Binocular beacons apply to animals with binocular vision, allowing depth perception. Binocular disparity, convergence, and accommodation are cues used by the human visual system to perceive shape, size, and distance of objects.
Binocular beacons is a term usually applied to humans but could justifiably apply to any animal with binocular vision, i.e. any animal whose eyes are fixed in such a way as to permit depth perception. These cues are cues to the visual processing system that enable and create the sensation of depth perception, primarily due to how our sense organs relay information to the brain and how that information is interpreted. Without binocular cues, we would lose most of our depth perception.
Most important of all binocular cues is binocular disparity, sometimes called binocular parallax. Binocular means having two sources of vision. Since humans have two eyes, the slight difference in their position causes each eye to perceive what it sees slightly differently from the other. The human brain is able to combine the signals from each eye into a single perception of what we are seeing. This difference in the position of our eyes and the resulting difference in how objects are perceived by each eye is known as binocular disparity and is interpreted by the brain to allow us to perceive the shape and size of objects.
The human vision system also uses other binocular cues. Binocular convergence is the mechanism by which relative proximity and distance are perceived. This signal comes from the fact that the visual receptors in our eyes are mostly centered in a location near the center of the back of the eyeball. This means that in order to focus on an object, both eyes must be on an object. As an object approaches, the eyes must turn towards each other to some extent to stay focused on the object, and they must move away from each other as the object moves away. The brain is able to process information about the position of our eyes and interpret relative distance, a key factor in depth perception involving objects that are relatively close to us.
A third binocular signal, related to the previous two, is binocular accommodation. This signal comes from the nature of the human eye and from the fact that to focus on an object, the tiny muscles that control the eye cause the lens, or cornea, to change shape. The brain is able to perceive this and use this information, together with that from other binocular signals, to quantify the movement, position and distance of multiple objects.
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