How do we perceive a three-dimensional world when the picture on the retina is a two-dimensional one?

The visual world is not composed of figures and objects organized two-dimensionally on a plane surface. Visual space is three-dimensional. Objects have depth; they are solid, and are located phenomenally as well as physically at various distances from the individual. A solid object forms a two-dimensional image on the retina of our eye, as our receptor cells are not capable of locating a third dimension.

The question is; how do we perceive a three-dimensional world when the picture on the retina is a two-dimensional one?

Some psychologists are of the opinion that as human beings we have an innate tendency to see depth and distance. The visual cliff experiments and experiments with visually handicapped children suggest that depth and distance perception is to some extent innate in character. However, it does not adequately explain the phenomenon of depth and distance perception. We make use of many different cues in forming our judgments about depth and distance perception. These cues may be monocular or binocular depending upon whether they can be seen with one eye or require the use of both the eyes.

The monocular cues have been so widely used by artists that they have become known as pictorial cues. The monocular cues to depth and distance include the following:

Monocular Cues

1. Proximal Size:

Proximal size refers to the size of the image on the retina in relation to the distance of the object. It proposes that all other factors remaining constant if the size of the image on the retina is larger, the object looks nearer, and vice versa. In other words, as the object moves farther and farther, the retinal images become smaller and smaller. The object is perceived as farther away with the decrement in the size of the retinal image.

Proximal size is one of the factors of monocular depth perception. It does not hold true in all cases. For example, what happens to our retinal images of a person whom we know well when he is 100 meters away from us and when he is very nearer to us. The retinal images of the person must be different when he is at different distances. However, we do not perceive any change in the size of the person. Size constancy is maintained here. Because of size constancy, which we develop by our interactions and communications with various objects, the objects maintain their size even if their retinal images vary.

2. Brightness:

It proposes that the brighter the object, the nearer it appears. Thus, a distant mountain looks farther away in a hazy day than on a right day. It is because haziness in the atmosphere blurs fine details from the view of the observer. Ordinarily if we see the fine details of an object, we [erceive the object as nearer than in which we fail to see the fine details. In ether words, nearer objects look clearer than distant objects, and this awareness provides clues for distance perception.

3. Shading:

Shadowing also provides information about depth. It is based on the fact that opaque objects block light and produce shadows. Shadows and highlights give us information about the object's three-dimensional shapes and about their relationship to the source of light.

4. Texture Gradient:

It is a monocular cue for depth perception based on the fact that the closer objects appear to have rougher or more detailed surfaces. Gradient is a continuous change in something, a change without abrupt transitions. In some situations, this gradation in texture in the visual field may re used as a cue for depth perception (Gibson, 1950). For example, when we look at a paddy field, we can see the details of the trees nearer to us. But as we look towards the distant field, it becomes fainter and no details of the field are visible. The continuous gradation of texture gives clues to the eye and the brain that can be used to experience depth perception.

5. Linear perspective:

It is a monocular cue for depth perception based on the fact that the distance separating the images of far away objects appear to be smaller than the distance separating closer objects. For example, if we stand near rail tracks and look at a distance, the gap between tracks would appear smaller, and tracks would seem to run closer. This perspective provides clue for depth and distance perception.

6. Interposition.

It occurs when one object obstructs our view of another. Out of two objects, if one object is completely visible and the other is partly covered by it, the first object is perceived as nearer.

7. Movement Parallax:

It is a monocular cue for depth perception based on the fact that nearby objects appear to move faster in relation to our own motion. Every one of us must have noticed that when we move in a bus or train, the distant objects such as mountains, stars or sun appear to move along with us. Objects at a medium distance seem to be stationary, while nearby objects, such as trees, roadside houses, and people moving on the road, etc. seems to move faster in the opposite direction. Thereby, we learn to perceive objects that appear to move with us as being at greater distances, and the objects that appear to move backward as being nearer to us.

Besides the monocular cues, we also rely heavily on the binocular cues for depth information based on the coordinated efforts of both eyes. The following are the two binocular cues.

Binocular Cues

1. Retinal Disparity:

Some psychologists are of the opinion that the retinal disparity is the main cause of depth and distance perception. By retinal disparity, it is meant that the two eyes produce two different and separate pictures of an object as viewed from different positions relating to each eye. That is, the retinal images of the two eyes for the same object are different from each other. With the right eye, we see more of the right side of the object; with the left eye, we see more of the left side of the object. We interpret distance by availing clues from the two different images of the same object. Retinal disparity as a cause of depth and distance perception has been proved by Thorner (1938), Martins (1939) and many others in a series of experiments. In these experiments, two pictures of the same three-dimensional object were taken simultaneously by a camera with two lenses positioned at a slight distance from each other. These two pictures, called the stereo pairs, were then presented to the subjects with the help of a stereo-gram. Subjects witnessed the third dimension of the object because of stereo fusion.

The above opinion maintains that two eyes are essential for visual depth perception as stereoscopic fusion would be possible only with two eyes. One- eyed persons cannot have depth perception. If they have, they simply have it in course of their learning and knowledge of depth of different objects. But experiments by Hofman (1939), and Engel (1966) proved that one-eyed persons could have depth perception also.

2. Convergence:

This is a binocular cue for depth perception based on the difference in the image cast by an object on the retinas of the eyes as the object moves closer or farther away (Rathus, 1994).

In addition to retinal disparity, angular convergence of the eyeball has an important function in providing binocular cues for depth perception. It is the cue for depth perception based on the inward movement of the eyes as they attempt to focus on an object that is drawing nearer. For example, if we try to look to the top of our nose, our eyes turn inward, or converge on it, giving us a cross-eyed look, but they are less converged when we look towards objects, which are at distant places. The binocular cues of retinal disparity and convergence are most efficient for depth and distance perception of objects, which are not too far away from the observer (Rathus, 1994).