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Monocular and binocular vision each serve a unique purpose. The difference between the two is the ability to judge distances or have depth perception. In binocular vision, two eyes work together to focus on a single point. The brain then processes that information to determine depth or distance to that point. Monocular vision exists in animals with eyes on opposite sides of the head, which prevents the two eyes from ever having a common focal point. It also exists in animals who may have formerly had binocular vision, but have lost vision in one eye.
Both monocular and binocular vision have evolved among different species. Each type of vision plays an important role in the interactions and interdependence of animals on one another. Monocular and binocular vision are not the most common forms of vision among animals, however. Insects, the most numerous of all species, often have compound vision.
Most carnivorous mammals and all birds of prey have binocular vision, in which two eyes both face forward. Binocular vision allows predators to hone in on prey using both eyes. They can then quickly and accurately determine striking distance. This ability is sometimes referred to as binocular acuity, and it is common to nearly all predators, including humans. A few predators that do not rely on binocular vision are bats, dolphins, and some whales, all of which use echo location to spot prey. Snakes use tongue flicking.
Prey animals like deer, who have eyes on opposite sides of the head, need a wide range of vision to inform them when a predator is near. Most prey animals have monocular vision. This type of vision does not prevent the animal from seeing an object in the distance, but it does prevent them from being able to tell exactly how far away the object is. Prey animals do not need the ability to precisely assess a predator’s location, but are better equipped to survive by having the increased field of view that monocular vision offers them. In fact, animals with monocular vision can see two completely different scenes out of each eye simultaneously, allowing for an increased chance of spotting a predator.
Acquired monocular vision occurs when an animal with binocular vision loses vision in one eye. Humans who have lost one eye are aware of the difficulties of living without the ability to determine depth perception. Driving an automobile or even stepping off the porch can be treacherous for people who have acquired monocular vision. Monocular and binocular vision both depend on the brain's ability to process the images coming in. Luckily, the brain is highly adaptable and able to compensate in cases of acquired monocular vision. Ultimately, it is the brain that drives all forms of vision.