A monocular cue is a visual cue for depth perception that only requires one eye. People with vision loss in one eye can still rely on these cues to navigate the world, although their depth perception will be impaired. Some examples include motion parallax, interposition, and linear perspective. Many of these cues can be seen in works of art, where artists rely on visual tricks to add depth and texture to visual scenes so that viewers feel like they are looking at a three-dimensional environment.
An example of a monocular signal is differential size and height. People rely on known data about the relative size of objects to orient themselves; a small car is interpreted as further away, for example, relying 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 of 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 of distant objects to move more slowly when people are in motion, is another one of the monocular cues people use to determine the position of objects in the environment. A person on a train can see a distant mountain for several minutes or hours, while a power pole flies by in seconds. That person knows that the mountain is further away.
Other monocular cues include texture gradient, where textures appear more detailed and accurate when closer together, along with atmospheric perspective. Distant objects can appear blurry, pale, or different due to atmospheric disruptions such as dust, and these visual distortions can provide clues about the distance of objects. Interposition is another monocular signal; the eyes assume that if one object is superimposed on another, the superimposed 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 binoculars, which require both eyes to fixate on objects in the environment.