Stereopsis

The word stereopsis comes from the Greek stereós meaning 'solid' and ópsis meaning 'appearance, sight'. Together, these indicate seeing the outside of three-dimensional, "solid" objects.

Therefore, stereopsis is the perception of depth produced by the reception in the brain of visual stimuly from the combination of both eyes. That's how human beens process that that objects in space are not flat but extend into depth, and that objects are at different distances from each other.

There are two mechanisms that produce the sensation of stereopsis:

1. Binocular depth vision, where the sensation comes from processing differences in retinal images resulting from the two eyes looking from different directions.
2. Monocular motion vision where the sensation arises from processing motion information when the observer moves.

The brain uses as many clues as possible to understand depth, so the sensation of stereopsis is similar in both cases. The main difference is that, with binocular depth motion, we don't need motion.

Binocular depth vision comes in two qualities:

1. Fine stereopsis plays a role in the recognition of shapes and objects.
2. Coarse stereopsis in spatial localization.