Illusions

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Dictionary of Biological Psychology

We see far more than meets the eyes, though not always correctly, for we experience various phenomena of illusion. Although they are errors, and can be misleading for the physical sciences as well as dangerous in skills such as driving, illusions are useful evidence of how eyes and brains normally work. ‘Illusion’ is hard to define. Illusions of the senses are departures from physical reality, but 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 largely wrong. Certainly physics frequently changes its mind. If we take modern physics’ accounts of matter, as atoms and molecules 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 no more helpful than to say that all perceptions are dreams.

We are pushed into thinking that visual and other illusions are departures from quite simple-minded physics—as measured with rulers, protractors, clocks and so on. Illusions include distortions of length, angles and time. There are also phenomena of ghostly fictions, and some figures or objects appear impossible. Others seem to flip from one perception to another, though there is no change at the eyes: they are ambiguous. These are weird and wonderful phenomena, which are central to art and hazards to the physical sciences. They are well worth explaining. It seems useful to try to classify phenomena of illusions, as classifications are important for all science. Chemistry was transformed by the PERIODIC TABLE OF THE ELEMENTS, biology by names of classes of species and varieties (see TAXONOMY). 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, fictions.

It is intriguing that these apply to vision and to language, for it is more than possible that language grew from pre-human classifications of objects and actions, over many millions of years. This would explain how language developed so rapidly in biological time for early humans—in only tens of thousands of years. Let’s compare errors of language with kinds of illusions:

The skeletal Necker cube switches in depth; the upper line of the Ponzo figure looks too long; the Tri-bar looks impossible; the ghostly square (a Kanizsa figure) does not exist.

To discover causes of illusions we need experiments, just as for any science, as well as appropriate explanatory concepts within a general theory or paradigm. Perception starts from what are called bottom-up signals from the eyes and the other senses. After a great deal of signal processing they are ‘read’ from topdown knowledge of objects, and from general rules (see TOP-DOWN vs. BOTTOM-UP PERCEPTUAL/NEURAL PROCESSING). When these are inappropriate, cognitive illusions occur, though there is nothing wrong with the physiological functioning. (This is rather like ‘software’ and ‘hardware’ errors of a computer; though the brain is not very like a digital computer, as it is most probably analogue.) Not all illusions are cognitive in origin. There seem to be four principal kinds of causes of illusions. The first is physical, for vision, optical disturbance between objects and the eyes. The second is physiological disturbance of neural signals, to and in the brain. The third is extremely different: cognitive errors of reading neural signals in terms of external objects. This occurs in two ways: errors of inappropriate knowledge of objects, and inappropriate general rules (such as depth-perspective applied to the flat surfaces of pictures, and the GESTALT laws of perceptual organisation). When either is inappropriate it can mislead perception, much as for rules and knowledge for thinking. (There are more or less corresponding illusions for the other senses, especially hearing). Here are examples, just one for each kind of visual illusion.

The ‘Cafe Wall’ distortion occurs with a chess board-like figure, when alternate rows of squares are displaced by half a square width. The rows become long wedges—as the retinal signals are upset by the brightness contrast across each half-square. The Ponzo illusion distortion is very different—perspective convergence of lines in a picture enlarge features that ‘should’ be more distant.

Distinguishing between objects and spaces between objects (‘figure-ground’) is the first most basic visual decision: it can be ambiguous. A demonstration of the power of topdown knowledge, here resolving ambiguities, is the hollow face. Though hollow it appears as a normal convex nose-sticking-out face. This is because convex faces are very familiar, and hollow faces extremely unlikely. Knowledge of faces is so strong it overcomes the bottom-up evidence that this face is truly hollow—countering and dominating bottom-up evidence, signalled by the eyes.

The left face is normal; but the right face is a hollow mould. The lighting is the same for both. Through the power of top-down knowledge, the right actually hollow face looks convex, like a normal face.

We always see the present in terms of past experience. We experience cognitive illusions when the past is not appropriate for seeing the present—for survival into the immediate future.

RICHARD L.GREGORY

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