Depth perception is the ability to visually determine the distance between objects, as well as to see in three dimensions.
The ability to perceive depth becomes operative relatively early in life. Research with infants revealed that by two months of age, visually and neuro-typical babies can perceive depth at close distances. Prior to that, they may be unable to do so, in part due to weak, undeveloped ocular muscles that do not permit use of binocular depth cues.
Of the monocular depth cues, motion parallax is used when the viewer is in motion and directly facing multiple objects. Parallax causes objects close to the viewer to appear to pass across the visual field more quickly than those farther away. Kinetic depth perception is used to judge the speed of moving objects when they are approaching or moving away from the viewer. Approaching objects appear to grow larger; receding objects seem to shrink as they get farther away. Linear perspective is the phenomenon that occurs when looking at parallel lines disappearing at the horizon line, such as railroad tracks. The nearer the parallel lines get to the horizon, the closer together they appear, until they seem to converge or touch. When objects are close to the horizon line, they appear to fade and wash out, losing contrast, while nearby objects appear more vivid and intensely colored. As objects move progressively farther away from the viewer, they also appear to lose texture and seem smoother. Humans also use the presence, angle, and size of an object's shadow to judge distance. When two distant objects are aligned, the viewer often sees the closer one covering (occluding) the one that is at the greater distance. Two additional monocular depth cues are peripheral vision and accommodation: The far outer edges of the visual field appear distorted or sharply curved because of the ways in which the eye's lens alters shape and curvature. The brain allows for this by ignoring the curve and perceiving objects accurately. When an object is extremely close to the viewer, it is seen clearly because the retina and brain accommodate and correct the visual image.
Binocular cues require the use of both eyes. Retinal disparity acknowledges that there is distance between the eyes, resulting in slightly different images being produced on each retina when looking at a single object. The brain combines the images and uses the differences to estimate size (height and width), distance, and depth. Convergence is a depth cue resulting from the movement of each retina required in order to focus on an object. The more the retinas need to turn inward (toward the nose), the closer the object is.
Animals such as primates that have eyes on the front of the face use binocular depth cues because the two eyes see two slightly disparate versions of the same subject. Animals with eyes on the sides of the head, like most birds, fish, and horses are less able to use binocular cues because the visual fields of the two eyes overlap minimally, so each eye sees a significantly different scene.
See also Binocular depth cues ; Figure-ground perception ; Perception; Vision .
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