Monocular Cues for Depth Perception

One way that we perceive depth in the world around us is through the use of what are known as monocular cues. These are clues that can be used for depth perception that involves using only one eye. If you try closing one eye, it might be more difficult to judge depth, but you're still able to detect how near or far objects are in relation to your position.

Depth perception allows us to perceive the world around us in three dimensions and to gauge the distance of objects from ourselves and from other objects. You can contrast monocular cues with binocular cues, which are those that require the use of both eyes.

These are some of the common monocular cues that we use to help perceive depth.

The relative size of an object serves as an important monocular cue for depth perception. It works like this: If two objects are roughly the same size, the object that looks the largest will be judged as being the closest to the observer. This applies to both three-dimensional scenes as well as two-dimensional images. Two objects on a piece of paper are the same distance away, yet size difference can make the larger object appear closer and the smaller object appear farther away.

Absolute size, or the actual size of an object, also contributes to the perception of depth. Smaller objects, even if we don't know exactly how big they are, will look farther away than a large object placed in the same spot.

An object position in relation to the horizon can also serve as a type of monocular cue. Objects located closer to the horizon tend to be perceived as farther away, while those that are farther from the horizon are usually seen as being closer.

Another essential monocular cue is the use of texture to gauge depth and distance. When you're looking at an object that extends into the distance, such as a grassy field, the texture becomes less and less apparent the farther it goes into the distance. As you look out over a scene, the objects in the foreground have a much more apparent texture. The asphalt of the road looks rough and bumpy. The vegetation in the field looks distinctive, and you can easily distinguish one plant from another.

As the scene recedes into the distance, these texture cues become less and less apparent. You cannot detect every single tree on the mountain in the distance. Instead, the vegetation covering the mountains simply looks like an indistinct patch of green color. These texture differences serve as important monocular cues for gauging the depth of objects that are both near and far.

The perception of moving objects can also serve as a monocular cue for depth. As you're moving, objects that are closer seem to zoom by faster than do objects in the distance. When you're riding in a car, for example, the nearby telephone poles rush by much faster than the trees in the distance. This visual clue allows you to perceive the fast moving objects in the foreground as closer than the slower moving objects off in the distance.

Objects that are farther away seem to be blurred or slightly hazy due to the atmosphere. As you look off into the horizon, closer objects seem more distinct while those in the distance might be obscured by dust, fog, or water vapor. Because objects in the distance tend to appear hazier, this cue tells us that blurry objects tend to be further away.

Parallel lines appear to meet as they travel into the distance. For example, the outer edges of a road seem to grow closer and closer until they appear to meet. The closer together the two lines are, the greater the distance will seem.

When one object overlaps another, the object that is partially obscured is perceived as being farther away. For example, if you see two figures standing in the distance and one figure overlaps and occludes the other one, you will perceive the occluded figure as being behind the non-occluded one. This allows you to judge how objects are placed in relation to one another and contributes to your experience of depth in the world around you.

The way light falls on objects and the amount of shading present can also be an important monocular cue. Objects that are darkened and obscured may appear further off in the distance than those that are brightly lit.

In order to focus on close-up objects, certain muscles in your eye contract, altering the shape of your lens. When looking at objects that are far away, these same muscles relax. This accommodation can serve as a monocular cue, even though we are often unaware of it.

When perceiving the world around us, many of these monocular cues work together to contribute to our experience of depth.

The corner of a building looks larger and more textured, causing it to seem closer. Objects further down the street appear smaller, so we judge them as being farther away. The parallel lines of the highway appear progressively closer as they disappear in the distance, and the mountains in the distance seem fuzzy and indistinct.

Monocular cues can play an important role in the detection of depth in the world around us. Unlike binocular cues, which involve the use of both eyes, monocular cues only require the use of one eye and can be presented in two dimensions. Because of this, many of these cues are used in art to create the illusion of depth in a two-dimensional space.