Depth perception

Depth perception

Monocular cues

Binocular cues

Auditory depth cues

Development of depth perception

Current research/future developments

Resources

Depth perception is the ability to see in three dimensions and to estimate the spatial distances of objects from oneself and from each other. Without depth perception we would be unable to tell how far objects are from us, and thus how far we would need to move to reach or avoid them.

Our ability to perceive depth includes space perception, or the ability to perceive the differential distances of objects in space. While researchers have discovered much about depth perception, numerous interesting questions remain. For instance, exactly how are we able to perceive the world in three dimensions when the images projected onto the retina are two-dimensional? And how much of a role does learning play in depth perception? While depth perception results primarily from our sense of vision, our sense of hearing also plays a role. Two broad classes of cues used to aid visual depth perception have been distinguished—the monocular (requiring only one eye), and the binocular (requiring both eyes working together).

Monocular cues

The following cues require only one eye for their perception. They provide information that helps us estimate spatial distances and to perceive in three dimensions.

Interposition

Interposition refers to objects appearing to partially block or overlap one another. When an object appears partially blocked by another, the fully visible object is perceived as being nearer, and this generally corresponds to reality.

Shading and lighting

In general, the nearer an object is to a light source, the brighter its surface appears to be, so that with groups of objects, darker objects appear farther away than brighter objects. And in looking at single objects, the farther parts of an object’s surface are from the source of light, the more shadowed and less bright they will appear. Varying shading and lighting then provide information about distances of objects from the source of light, and may serve as a cue to the distance of the object from the observer. In addition, some patterns of lighting and shading seem to provide cues about the shapes of objects.

Aerial perspective

Generally, objects having sharp and clear images appear nearer than objects with blurry or unclear images. This occurs because light is scattered or absorbed over long distances by particles in the atmosphere such as water vapor and dust which to a blurring of objects’ lines. This is why on clear days, very large objects such as mountains or buildings appear closer than when viewed on hazy days.

Elevation

This cue, sometimes referred to as “height in the plane” or “relative height,” describes how the horizon is seen as vertically higher than the foreground. Thus objects high in the visual field and closer to the horizon line are perceived as being farther away than objects lower in the visual field and farther away from the horizon line. Above the horizon line this relationship is reversed, so that above the horizon, objects that are lower and nearer to the horizon line appear farther away than those up higher and at a greater distance from the horizon line.

Texture gradients

Textures that vary in complexity and density are a characteristic of most object surfaces and they reflect light differentially. Generally, as distance increases, the size of elements making up surface texture appear smaller and the distance between the elements also appears to decrease with distance. Thus if one is looking at a field of grass, the blades of grass will appear smaller and arranged more closely together as their distance increases. Texture gradients also serve as depth and distance cues in groupings of different objects with different textures in the visual field, as when looking at a view of a city. Finally, abrupt changes in texture usually indicate an alteration in the direction of an object’s surface and its distance from the observer.

Linear perspective

Linear perspective is a depth cue based on the fact that as objects increase in distance from the observer their images on the retina are transformed so that their size and the space separating them decrease until the farthest objects meet at what is called the vanishing point. It is called the vanishing point because it is the point where objects get so small that they are no longer visible. In addition, physically parallel lines such as those seen in railroad tracks are perceived as coming closer together until they meet or converge at the vanishing point.

Motion parallax

Whenever our eyes move (due to eye movement alone, or head, or body movement) in relation to the spatial environment, objects at varying distances move at different rates relative to their position and distance from us. In other words, objects at different distances relative to the observer are perceived as moving at different speeds. Motionparallax refers to these relatively perceived object motions which we use as cues for the perception of distance and motion as we move through the environment.

As a rule, when the eyes move, objects close to the observer seem to move faster than objects farther away. In addition, more distant objects seem to move smaller distances than do nearer objects. Objects that are very far away, such as a bright star or the moon, seem to move at the exact same rate as the observer and in the same direction.

The amount and direction of movement are relative to the observer’s fixation point or where they are focusing. For instance, if you were traveling on a train and focusing on the middle of a large field you were passing, any objects closer to you than your fixation point would seem to be moving opposite to your direction of movement. In addition, those objects beyond your fixation point would appear to be moving in the same direction as you are moving. Motion parallax cues provide strong and precise distance and depth information to the observer.

Accommodation

Accommodation occurs when curvature of the eye lens changes differentially to form sharp retinal images of near and far objects. To focus on far objects the lens becomes relatively flat and to focus on nearer objects the lens becomes more curved. Changes in the lens shape are controlled by the ciliary muscles and it seems that feedback from alterations in ciliary muscle tension may furnish information about object distance.

Retinal size

As an object’s distance from the viewer increases, the size of its image on the retina becomes smaller. And, generally, in the absence of additional visual cues, larger objects are perceived as being closer than are smaller objects.

Familiarity

While not exactly a visual cue for perceiving space or depth as are the previous ones discussed, our familiarity with spatial characteristics of an object such as its size or shape due to experience with the object may contribute to estimates of distance and thus spatial perception. For instance, we know that most cars are taller or higher than children below the age of five, and thus in the absence of other relevant visual cues, a young child seen in front of a car who is taller than the car would be perceived as being closer than the car.

Binocular cues

Monocular cues certainly provide a great deal of spatial information, but depth perception also requires binocular functioning of the eyes, that is, both eyes working together in a coordinated fashion. Convergence and retinal disparity are binocular cues to depth perception.

Convergence

Convergence refers to the eyes’ disposition to rotate inward toward each other in a coordinated manner in order to focus effectively on nearby objects. With objects that are farther away, the eyes must move outward toward one’s temples. For objects further than approximately 20 ft (6 m) away no more changes in convergence occur and the eyes are essentially parallel with each other. It seems that feedback from changes in muscular tension required to cause convergence eye movements may provide information about depth or distance.

Retinal disparity and stereopsis

Retinal disparity refers to the small difference between the images projected on the two retinas when looking at an object or scene. This slight difference or disparity in retinal images serves as a binocular cue for the perception of depth. Retinal disparity is produced in humans (and in most higher vertebrates with two frontally directed eyes) by the separation of the eyes which causes the eyes to have different angles of objects or scenes. It is the foundation of stereoscopic vision.

Stereoscopic vision refers to the unified three-dimensional view of objects produced when the two different images are fused into one (binocular fusion). We still do not fully understand the mechanisms behind stereopsis but there is evidence that certain cells in some areas of the brain responsible for vision are specifically responsive to the specific type of retinal disparity involving slight horizontal differences in the two retinal images. This indicates that there may be other functionally specific cells in the brain that aid depth perception. In sum, it seems that we use numerous visual depth cues, binocular vision, and functionally specific cells in the nervous system to make accurate depth judgments.

Auditory depth cues

Auditory depth cues are used by everyone who can hear but are especially important for the blind. These include the relative loudness of familiar sounds, the amount of reverberation of sounds as in echoes, and certain characteristics of sounds unique to their frequency. For instance, higher frequency sounds are more easily absorbed by the atmosphere.

Development of depth perception

A theme running throughout the study of perception in general since the time of the ancient Greeks has been whether perceptual processes are learned (based on past experience) or innate (existent or potential at birth). In terms of depth perception, research using the visual cliff with animals and human infants too young to have had experience with depth perception indicates that humans and various species of animals are born with some innate abilities to perceive depth.

The visual cliff is one the most commonly used methods of assessing depth perception. It is an apparatus made up of a large box with a clear or see-through panel on top. One side of the box has a patterned surface placed immediately under the clear surface, and the other side has the same patterned surface placed at some distance below the clear surface. This latter side gives the appearance of a sharp drop-off or cliff. The subject of the study will be placed on the glass and consistent movement toward the shallow side is seen as an indication of depth perception ability. Newborn infants who cannot crawl commonly show much distress when placed face down over the “cliff” side.

KEY TERMS

Accommodation— Changes in the curvature of the eye lens to form sharp retinal images of near and far objects.

Aerial-perspective— A monocular visual cue referring to how objects with sharp and clear images appear nearer than objects with blurry or unclear images.

Binocular cues— Visual cues that require the coordinated use of both eyes.

Convergence— The tendency of the eyes to rotate toward each other in a coordinated manner in order to focus effectively on nearby objects.

Elevation— A monocular visual cue referring to an object’s placement in relation to the horizon.

Interposition— A monocular cue referring to how when objects appear to partially block or overlap with each other, the fully visible object is perceived as being nearer.

Linear perspective— A monocular depth cue involving the apparent convergence of parallel lines in the distance, as well as the perceived decrease in the size of objects and the space between them with increasing distance from the observer.

Monocular cues— Visual cues that one eye alone can perceive.

Motion parallax— The perception of objects moving at different speeds relative to their distance from the observer.

Retina— An extremely light-sensitive layer of cells at the back part of the eyeball. Images formed by the lens on the retina are carried to the brain by the optic nerve.

Stereoscopic vision— The unified three-dimensional view of objects produced when the two slightly different images of objects on the two retinas are fused into one.

Texture gradient— A monocular visual cue referring to how changes in an object’s perceived surface texture indicate distance from the observer and changes in direction of the object.

Research with animals raised without opportunities to see (for example if reared in the dark) sustain long-lasting deficits in their perceptual abilities. Indeed, such deprivation may even affect the weight and biochemistry of their brains. This research indicates that while humans and some animal species have innate mechanisms for depth perception, these innate abilities require visual experience in order to develop and become fully functioning. This research also suggests that animals and humans may have developmentally sensitive periods in which visual experience is necessary or permanent perceptual deficits may occur.

Current research/future developments

In sum, while environmental cues, binocular vision, and physiological aspects of the nervous system can account for many aspects of depth perception, numerous questions remain. Advances in understanding the physiological basis of vision have been great since the 1950s and this has greatly influenced research and theorizing in perception in general, and depth perception in particular. Researchers are eagerly looking at the structure of the nervous system to see if it might explain further aspects of depth perception. In particular, researchers continue to explore the possibility that additional fine tuned detector cells may exist that respond to specific visual stimuli. Finally, some psychologists have begun using certain basic principles of associative learning theory to explain a number of well known yet poorly understood elements of perceptual learning. Both of these approaches show great potential for furthering our understanding of many processes in perception.

Resources

BOOKS

Goldstein, E. Bruce. Sensation and Perception. Belmont, CA: Wadsworth Publishing, 2006.

Snowden, Robert, et al. Basic Vision: An Introduction to Visual Perception. New York: Oxford University Press, 2006.

Wolfe, Jeremy M., et al. Sensation and Perception. Washington, DC: Sinauer Associates, 2005.

Marie Doorey