Depth perception is the ability to perceive three-dimensional space and to accurately judge distance. Without depth perception, we would be unable to drive a car, thread a needle, or simply navigate our way around the supermarket (Howard & Rogers, 2001). Research has found that depth perception is in part based on innate capacities and in part learned through experience (Witherington, 2005).
Psychologists Eleanor Gibson and Richard Walk (1960) tested the ability to perceive depth in 6- to 14-month-old infants by placing them on a visual cliff, a mechanism that gives the perception of a dangerous drop-off, in which infants can be safely tested for their perception of depth (Figure \(\PageIndex{1}\)). The infants were placed on one side of the “cliff,” while their mothers called to them from the other side. Gibson and Walk found that most infants either crawled away from the cliff or remained on the board and cried because they wanted to go to their mothers, but the infants perceived a chasm that they instinctively could not cross. Further research has found that even very young children who cannot yet crawl are fearful of heights (Campos et al., 1970). On the other hand, studies have also found that infants improve their hand-eye coordination as they learn to better grasp objects and as they gain more experience in crawling, indicating that depth perception is also learned (Adolph, 2000).
Figure \(\PageIndex{1}\): A mother urging her child from across the deep side of the visual cliff. Despite a transparent surface covering the cliff, the child hesitates to move forward. [“NIH visual cliff experiment” by U.S. National Institutes of Health/Wikimedia Commons is licensed under CC-BY-SA 4.0. From Gibson and Walk (1960). Copyright 1960 Nature Publishing Group.]
Depth perception is the result of our use of depth cues, messages from our bodies and the external environment that supply us with information about space and distance. Binocular depth cues are depth cues that are created by retinal image disparity—that is, the space between our eyes—and thus require the coordination of both eyes. One outcome of retinal disparity is that the images projected on each eye are slightly different from each other. The visual cortex automatically merges the two images into one, enabling us to perceive depth. Three-dimensional movies make use of retinal disparity by using 3-D glasses that the viewer wears to create a different image on each eye. The perceptual system quickly, easily, and unconsciously turns the disparity into 3-D.
An important binocular depth cue is convergence, the inward turning of our eyes that is required to focus on objects that are less than about 50 feet away from us. The visual cortex uses the size of the convergence angle between the eyes to judge the object’s distance. You will be able to feel your eyes converging if you slowly bring a finger closer to your nose while continuing to focus on it. When you close one eye, you no longer feel the tension—convergence is a binocular depth cue that requires both eyes to work.
The visual system also uses accommodation to help deter- mine depth. As the lens changes its curvature to focus on distant or close objects, information relayed from the muscles attached to the lens helps us determine an object’s distance. Accommodation is only effective at short viewing distances, however, so while it comes in handy when threading a needle or tying shoe- laces, it is far less effective when driving or playing sports.
Although the best cues to depth occur when both eyes work together, we are able to see depth even with one eye closed. Monocular depth cues are depth cues that help us perceive depth using only one eye (Sekuler & Blake, 2006). Some of the most important are summarized in Table \(\PageIndex{1}\).
Name
Description
Example
Image
Position
We tend to see objects higher up in our field of vision as farther away.
In the image, the fence posts at left appear farther away not only because they become smaller but also because they appear higher up in the picture.
Relative size
Assuming that the objects in a scene are the same size, smaller objects are perceived as farther away.
At right, the cars in the distance appear smaller than those nearer to us.
Linear perspective
Parallel lines appear to converge at a distance
We know that the tracks at right are parallel. When they appear closer together, we determine they are farther away.
Light and shadow
The eye receives more reflected light from objects that are closer to us. Normally, light comes from above, so darker images are in shadow.
We see the ovals at right as extended and indented according to their shadowing. If we invert the picture, the images will reverse.
Interposition
When one object overlaps another object, we view it as closer.
At right, because the orange star covers the blue bar, it is seen as closer than the yellow moon.
Aerial perspective
Objects that appear hazy, or that are covered with smog or dust, appear farther away.
The artist who painted the picture on the right used aerial perspective to make the distant hills more hazy and thus appear farther away.
