What Are Monocular Cues?

An eye observing monocular cues
Angela Lumsden / Moment Open / Getty Images

How exactly do we perceive depth in the world around us? One way is through the use of what are known as monocular cues. These are clues that can be used for depth perception that involve using only one eye. If you try closing one eye, it might be more difficult to judge depth, but you are still able to detect depth with the use of one eye.

Depth perception allows us to perceive the world around us in three dimensions and to gauge the distance of object from ourselves and from other objects.

You can contrast monocular cues with binocular cues, which as you might expect, are those that require the use of both eyes. 

The following are six common monocular cues:

1. Relative Size

The relative size of object serve as an important monocular cue for depth perception. How does this work? 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 appears farther away.

2. Texture Gradient

Another essential monocular cue is the use of texture to gauge depth and distance. When you are 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 roughs 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 difference serve as important monocular cues for gauging the depth of objects that are both near and far.

3. Motion Parallax

The perception of moving object can also serve as a monocular cue for depth. As you are moving, objects that are closer seem to zoom by faster than do objects in the distance. When you are 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.

4. Aerial Perspective

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.

5. Linear Perspective

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.

6. Overlap (or Interposition)

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.

Monocular Cues In Action

When perceiving the word 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.

All of the monocular cues contribute to our total experience of the scene, our perception of depth and distance and our interpretation of our position in relation to other objects in the scene.

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