What monocular depth you can best explain why railroad tracks appear to come together in the distance?

Modified: 2020-03-18

Linear perspective is the monocular cue provided by the convergence of lines toward a single point of the horizon. Looking down a set of railroad tracks is a good example. We know that the tracks do not converge; they are parallel throughout, but when we look down the tracks, it appears that they converge to a single point. Renaissance artists discovered that they could reproduce this phenomenon on a two-dimensional canvas. Artists have been using that representation since, and "primitive art" is often described as art that does not use perspective.

Texture is the monocular cue provided by our proximity to an object. The closer one is to something, the more detail or texture one can see. For example, if I look at a wall from 20 feet away, it will look fairly smooth. But, as I approach the wall, I begin to see more and more detail or texture. When I am right up against the wall, I can see things that I could not see before when I was further away. That correlation between distance and texture is interpreted as a distance cue.

Haze is similar to texture, but is broader. Aerial perspective acts as a depth cue over long distances when we are outside. The same scene will provide quite different perceptual experiences depending on the presence or absence of haze. For example, you rent a condominium in Colorado, and you look out from your porch at the mountains. On a clear day (no haze), the mountains seem so close that you can touch them. Naively, you might even suggest that you walk to the mountain. But you are told it is 75 miles away. Had you first looked at the mountain on a hazy day, it would not have seemed so close.

Interposition is a depth cue derived from the overlapping position of objects. Objects that are in front of other objects may partially block our view of the rearmost object. Because we know what the object should look like, and because we see only part of it, we interpret the obstructed object as being farther away.

Relative size is another interesting monocular cue. Our learning contributes heavily to this cue. Over the years, we have learned that objects on our planet change size slowly, if at all. In other words, it is not the case that people shrink to half their size, or double their size in an eye blink. They do not. If they did, we would not be able to tell whether they were shrinking or moving away, or whether they were growing or coming closer. But, we do not have that problem. Instead, when the image of an object gets larger on the retina, we interpret that as a distance cue (closer). Conversely, when the image on the retina gets smaller, we interpret that as the object becoming farther away. So, if we are in a tall building looking down at some people, we do not say, "Look at those teensy people just outside the window." Rather, we say, "Look at those normal-sized people far away." Our knowledge of the world, then, dictates our perceptions.

Accommodation occurs with both eyes, but it is still a monocular cue, because one eye alone would give the same information as would both. Accommodation refers to the feedback we receive from the muscles in the eye that cause the lens to bulge or get thinner. Remember that as we focus on distant objects, our lens is made thinner, and as we focus on near objects, our lens is made thicker. Try the following. Hold a pencil in front of you at arm's length, cover one eye with your other hand. Now, gradually bring the pencil closer to your uncovered eye. You will find a point about a foot or less away from your eye where you can feel those eye muscles strain. Move your pencil back and forth with a six-inch distance. Can you feel the muscles working? That is accommodation and you have learned, perhaps unknowingly, to use it as a depth perception cue.

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 involve 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.

Relative Size

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 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 and Familiar Size

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.

Our familiarity with an object affects our perceptions of size and distance. While driving, your familiarity with the typical size of a car helps you determine how close or far away other vehicles on the road are from your location.

Elevation

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.

Texture Gradient

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.

Motion Parallax

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.

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.

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.

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.

Shading and Lighting

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.

Accommodation

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.

How Monocular Cues Are Used

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.

All of these 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.

A Word From Verywell

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.

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Additional Reading

• Coon D, Mitterer JO. Introduction to psychology: Gateways to mind and behavior. 13th ed. Wadsworth Cengage Learning; 2012.

• Goldstein EB. Sensation and perception. Cengage Learning; 2014.

By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.

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