has been supported by JSPS Kakenhi JP20H05957.