Depth perception
Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis and accommodation of the eye.
For objective comparisons of size, see Orders of magnitude (length).
Depth sensation is the corresponding term for non-human animals, since although it is known that they can sense the distance of an object, it is not known whether they perceive it in the same way that humans do.[1]
Depth perception arises from a variety of depth cues. These are typically classified into binocular cues and monocular cues. Binocular cues are based on the receipt of sensory information in three dimensions from both eyes and monocular cues can be observed with just one eye.[2][3] Binocular cues include retinal disparity, which exploits parallax and vergence. Stereopsis is made possible with binocular vision. Monocular cues include relative size (distant objects subtend smaller visual angles than near objects), texture gradient, occlusion, linear perspective, contrast differences, and motion parallax.[4]
Context-dependent interpretation of the size
Shots at different distances
The horizon line is at the height of the armrests.
View from a window on the 2nd floor of a house
Theories of evolution[edit]
The law of Newton–Müller–Gudden[edit]
Isaac Newton proposed that the optic nerve of humans and other primates has a specific architecture on its way from the eye to the brain. Nearly half of the fibres from the human retina project to the brain hemisphere on the same side as the eye from which they originate. That architecture is labelled hemi-decussation or ipsilateral (same sided) visual projections (IVP). In most other animals, these nerve fibres cross to the opposite side of the brain.
Bernhard von Gudden showed that the OC contains both crossed and uncrossed retinal fibers, and Ramon y Cajal[26] observed that the grade of hemidecussation differs between species.[27][26] Gordon Lynn Walls formalized a commonly accepted notion into the law of Newton–Müller–Gudden (NGM) saying: that the degree of optic fibre decussation in the optic chiasm is contrariwise related to the degree of frontal orientation of the optical axes of the eyes.[28] In other words, that the number of fibers that do not cross the midline is proportional to the size of the binocular visual field. However, an issue of the Newton–Müller–Gudden law is the considerable interspecific variation in IVP seen in non-mammalian species. That variation is unrelated to mode of life, taxonomic situation, and the overlap of visual fields.[29]
Thus, the general hypothesis was for long that the arrangement of nerve fibres in the optic chiasm in primates and humans has developed primarily to create accurate depth perception, stereopsis, or explicitly that the eyes observe an object from somewhat dissimilar angles and that this difference in angle assists the brain to evaluate the distance.
The eye-forelimb (EF) hypothesis[edit]
The eye-forelimb (EF) hypothesis suggests that the need for accurate eye-hand control was key in the evolution of stereopsis. According to the EF hypothesis, stereopsis is evolutionary spinoff from a more vital process: that the construction of the optic chiasm and the position of eyes (the degree of lateral or frontal direction) is shaped by evolution to help the animal to coordinate the limbs (hands, claws, wings or fins).[30]
The EF hypothesis postulates that it has a selective value to have short neural pathways between areas of the brain that receive visual information about the hand and the motor nuclei that control the coordination of the hand. The essence of the EF hypothesis is that evolutionary transformation in OC will affect the length and thereby speed of these neural pathways.[31]
Having the primate type of OC means that motor neurons controlling/executing let us say right hand movement, neurons receiving sensory e.g. tactile information about the right hand, and neurons obtaining visual information about the right hand, all will be situated in the same (left) brain hemisphere. The reverse is true for the left hand, the processing of visual, tactile information, and motor command – all of which takes place in the right hemisphere. Cats and arboreal (tree-climbing) marsupials have analogous arrangements (between 30 and 45% of IVP and forward-directed eyes). The result will be that visual info of their forelimbs reaches the proper (executing) hemisphere.
The evolution has resulted in small, and gradual fluctuations in the direction of the nerve pathways in the OC. This transformation can go in either direction.[30][32]
Snakes, cyclostomes and other animals that lack extremities have relatively many IVP. Notably these animals have no limbs (hands, paws, fins or wings) to direct. Besides, the left and right body parts of snakelike animals cannot move independently of each other. For example, if a snake coils clockwise, its left eye only sees the left body-part and in an anti-clock-wise position the same eye will see just the right body-part. For that reason, it is functional for snakes to have some IVP in the OC (Naked). Cyclostome descendants (in other words, most vertebrates) that due to evolution ceased to curl and, instead developed forelimbs would be favored by achieving completely crossed pathways as long as forelimbs were primarily occupied in a lateral direction. Reptiles such as snakes that lost their limbs, would gain by recollecting a cluster of uncrossed fibres in their evolution. That seems to have happened, providing further support for the EF hypothesis.[30][32]
Mice' paws are usually busy only in the lateral visual fields. So, it is in accordance with the EF hypothesis that mice have laterally situated eyes and very few crossings in the OC. The list from the animal kingdom supporting the EF hypothesis is long (BBE). The EF hypothesis applies to essentially all vertebrates while the NGM law and stereopsis hypothesis largely apply just to mammals. Even some mammals display important exceptions, e.g. dolphins have only uncrossed pathways although they are predators.[32]
It is a common suggestion that predatory animals generally have frontally-placed eyes since that permit them to evaluate the distance to prey, whereas preyed-upon animals have eyes in a lateral position, since that permit them to scan and detect the enemy in time. However, many predatory animals may also become prey, and several predators, for instance, the crocodile, have laterally situated eyes and no IVP at all. That OC architecture will provide short nerve connections and optimal eye control of the crocodile's front foot.[32]
Birds, usually have laterally situated eyes, in spite of that they manage to fly through e.g. a dense wood.
In conclusion, the EF hypothesis does not reject a significant role of stereopsis, but proposes that primates' superb depth perception (stereopsis) evolved to be in service of the hand; that the particular architecture of the primate visual system largely evolved to establish rapid neural pathways between neurons involved in hand coordination, assisting the hand in gripping the correct branch[31]
Most open-plain herbivores, especially hoofed grazers, lack binocular vision because they have their eyes on the sides of the head, providing a panoramic, almost 360°, view of the horizon – enabling them to notice the approach of predators from almost any direction. However, most predators have both eyes looking forwards, allowing binocular depth perception and helping them to judge distances when they pounce or swoop down onto their prey. Animals that spend a lot of time in trees take advantage of binocular vision in order to accurately judge distances when rapidly moving from branch to branch.
Matt Cartmill, a physical anthropologist and anatomist at Boston University, has criticized this theory, citing other arboreal species which lack binocular vision, such as squirrels and certain birds. Instead, he proposes a "Visual Predation Hypothesis," which argues that ancestral primates were insectivorous predators resembling tarsiers, subject to the same selection pressure for frontal vision as other predatory species. He also uses this hypothesis to account for the specialization of primate hands, which he suggests became adapted for grasping prey, somewhat like the way raptors employ their talons.
In robotics and computer vision[edit]
In robotics and computer vision, depth perception is often achieved using sensors such as RGBD cameras.[41]