Monocular Cues in Depth Perception

Modified: 2013-12-30

Another set of depth cues is available to us with just one eye. (If you have two eyes, the monocular cues still work.) These cues are less powerful than retinal disparity, but they still provide us with solid depth-perception information.

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.

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