Perceived depth of curved lines in the presence of cyclovergence

Mitsudo [Mitsudo, H. (2007). Illusory depth induced by binocular torsional misalignment. Vision Research, 47, 1303-1314] reported a new depth illusion in which a static flat pattern consisting of curved lines appears stereoscopically stratified when viewed with eccentric elevated gaze. He proposed a hypothesis that the illusory depth produced with the curved-line stereogram might originate in a failure to counteract the effect of cyclovergence (i.e., the binocular misalignment of the eyes about the lines of sight). To test this hypothesis, we measured observers' cyclovergence with a video-based eye tracker while they were making a depth judgment of the curved-line stereogram. The observers' cyclovergence was induced by the elevation of gaze (Experiment 1) and by cyclorotated random dots (Experiment 2). The results showed that the magnitude of perceived depth correlated well with the measured cyclovergence for the curved-line stereogram. In contrast, when similar stimuli contained more dot-like elements, perceived depth was relatively independent of cyclovergence. These results support Mitsudo's hypothesis and are consistent with the notion that the stereo system requires unambiguous image cues-e.g., spatially distributed dot-like elements-to counteract the retinal cyclodisparity and produce perceived depth. A computational model was proposed to account for the results.