The effect of binocular eye position on stereopsis with double images1

:  We examined the hypothesis (Kaufman, 1976) that perceived relative depth between a stereoscopically fused stimulus and two nonfused stimuli (or double images) is due to vergence-induced disparity (or disparity between the fixation plane and the fused stimulus). Twenty-four observers reported their perceived depth between fused and nonfused stimuli with seven different vergence-induced disparities, the sizes of which were controlled using Nonius alignment. We found that the magnitude of perceived depth covaried with the vergence-induced disparity size and that it also depended on the binocular disparity size of the stereoscopic stimuli. This finding suggests that both vergence-induced disparity and binocular disparity can be depth cues in stereopsis with double images.