Stereopsis () is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image because they are in slightly different positions on one's head (left and right eyes). These positional differences are referred to as "horizontal disparities" or, more generally, "binocular disparities". Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real three-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".
The perception of depth and three-dimensional structure is, however, possible with information visible from one eye alone, such as differences in object size and motion parallax (differences in the image of an object over time with observer movement), though the impression of depth in these cases is often not as vivid as that obtained from binocular disparities. Therefore, the term stereopsis (or stereoscopic depth) can also refer specifically to the unique impression of depth associated with binocular vision (colloquially referred to as seeing "in 3D").
It has been suggested that the impression of "real" separation in depth is linked to the precision with which depth is derived, and that a conscious awareness of this precision – perceived as an impression of interactability and realness – may help guide the planning of motor action.
Distinctions
Coarse and fine stereopsis
There are two distinct aspects to stereopsis: coarse stereopsis and fine stereopsis, and provide depth information of different degree of spatial and temporal precision. Coarse stereopsis (also called gross stereopsis) appears to be used to judge stereoscopic motion in the periphery. It provides the sense of being immersed in one's surroundings and is therefore sometimes also referred to as qualitative stereopsis. Coarse stereopsis is important for orientation in space while moving, for example when descending a flight of stairs. Fine stereopsis is mainly based on static differences. It allows the individual to determine the depth of objects in the central visual area (Panum's fusional area) and is therefore also called quantitative stereopsis. It is typically measured in random-dot tests; persons having coarse but no fine stereopsis are often unable to perform on random-dot tests, also due to visual crowding which is based on interaction effects from adjacent visual contours. Fine stereopsis is important for fine-motor tasks such as threading a needle.
The stereopsis which an individual can achieve is limited by the level of visual acuity of the poorer eye. In particular, patients who have comparatively lower visual acuity tend to need relatively larger spatial frequencies to be present in the input images, else they cannot achieve stereopsis. Fine stereopsis requires both eyes to have a good visual acuity in order to detect small spatial differences, and is easily disrupted by early visual deprivation. There are indications that in the course of the development of the visual system in infants, coarse stereopsis may develop before fine stereopsis and that coarse stereopsis guides the vergence movements which are needed in order for fine stereopsis to develop in a subsequent stage. Furthermore, there are indications that coarse stereopsis is the mechanism that keeps the two eyes aligned after strabismus surgery.
Static and dynamic stimuli
It has also been suggested to distinguish between two different types of stereoscopic depth perception: static depth perception (or static stereo perception) and motion-in-depth perception (or stereo motion perception). Some individuals who have strabismus and show no depth perception using static stereotests (in particular, using Titmus tests, see this article's section on contour stereotests) do perceive motion in depth when tested using dynamic random dot stereograms. One study found the combination of motion stereopsis and no static stereopsis to be present only in exotropes, not in esotropes.