Monocular cues for depth perception, such as motion parallax and linear perspective, can be used by people with vision loss in one eye. These cues can also be seen in artwork and include size and height differences, texture gradient, and atmospheric perspective. The brain uses these cues to interpret the two-dimensional image projected on the retinas.
A monocular cue is a visual cue for depth perception that requires only one eye. People with vision loss in one eye may still rely on these cues to navigate the world, although their depth perception will be impaired. Some examples include motion parallax, easing, and linear perspective. Many of these cues can be seen in artwork, where artists rely on visual tricks to add depth and texture to visual scenes so that viewers feel as if they are looking at a three-dimensional environment.
An example of a monocular splint is the difference in size and height. People rely on known data about the relative size of objects for orientation; a small car is interpreted as more distant, for example, based on what is known about the size of the car. Similarly, even if the exact size or height of an object is not known, surrounding objects can be used as a general reference. Two trees of the same type and shape but different sizes will be perceived at different distances assuming the larger tree is closer, for example.
Linear perspective, the tendency for distant lines to appear to converge, is an important monocular cue for depth perception. The position of objects relative to those lines can also be judged. The apparent convergence of train tracks on the horizon is one example. Motion parallax, the tendency for distant objects to move more slowly when people are in motion, is another monocular cue people use to determine the location of objects in the environment. A person on a train can see a distant mountain for several minutes or hours, while an electric pole whizzes by in seconds. That person knows the mountain is further away.
Other monocular cues include texture gradient, where textures appear more detailed and precise the closer they are, along with atmospheric perspective. Distant objects can appear blurry, pale, or otherwise different due to atmospheric disruptions such as dust, and these visual distortions can provide clues as to the distance of objects. Interposition is another monocular cue; the eyes assume that if one object overlaps another, the overlapping object is further away.
Each monocular signal can help the brain interpret the image projected on the retinas. Although the world is three-dimensional, the eyes actually see in two dimensions and the brain relies on visual cues to provide three-dimensional feedback. Other depth perception cues are binocular, requiring both eyes to fix the position of objects in the environment.
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