Illusions are a natural part of consciousness

The most remarkable thing in a remarkable universe is so commonplace that it is accepted without wonder or understanding. This is the appearance of reality and solidity that surrounds us when our eyes are open, which we call simply vision. Intellectual effort is required to realize that this seeming reality is within us, distinct for each individual, but so concordant with reality and with each other, and so stable, that it is accepted without question. The explanation for this wonderful aspect of consciousness is completely unknown.

The information that allows the mind to create and maintain this appearance is collected entirely by the visual system. When the eyes are closed, the appearance vanishes. At least, it does for me. Some people may be able to create realistic pictures with their mind's eye, but I cannot. I can still imagine my environment more or less accurately, but the vivid picture is gone.

The ability to make an accurate visual model of the external world is learned, not innate. The necessary materials are there at birth, of course, but they must be trained, or programmed, before the skill is perfected. This seems to be done in the first instance by comparing the chaotic impressions of light with the solid evidence of touch, which gives the perception depth and form. The visual sense ever after shows subtle indications of its origin in touch, though it becomes completely independent of touch after perfection. The ability to acquire this skill vanishes early in mental development. A person totally blind from birth whose vision may become normal at a later age can never make sense of the visual information and arrange it in a consistent manner.

There are three links in the chain of perception. The first is external and physical: the propagation of electromagnetic waves from the object to the eye. The second is the physical visual apparatus, from eye to brain, consisting of nervous tissue, although some important preliminary processing takes place. The third, and most complex, is the interpretation of the visual stimulus and the creation of the internal model of the world that is used by the consciousness.

In the third step, the visual stimulus received from outside is combined with information from the memory to create the picture. This is the most important part of vision, and how it is done is unknown. All the really interesting parts of vision occur here. The physical visual system from eye to brain has been studied in exquisite detail, its parts examined and described, and even the nerve impulses observed and measured, but all this gives no satisfying explanation of vision. It has been established, however, that important preliminary processing takes place here, including the differencing and the coding of stimuli. Coding is necessary to reduce the flood of information to a manageable amount. The visual system has a bandwidth problem, indeed.

The world picture must be constructed from incomplete information, in fact inferred from clues. The three-dimensional world is sensed by the two- dimensional retina, emphasizing the central role of depth clues. The picture depends on the unconscious recognition of objects, so that the remembered properties of objects can be transferred to those they seem to be on the basis of visual hints. Recognition is what gives vision its reality, showing the central role of mind.

A picture so assembled on the basis of partial information must be expected to occasionally be in error. The mind will always try to match stimulus and memory to create a picture. It will make what seems to be the most likely choice, and present that to the consciousness. An illusion occurs when the choice is incorrect. If a picture is created solely from memory, without visual stimulus (or with only a minimal visual stimulus) the result is hallucination, with which we shall not be concerned here, since it is a disorder of perception, not a normal or intended part of it. Things that are not there can also appear in illusion, it must be emphasized, but here it is normal.

An illusion can arise in any of the three links of visual perception. The mirage is an example of an external illusion, created in the first, physical link of light rays. It is visually interpreted as an actual scene, though we consciously recognize it as an illusion, and understand its cause. When we stare at a brightly illuminated red disk for a time, then transfer our attention to a white paper, we see a green disk as a result of what is called rather inaccurately fatigue. The green disk is an illusion created in the second partly physical, partly mental link. When the full moon is seen at the horizon, it seems much larger than when riding high in the sky, though physically it subtends exactly the same angle at the eye. This familiar illusion occurs in the third, mental link of vision, and a satisfying explanation of it is unknown.

Illusions occurring in the third link are those most generally recognized as optical illusions. Their scientific study began with J. Oppel in Jahresberichte des physikalisches Vereins zu Frankfurt, p. 138 (1854). Much work was done later in the century, but tapered off after 1900, although the subject is still actively researched by psychologists. Recent work deals largely with color and motion illusions, not on the static, black- and-white illusions that dominated earlier work. Popular interest in optical illusions has been sustained. The books by M. Luckeish (Visual Illusions, 1920), S. Tolansky (Optical Illusions, 1964), and M. Fineman (The Nature of Visual Illusion, 1981) are evidence of the continuing fascination. Each of these books gives references to further information. All theories of optical illusion in the third link are mere jejune speculation. Feel free to create your own theories; they will be as valid as those created by many a psychologist!

Sometimes a phenomenon is called an illusion when it really is not, but is simply a true picture of an unexpected observation. An example is the searchlight illusion described by Luckeish. The beam of a bright searchlight is visible because of scattering by dust and fog in its path, so that it seems practically a physical object. When the beam is projected up into the sky, it seems to vanish abruptly while still in full glory. When you look at this apparent end of the beam, you are looking in the direction in which the beam is pointed. If the beam were parallel (as your mind expects) it would, by perspective, narrow to a point. However, a searchlight beam is actually more or less divergent, fooling this expectation. It is only one's mental interpretation that is an illusion in this case, not the observation. Stars can be pointed out to others by means of a strong laser using this effect. If you view the searchlight beam from a distance, you see it diverge and become attenuated, and perhaps penetrating the layer of dusty air.