Depth Perception and Constancy Phenomena.

Have you ever noticed how large a full moon appears just after it rises, and how much smaller it seems when it is right overhead? The next time you see a full moon that has just risen, look at it and note the size. Then turn your back, bend over, and look at the moon through your legs. You will perceive it as smaller! Then later the same night check the small overhead moon. Lie on your back and it becomes larger! This "moon illusion" has puzzled man for centuries; you will not find its explanation in any book. But it must depend, in some way, on me perceptual cues that contribute to size constancy.

As you read about perceptual cues and constancy phenomena in this module, try to answer these questions.


Understanding the human behavior of perceiving three dimensions is one of the most fundamental problems in visual space perception.

Perception of space involves several senses (vision, hearing, kinesthesis, and touch), the most important of which is vision. The retina of the eye, though virtually a flat surface that spreads falling light energies into two dimensions, apparently always "sees" three-dimensional space oven under very simple stimulus circumstances. Its image is curved to conform to the curvature of the eyeball. However, since the retinal curvature is similar to that of a bent photograph, no depth is added to the image because of this.

Images are perceived in the cerebral cortex, not in the retina of the eye

As transmitted to the brain, an image on the retina is not a picture; rather it is a pattern of nerve impulses, aroused by a light pattern that terminates in the visual area of the cerebral cortex. Through some activity of the occipital lobes of the cerebral cortex, human beings apparently perceive the external world in a three-dimensional manner that is correlated with the retinal-image pattern in some orderly manner.

Psychologists are particularly interested in the cues which enable people to perceive depth and distance. Stimulus patterns for arousing a depth experience occur when individuals are given specific cues. The cues may be monocular, effective when using one eye as well as two, or binocular, requiring the usage of both eyes. The cues may also be psychological, depending only on the visual image, or physiological, originating from the structure and movement of the eyes.

Some perceptual ability may be inherited rather than learned

The question of whether individuals learn or inherit the ability to interpret these cues is a persistent controversy among psychologists. Some psychologists maintain that human perceptions of space, at least to a certain extent, are sensed directly by visual cues beginning at birth. Research involving a visual cliff shows that, by the time infants are crawling, they already avoid tumbling off a visual dropoff. Some animals, born with open eyes and walking ability, also avoid visual cliffs, apparently without "learning." It is, of course, difficult to prove that this behavior is not learned, since both animals and human infants are exposed to an environment involving depth and space before they can be tested on the visual cliff.

Some psychologists contend that the process of defining and using visual cues results from our learning about the world through our other senses. People may learn to interpret these cues through associated tactile and motor processes. For example, an object looks round because it Feels" round, or an object looks nearer to someone because the muscular effort used to touch it is less than the effort expended reaching for an object that looks farther away (Goodenough, 1934). One psychologist has written that this type of learning is not an either-or proposition, because innate organization and acquired meaning may both be involved (Gregory, 1966).



In many cases depth can be perceived with one eye

In a human beings view of the three-dimensional environment, various surfaces are seen extending outward in depth, such as the floor below, the walls adjacent, and the ceiling above. When projected as flat images on the retina of the eye, these surfaces typically have "textures." According to J. J. Gibson (1950), a uniform texture of-the physical surface is projected on the retina so that the greater the distance away, the greater the density of texture in the retinal pattern. That is, the flat retinal pattern consists of gradients of texture that correspond to the real physical distance dimension. Gibson indicates that these texture gradients provide adequate stimulus for perception of depth.

Many of the visual cues used in perceiving depth or relative distance require only one eye; they are called monocular cues. In general six monocular cues are categorized as ``psychological," rather than physiological. This means that depth perception is achieved with cues pertaining to the visualpattem, rather than muscle adjustments of the eye or head movements. A flat visual pattern can appear to have depth if the objects within it look as though they are at varying distances from the observer. Some monocular cues are illustrated in Figure 17.

Figure 17 Some monocular cues for depth perception

Interposition is a distance cue in which one object's retinal image partially obscures another's so that the whole image appears closer to the observer.

The relative size of a retinal image is a distance cue making the larger of two objects appear closer to the observer. For example if an individual sees a particular species of bird as a small retinal image, when he knows the species to have a fairly constant size, he would judge it to be a greater distance from him than if the image were larger. However, with unfamiliar objects, the size of the retinal image has little value in achieving depth perception; the distance of a number of clouds is difficult to determine using only their size, since clouds can be of any size.

The relative height of an image is another cue if one object is lower than another it appears to be nearer the observer. Aerial perspective refers tooths clearness of distant details under different atmospheric conditions. If the actual distance of two objects is unknown, the object that is clearer and more detailed will appear closer to an observer. For example a mountain which stands out from the range surrounding it seems closer on a clear day than on a hazy or foggy day. The contrast of light and shadow indicates something of the shape and depth of objects.

Linear perspective is the decrease in size and separation of objects as they become more distant. This cue is often used by artists to represent distance; lines converging toward the horizon give an impression of increasing distance. Trees, telephone poles, and other objects seem to decrease in size, and railroad tracks appear to converge, as they recede into the distance.

Movement cues are often provided by the relative velocity of moving objects. A stationary observer, watching an object moving rapidly past him, usually judges that object to be closer to him than one moving more slowly. If the observer is moving, nearby objects seem to be moving in the opposite direction, while distant objects appear to move in the same direction.


In addition to these monocular "psychological" cues, one monocular cue accommodation is physiological. In an observer's eye the lens system changes its curvature in order to focus on the light rays coming from an object. If he focuses on a distant object, the lens flattens; it rounds out to focus on something nearby. Since this adjustment is accomplished by muscles and ligaments, it is possible that nerve impulses activated by it provide the brain with information about the relative distances of perceived objects. 116


Both of the two generally recognized binocular cues, convergence and retinal disparity, are physiological.

In some cases, depth perception requires the coordinated action of both eyes

In convergence, the eyes turn inward more when the observer is looking at a nearby object than when he is looking at something farther away. If the objects are far away the lines of sight become almost parallel and the amount of convergence is insignificant; for distances greater than 50 or 60 feet, convergence is not effective in aiding in the perception of depth. These reactions are controlled by muscles attached to the eyeball, and the nerve impulses generated in the muscles during this adjustment may also serve as distance cues.

Retinal disparity occurs because the two eyes have different locations in the head, thereby obtaining slightly different images of the same object. The perceiver seeks to harmonize these images to the greatest possible degree (though not consciously). However, the images may not fuse and will instead oscillate from one to another (retinal rivalry). The degree to which fusion, rather than rivalry, occurs depends on the consonance or dissonance (agreement or disagreement) of the two images.

Because this cue is a powerful one, scientists have succeeded in developing artificial impressions of depth by giving each eye a slightly different picture and making the eyes converge. The principle of stereoscopic vision and stereoscopic devices was developed by the English physicist Wheatstone in 1838.

Depth perception based on retinal disparity can be demonstrated in children as young as one year (Johnson and Beck, 1941). A doll is pictured as it would be seen by the right and left eyes. When the pictures are projected so that they coincide on the screen, an observer wearing suitable Polaroid lenses sees the doll as being between himself and the screen. A child in this setting, with similar lenses, reaches out for the doll.

Even though monocular and binocular cues have been described separately, the most effective use of them in perceiving depth is when they work simultaneously. However, the perception of depth is not a simple summation of the various cues. In general the perceptual process seems to weigh the various cues and derive a "reasonable" percept of spaceÄa process that is not conscious to the observer.


Constancy refers to the tendency to respond in a fixed way under conditions of stimulus change. We have already looked at some of the properties of stimulus objects that cause us to see them as we do. Now we will see why we respond as if these properties remained the same under different conditions.

Perceptual constancy phenomena are classified under the headings of size, shape, brightness, and color.


The tendency of objects to be perceived as constant in size when the size of the retinal image changes is called size constancy. Figure 18 shows two objects (the thumb and the building) that are perceived as being of different sizes, yet the retinal images are both the same size.


Figure 18. Size constancy

No one, however, would perceive the thumb and the building as the same size. Perceived size is sometimes considered to be a function of familiar size. This theory breaks down, however, in experiments on the perceived size of unfamiliar objects. Gibson (1950) has suggested texture density as a factor in size constancy. This refers to the fact that, since most objects have a fairly uniform texture, the density can tell us about their distance and size. The elements that surround an object and most of the depth perception cues also contribute to the phenomenon of size constancy.


When you stand directly in front of a window, the retinal image of the window is a rectangle. When you move. off to one side, however, the retinal image is a trapezoid. Despite the change in shape of the retinal image, the window is still perceived as a rectangle. The perceived shape of objects tends to remain the same, regardless of the direction from which they are viewed.

One researcher (Gibson, 1950) has suggested that slant may be a factor in shape constancy. He implies that the shape of an object is constant so long as the observer can distinguish the amount of tilt or slant. Not much research has been done, however, on shape constancy.



Light-colored things look light, even in dimly lit surroundings. Dark-colored things look dark, even on the brightest days. A piece of coal in the sunlight may reflect 10 times as much light as snow in the shade, yet the brightness does not affect our perception of them. One of the earliest explanations for this was that of familiarityÄwe know what colors coal and snow are, so the brightness or amount of light does not affect our perception. Experiments with unfamiliar objects, however, show that constancy is still maintained.

One experimenter (Wallach, 1948) held the illumination of a test surface constant and varied the illumination of surrounding areas. He found that the brightness of the test surface appeared to vary. It may be, then, that ratios rather than absolute intensities maintain brightness constancy.


As with brightness, we tend to perceive objects as being of a constant color. On the stage red and blue spotlights are often used to establish a mood. Yet we do not perceive the flesh of the actors or singers to be red or blue. Explanation in terms of a learned color, as with brightness, has proven inadequate to explain color constancy. Again the total visual field must be considered. Color constancy causes some strange effects. For example, a shadow falling on red light looks green, even though none of the light is of the wavelengths we see as green. One explanation of this is that the apparent hue of the grey shadow is determined at least in part by the surrounding red light.



Now test yourself without looking back.

1. A tin can is viewed as cylindrical, no matter what position it is in, because of

2. The fact that distant objects are hazy is the basis of which of the following cues?
a. Linear perspective
b. Aerial perspective
c. Relative size
d. Convergence

3. A cue for depth perception is physiological if:
a. it depends only on learning.
b. it depends only on the structure and movement of the eye.
c. it is a function of the organism's history.
d. (none of these)

4. Visual cues for depth perception are either:
a. physiological or anatomical.
b. physiological or psychological.
c. monocular or binocular.
d. constancies or inconstancies.

5. Retinal disparity is a__________________ (physiological /psychological),
_______________________ (monocular/binocular) cue for depth perception.

6. What monocular cue for depth perception is physiological?

7. Define color constancy.





Cues for Depth Perception

This diagram illustrates the depth perception cue of______________________________1

Write the type of cue after each of the following.

a. The clearness of details depends on atmospheric conditions._____________________________

b. An object seems to decrease in size as it gets farther away._________________________________

c. When two cars are traveling at the same speed, the one that seems to be going more slowly appears more distant.__________________________

d. The eyes turn inward more for near objects than they do for distant ones.__________________________

e. The separation of the two retinas gives two slightly different pictures.______________________________


This diagramillustrates the depth perception cue of ____________________________4

Check the monocular cue(s) below.
a. Interposition
b. Convergence
c. Relative Movement
d. Retinal Disparity
e. Accommodation


1 relative size
2 a. aerial perspective
b. linear perspective, relative size
c. relative movement
d. convergence
e. retinal disparity

3 a,c,e

4 interposition


Some depth perception cues depend only on the visual image that is present on the retina. Perspective and relative size are cues of this type. A cue that depends only on the visual image is:
a. physiological.
b. psychological.

Physiological arise from the structure and movement of the eyes. Some cues that depend on the structure and movement of the eyes are:
a. relative movement.
b. accommodation.
c. retinal disparity.
d. convergence.

Which monocular cues are physiological?
_____________________________________8 Which binocular cues are psychological?
__________________________________________ 7

Size and shape constancy depend on many of the visual cues for depth perception. Which of the following might depend on visual cues for depth perception?
a. The tendency to interpret nonsense words as having meaning
b. The tendency to perceive an object as having the same shape regardless of viewing angle
c. The tendency to perceive a man as the same size at a distance of six feet as at a distance of sixty feet


Match the following, referring to the chart at the beginning of the exercise, if necessary. 1) Shape constancy ____
2) Size constancy ______________
3) Brightness constancy_________
4) Color Constancy_____________

a. The tendency to perceive an object as of the same size regardless of viewing distance

b. The tendency to perceive an object as of the same brightness regardless of light and shadow

c. The tendency to perceive an object as of the same shape regardless of viewing angle

d. The tendency to perceive an object as of the same color regardless of illuminabon

______________________________________________ 4

Complete the following definitions:

a. Shape constancy is the tendency to perceive an object as of the same shape regardless of___________________________________

b. Size constancy is the tendency to perceive an object as the same size regardless of____________________________

c. Color constancy is the tendency to perceive an object as the same color regardless of_________________________

d. Brightness constancy is the tendency to perceive an object as the same brightness regardless of____________________________


1 b, c, d

2 a. viewing angle
b. distance
c. illumination
d. light and shadow

3 b

4 1)c 2) a 3) b. 4) d

5 perceive

6 b, c

7 none

8 accommodation




1. What depth perception cues are illustrated by each of the following?

2. All of the cues illustrated in question 1 are:
a. physiological.
b. psychological.
c. monocular.
d. binocular.

3. What is the difference between physiological and psychological cues?

4. A man seen at a distance of 100 yards is perceived as a normal-sized man. This is an example of

5. Which of the following cues are binocular?
a. Accommodation
b. Interposition
c. Convergence
d. Retinal disparity

6. Name three monocular, psychological cues for depth perception.

7. Define brightness constancy.


UNIT 6 Table of Contents