Depth Perception

depth perception

Our ability to perceive spatial relationships in three-dimensional (3-D) space is known as depth perception. With depth perception, we can describe things as being in front, behind, above, below, or to the side of other things.

Our world is three-dimensional, so it makes sense that our mental representation of the world has three-dimensional properties. We use a variety of cues in a visual scene to establish our sense of depth.

binocular cues

Binocular cues rely on the use of both eyes. One example of a binocular depth cue is binocular disparity, the slightly different view of the world that each of our eyes receives.

To experience this slightly different view that results from binocular disparity, do this simple exercise: extend your arm fully and extend one of your fingers and focus on that finger. Now, close your left eye without moving your head, then open your left eye and close your right eye without moving your head. You will notice that your finger seems to shift as you alternate between the two eyes because of the slightly different view each eye has of your finger.

A 3-D movie works on the same principle: the special glasses you wear allow the two slightly different images projected onto the screen to be seen separately by your left and your right eye. As your brain processes these images, you have the illusion that the leaping animal or running person is coming right toward you.

Although we rely on binocular cues to experience depth in our 3-D world, we can also perceive depth in 2-D arrays.

monocular cues

Monocular cues require only one eye. Think about all the paintings and photographs you have seen. Generally, you pick up on depth in these images even though the visual stimulus is 2-D. When we do this, we are relying on a number of monocular cues.

If you think you can’t see depth with one eye, note that you don’t bump into things when using only one eye while walking—and, in fact, we have more monocular cues than binocular cues.

An example of a monocular cue would be what is known as linear perspective. Linear perspective refers to the fact that we perceive depth when we see two parallel lines that seem to converge in an image (Figure 1).

Some other monocular depth cues are: interposition, the relative size and closeness of images to the horizon, relative size, and the variation between light and shadow.

• Interposition: When one object partially covers another object, the partially covered object is perceived as being farther away than the object that is covering it.
• Relative size: When two objects are of similar shape but different sizes, the smaller object is perceived as being farther away than the larger one.
• Closeness to the horizon: Objects that are closer to the horizon are perceived as being farther away than objects that are higher in the visual field.
• Variation in light and shadow: When there are differences in light and shadow on objects, our brain uses this information to perceive depth and shape. Because nearby objects reflect more light to our eyes, a dimmer object will be perceived as further away.

The following video of anamorphic art demonstrates how we rely on these monocular cues to see depth, even when the depth is only imagined.

You can view the text alternative for “Amazing Anamorphic Illusions!” (opens in new window).

Stereoblindness

Bruce Bridgeman was born with an extreme case of lazy eye that resulted in him being stereoblind, or unable to respond to binocular cues of depth. He relied heavily on monocular depth cues, but he never had a true appreciation of the 3-D nature of the world around him. This all changed one night in 2012 while Bruce was seeing a movie with his wife.

The movie the couple was going to see was shot in 3-D, and even though he thought it was a waste of money, Bruce paid for the 3-D glasses when he purchased his ticket. As soon as the film began, Bruce put on the glasses and experienced something completely new. For the first time in his life he appreciated the true depth of the world around him. Remarkably, his ability to perceive depth persisted outside of the movie theater.

There are cells in the nervous system that respond to binocular depth cues. Normally, these cells require activation during early development in order to persist, so experts familiar with Bruce’s case (and others like his) assume that at some point in his development, Bruce must have experienced at least a fleeting moment of binocular vision. It was enough to ensure the survival of the cells in the visual system tuned to binocular cues. The mystery now is why it took Bruce nearly 70 years to have these cells activated (Peck, 2012).

Photos in this activity are from GlacierNPS, Alicia Nijdam, KlipschFan, scillystuff, rhondawebber (CC-BY-2.0)