illusions We see far more than meets the eyes, though not always correctly, for we experience various phenomena of illusion — departures from physical reality. Although they are errors, illusions are useful evidence of how eyes and brains normally work.

But ‘illusion’ is hard to define adequately. If illusions are departures from reality, what is reality? This is difficult to answer, for we have many descriptions of reality, in science, in art, and so on. No doubt all are incomplete and perhaps largely wrong. Certainly physics frequently changes its mind. If we take the account offered by modern physics of matter as invisible atoms in violent motion, with weird effects of quantum mechanics, this is so different from how things appear that we might be tempted to say all perception is illusion! But this is not more helpful than to say that all perceptions are dreams.

We are pushed into thinking that illusions must be departures from quite simple-minded physics, as measured with rulers, protractors, and clocks. Illusions include distortions of length, angles, and time. There are also perceptual experiences that one might call ‘ghostly fictions’ (Table 1(d)). Some figures or objects appear impossible (Table 1(c)). Others seem to flip from one perception to another, though there is no change of the image at the eyes (Table 1(a)). These are weird and wonderful phenomena, which are central to art, but are hazards to the physical sciences, and can be dangerous in everyday living. They are well worth explaining.

Explaining illusions

It seems useful to try to classify phenomena of illusions, as classifications are important for all science. Chemistry and biology were transformed by the Periodic Table of the Elements, and by names for species and varieties. How shall we start for illusions? It is suggestive that illusions of seeing correspond with errors of language. Appearances of illusions fall into classes which may be named quite naturally from kinds of errors of language — ambiguities, distortions, paradoxes, and fictions.

It is intriguing that these apply both to vision and to language, for it is more than possible that language grew from pre-human classifications of objects and actions over millions of years. If the evolution of language were parasitic on these perceptual mechanisms, it might explain how language developed so rapidly for humans. It is a tricky question how much extra brain was needed for language, and how much language makes a brain more effective. This is rather like using any tool, such as an electric drill or a computer. One needs the skill to use it; but then the tool adds a lot of extra skill. Tools increase intelligence. Indeed our language, and tools of technology, are large parts of our intelligence, though we are never infallible.

Let us compare errors of language with kinds of illusions:

There seem to be four principal causes of illusions. The first is optical disturbance between objects and the eyes. The second is the physiological disturbance of signals in the ‘peripheral’ machinery of vision (in the retina and early areas of the visual cortex). The third and the fourth are extremely different — for they are cognitive. This is like mistakes of interpreting plainly visible words. This can happen by confusions of meanings (semantics), or by misapplied rules of grammar (syntactics). There are similar kinds of cognitive errors in vision and other perception, because perception is based on knowledge of the world and it works by applying general rules. Either kind of cognitive mistake can mislead, by not being appropriate to the situation. The same kinds of cognitive illusions can occur in computer vision.

Table 2 Classes of illusions and kinds of causes

Classes	Physical causes	Cognitive causes
	Optics	Physiology	Rules	Knowledge
Ambiguities	Looking	Pure yellow	Figure-ground	Hollow face
	through mist	versus	reversals
		red + green
Distortions	Mirages	Café wall	Perspective	Size-weight
		illusion	illusions	illusion
Paradoxes	Looking glass	Rotating spiral	Impossible	Magritte's
		after-effect	objects	painting of the
				mirror
Fictions	Rainbow	After-images	Ghostly shapes	Faces-in-the-fire

Here is one example of each kind:

These are examples of false perceptions of different origin. In a looking glass, one sees onself through it, yet knows one is in front of it — doubled, yet one person! Pure, monchromatic yellow light cannot be distinguished from a mixture of red and green light because they cause the same activation of the three types of colour-sensitive cones in the retina. The café wall illusion looks like a chess board, but with alternate rows displaced sideways by half a square: the horizontal rows of squares appear like long wedges, converging towards one side or the other. If you stare at a rotating spiral it appears either to shrink or expand, depending on which way it is rotating. If you then look away, whatever you are now looking at appears to do the opposite — to expand or to shrink — an example of an after-effect that is probably due to ‘fatigue’ of neurons in the visual cortex. After-images are ghost-like blobs that move with each eye movement, resulting from over-stimulation of one region of the retina. The most familiar after-images are the dark blobs that persist after looking at a light bulb or a camera flashlight. But after-images (of complementary colour and brightness) can be caused by staring intently at any pattern of high contrast. What is ‘figure’ (or object) and what is ‘ground’ (the background between objects) is the most basic perceptual decision. In ambiguous situations (such as the famous Rubin picture, with two silhouette profiles of faces, staring at each other, creating a vase shape between them) perception can flip from one interpretation to the other (from faces to vase). Perspective illusions are distortions produced by converging lines, which signal depth (even when depth is not perceived), e.g. the Ponzo illusion in Table 1. ‘Ghostly shapes’ are perceptual interpretations that go beyond the immediate information in the retinal image, as in the Kanizsa figure in Table 1. Impossible objects are representations that are not compatible with sensible ‘perceptual hypotheses’, such as the Penrose impossible triangle, or the Tri-bar in Table 1. The inside of a mask (a hollow face) appears as a normal face, with the nose sticking out, presumably because the improbable interpretation of a true hollow face is rejected. Small objects feel heavier than large objects of the same weight — because large objects are expected to be heavier. The Magritte painting shows a back view of a man looking into a mirror — but paradoxically we see the back of his head in the mirror. Faces-in-the-fire are fictional perceptual interpretations seen fleetingly in the flames of a fire.

Richard Gregory

Bibliography

Gombrich, E. (1960). Art and illusion. Phaidon, London.
Gregory, R. L. (1997). Eye and brain, (5th edn). Oxford University Press.
Robinson, J. O. (1972). The psychology of visual illusions. Hutchinson, London.

See also perception; vision.