Binocular depth mixture: an artefact of eye vergence?

Several studies by Foley et al. suggest that for particular stereoscopic stimulus conditions perceived depth depends on the luminance ratios of the stimuli. Perceived depth appears "shifted" in the direction of the most luminant stimulus. This finding has been interpreted as contradictory evidence against modern neuronal network or algorithmic models which are extensions or variants of so-called projection theories of stereopsis. The present study shows that this interpretation lacks a solid basis and that the apparent depth shift effect probably qualifies as an artefact caused by systematic fixation errors (fixation disparities). With a psychophysical methodology which aims at preventing the occurrence of systematic fixation errors no depth shift effect is found.