Stereopsis

In the science of vision, stereopsis is the sensation that objects in space are not flat but extend into depth, and that objects are at different distances from each other. This sensation is much stronger than the suggestion of depth that is created by two-dimensional perspective.

In humans, 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 directions (binocular disparity). And in motion vision, the sensation arises from processing motion information when the observer moves (optical flow, parallax). The sensation of stereopsis is similar in both cases. This is illustrated in the image below. The image alternates between the left and right images of a stereoscopic photograph. People closer to the image appear to move faster than those further away. This is perceived as depth perception: the subjects appear to be separated in depth. If the two images were viewed side by side in a stereoscope, the same 3D image would be perceived, but without motion.

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 the Greek stereós meaning 'solid' and ópsis meaning '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. There are two neurophysiological mechanisms present in the brain for this.

Binocular depth vision is a specialization of the ability to direction vision that is discussed in a separate article. Stereopsis is based on small differences (disparities) in the direction in which the left and right eyes see an object, which are the result of the fact that the two eyes are about 6.5 cm apart.

Conditions for the occurrence of binocular depth vision are that the visual directions in the left and right eyes have a certain similarity, are stimulated more or less at the same time, and the difference between the directions in the left and right eyes (horizontal disparity) is limited. The following describes in broad terms the knowledge about normal binocular depth vision in humans for the aspects mentioned, and explains the basic concepts that are necessary to understand the underlying source documents.

Research into binocular depth vision begins with Charles Wheatstone. 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.