In the science of visual perception, stereopsis is the sensation that objects in space extend into depth, and that objects have different distances from each other. This sensation is much stronger than the suggestion of depth that is created by two-dimensional perspective. In humans, at least two mechanisms produce the sensation of stereopsis: binocular depth vision and (monocular) motion vision. In binocular depth vision, the sensation arises from processing differences in retinal images resulting from the two eyes looking from different, but similar, directions (binocular disparity). In motion vision, the sensation arises from processing motion information when the observer moves (e.g. optical flow, parallax). The sensation of stereopsis is similar in both cases. In research on depth vision, the term stereopsis is primarily used for binocular depth vision and not for the sensation of depth resulting from motion vision. Sometimes the term "relative depth" is used. This term emphasizes that it refers not to the distance to the observer, but to the mutual depth relationships of the perceived objects. If the meaning is clear from the context, the single word "depth" is also used instead of "relative depth." The word stereopsis comes from Greek stereós 'solid' and ópsis 'appearance, sight'. Together, these indicate seeing the outside of three-dimensional, "solid" objects. Binocular depth vision comes in two qualities: coarse stereopsis and fine stereopsis. Fine stereopsis plays a role in the recognition of shapes and objects, and coarse stereopsis in spatial localization.
Summary of research Research into binocular depth vision begins with Charles Wheatstone, who was the first to make a stereoscope. At the end of the 19th century, he was the first to demonstrate that horizontal disparity of vertical lines is sufficient to evoke a sensation of depth. Bela Julesz showed in the 20th century that the sensation also occurs with dots (random dot stereogram) and that depth vision precedes the perception of forms. Jodi Krol showed around the same time that a light transition (edge) is necessary, but that two corresponding edges of opposite contrast do not give a depth sensation. He also found that the depth of the surfaces between these edges is an interpretation. The left and right visual directions should be stimulated at approximately the same time, but this does not have to happen at exactly the same time. This is illustrated by the Puflrich illusion. Kenneth Ogle (1950) found that the quality of depth perception differs for small and large disparities and on this basis distinguishes different types of stereopsis. It is generally accepted that in the absence of a sensation of stereopsis the perceived image is usually seen in the plane of the horopter. John Foley (1972) describes that in exceptional cases the image can also appear slightly behind or in front of the horopter. Jodi Krol (1982) shows that the latter happens when the eyes are unconsciously directed slightly in front of or behind the intended fixation point due to certain reflexes and the fixation point is therefore not on the horopter. Finally, nerve cells have been found in the visual cortex that are tuned to a certain disparity. These cells form the basis for neural models for binocular depth vision and for the solution of the correspondence problem to be discussed. These cells are part of two neurophysiological mechanisms that are specialized in shape and object recognition and in spatial localization, respectively.
Binocular disparity
When attention is directed to a point F in space, automatic eye movements are performed, causing the eye to rotate and point F is mapped onto the point of the eye with which it can see most sharply, the fovea. The direction in which the eye then looks is called the principle ocular direction. Every other point in space is seen by the eye in a certain direction that can be expressed as the angle that this direction makes with the principle direction. This is called visual direction or simply direction. The directions in which each eye sees the same object are sometimes the same, but usually not. The difference in directions is called disparity. The separate article on directional vision explains how the brain combines the directions that each eye sees into a combined image with single images, double images and fused images that are apparently seen from a point in the middle between both eyes (cyclopean eye).
Horizontal disparity
The horizontal distance of approximately 6.5 cm between the two eyes ensures that points in space that are at different depths relative to the fixation point have a horizontal difference in direction. This difference is called horizontal disparity and is expressed as the difference between the angles α and β in the figure. In addition to horizontal disparity, there is also vertical disparity. This term indicates a vertical disalignment between the two eyes which can be caused by vertical eye movement or tilting the head. The latter is usually partially or completely corrected with automatically performed eye movements. Vertical disparities can also evoke a sense of depth in some cases. The terms horizontal and vertical disparity only make sense when the observer is upright. Another term that is sometimes used and that better describes the load is binocular disparity. Binocular disparity has many similarities with motion parallax and is therefore sometimes also called binocular parallax. Motion parallax also evokes a depth sensation, but it requires the observer to move or the observed objects to move relative to each other. Under neurophysiological mechanisms it can be read that motion parallax is possibly processed in the same system as coarse stereopsis.
Horopter
Points at the same depth as the fixation point lie on a circle through both eyes and the fixation point. This circle is called the horopter. Points on this circle project onto corresponding points in both eyes, i.e. onto points that look in the same direction in the left and right eye. We perceive the horopter as a kind of screen on which we see the world.
Crossed disparity The directions in both eyes of a point C that is closer than the horopter are seen crossed on this screen: the direction in which the direction of the left eye is seen is to the right of the direction of the right eye. This is called crossed disparity or negative disparity.
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