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Binocular cues is a term usually applied to humans but that could rightfully apply to any animal with binocular vision, that is any animal whose eyes are set in such a way as to enable depth perception. These cues are signals to the visual processing system that allow and create the sensation of depth perception, primarily due to the way 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.
The 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 a little differently than the other eye. The human brain is able to combine the signals from each eye into one 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 in such a way as to allow us to perceive the shape and dimensions of objects.
The human vision system uses other binocular cues as well. Binocular convergence is the mechanism by which relative nearness and distance is perceived. This cue is derived from the fact that the visual receptors in our eyes are mostly centered in one location near the middle of the back of the eyeball. This means that to focus on an object, the eyes must both point at an object. As an object nears, the eyes must turn towards each other to a degree in order to stay focused on the object and must turn away from each other as the object moves farther away. The brain is able to process the information on the position of our eyes and to interpret relative distance, a key factor in depth perception involving objects that are relatively close to us.
A third binocular cue, which is related to the previous two is binocular accommodation. This cue is derived from the nature of the human eye and the fact that in order to focus on an object, the tiny muscles controlling the eye cause the lens, or cornea, to change shape. The brain is able to sense this and to use this information, along with that from other binocular cues, to quantify motion, position, and distance of multiple objects.
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