Sighted animals, including humans, see in 2D and use various depth cues to convert what they see into 3D perception. There are three main ways this is accomplished:
- Light transport. This conveys depth perception via shading – light creates shadows provides cues about depth.
- Perspective: The image of an object changes in size as the object approaches or recedes relative to an observer. this is called “Looming.”
- Triangulation: this provides depth cues by comparing views of an object from different locations.
Animals use a combination of the three methods to perceive depth and distance. As for triangulation, it can occur by (a) stationary eye(s) observing moving objects, (b) by moving eyes observing stationary objects or (c) by eyes spaced apart transmitting slightly different scenes to the brain.
Some animals appear to only be able to use triangulation when an object moves or when their own eyes move. Other animals, humans being a prime example, can triangulate using the third method which is called stereopsis. The exact definition of stereopsis is “the ability to gain information about the 3D structure of visual scenes by comparing information collected separately and simultaneously from different lines of sight to the same region of space.”
Thus, our eyes, spaced about six inches apart, provide slightly different information to our brains which then uses that disparity to calculate and perceive depth. This is called lack of correspondence and takes a lot of brainpower. “If you hold a finger in front of your face and close first one eye then the other, you notice that the finger occludes different parts of the background in the two eyes. Natural scenes typically contain many such occluded regions, which by definition have no corresponding match in the other eye. Humans can make qualitative depth judgements based on the position of these monocularly occluded regions.”
It used to be thought that only primates have stereo vision but this has been refuted, asstudies have now led to the conclusion that “stereopsis has evolved independently at least four times: in mammals, birds, amphibians and insects.” But not all mammals, or other
Predator vs Prey Eye Placement
A commonly espoused theory is that predators have
While this explanation is elegant, it isn’t universal as not all predators have front-facing eyes, such as mongooses, tree shrews
Additionally, many types of animals with eyes on the sides of their heads have stereopsis vision, such as horses and praying mantises. Thus, the exact evolutionary mechanisms for front facing vs. side facing eyes is still up for debate.
Source: Note that much of this IFOD was based upon, and quotes, where are not attributed elsewhere, from the excellent research paperStereopsis in animals: evolution, function