Seeing in 3D with just one eye: Stereopsis without binocular vision

Humans can perceive 3D structure with one eye, or even when viewing a picture of a 3D scene. However, the qualitative impression of three-dimensionality when viewing a real scene with both eyes is different. There is a compelling experience of immersive space and tangible solid objects. This phenomenon (stereopsis) is conventionally thought to be due to binocular disparity, or the geometrically equivalent information provided by motion parallax. Here we empirically establish, for the first time, the qualitative characteristics associated with stereopsis and show that they can occur in the absence of disparity or parallax. We show that the impression of stereopsis is a measurable attribute, and its absence when viewing pictorial images cannot be explained in terms of conflicting binocular cues or a reduced magnitude of perceived depth. These findings challenge conventional understanding and are discussed in the context of an alternative theory linking stereopsis to egocentric scaling of depth.