Apparent depth by the stereograms without binocular retinal disparity: Is Lau effect artifact?

We examined the idea that, when the Zollner figure in one eye and the principal lines in the figure in the other eye were presented, the perceived depth is due to “phenomenal” disparity. The phenomenal disparity is defined as apparent displacement between two illusory “oblique” principal lines in one eye and two “parallel” principal lines in the other. Observes were asked whether depth was seen or not at upper or lower part of the two perceived lines and, if seen, which line was closer. Seventeen observers showed that only 9% of their responses was consistent with the idea, although they reported depths for disprity stimuli. This suggests that the dichoptic presentation of such figures should not be considered as that requires the processing of binocular disparity.