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Do Animals have illusions?

J. J. Gibson, Cornell University

The World Wide Web distribution of James Gibson's "Purple Perils" is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.

The evolution of the senses (perceptual systems) must have been a continuous process of eliminating misperceptions. If, in general, "things were not what they seemed" to animals, they could not be coped with. Hence the class of illusions that (1) mislead the observer so as to arouse inappropriate behavior and that (2) occurs regularly in Nature should not be manifested in the perception of animals.
There is, of course, a class of experiences that does not elicit overt behavior. Afterimages are an example. They are "subjective" experiences or so called "private" experiences, and are therefore not perceptions in the common use of that term. It would be very difficult to determine whether animals have afterimages. If they do, it is likely that they would pay no attention to them.
There is another class of experiences which are not subjective or private, are similar to perceptions, but are nevertheless false. Images (virtual objects) in still water or in a mirror are examples. Such virtual objects are presumably falsely perceived by all seeing observers, animal or human. It may be that inappropriate behavior has to be unlearned (extinguished) in such cases. (Rainbows do not elicitbehavior.) Pictures are another example.
The straight stick which appears bent when partially immersed in water is similar to cases of the latter class. The "percept" is simply the result of the fact that the visual perception of objects depends on the optic array entering the eye. There is information in the light for a bent stick, and it can only be disallowed by what is rightly called inferential knowledge (i.e., knowledge about refraction). But this is true only so long as the stick is not moved.
When the stick is moved, and especially when it is rotated (as we have shown) there exists a different level of information in light: the invariants of changing perspectives over time. One of these invariants (information for straightness) as we have shown can be noticed by young children, who cannot possibly "know" about the laws of refraction.
The moral of all this is that invariant detection over time is a useful dimension of sensitivity and that the perceptual system will tend to develop such kinds of sensitivity. But, along with these, there will inevitably be useless dimensions of sensitivity that are merely incidental to the useful ones. Animals could not evolve the ability to detect solid tridimensional shapes without incidentally having the ability to detect flat bidimensional forms (frozen pictorial forms). But this so called "form sense" never did animals any good until man began to exploit it, quite recently, by making pictures on flat surfaces. Euclidean and Platonic forms are wonderful inventions for teaching mathematics but they have caused hopeless confusion in the problem of understanding visual perception. We have taken for granted that visual forms, perspectives, were the basic elements of object-perception ­ the sensory basis of object perception.

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