Illusory depth induced by binocular torsional misalignment.

This study reports a new depth illusion in which a static flat pattern appears stratified stereoscopically when viewed binocularly with an elevated gaze. Three psychophysical experiments measured perceived relative depth and fixational cyclodisparity (a rotation of one eye's view relative to the other eye's view about the line of sight) when flat patterns drawn with solid or dashed curved lines were fixated at various levels of gaze elevation. Experiments 1 and 2 showed that the patterns drawn with solid lines produced illusory depth only at large gaze elevations (downward and upward). Experiment 3 showed that the magnitude of the illusory depth was correlated with that of fixational cyclodisparity. These results suggest that the illusory depth originates in the binocular torsional misalignment generated by gaze elevation.