It is a curious fact that all humans perceive optical illusions in the same manner. That is, their brains misinterpret the same information in exactly the same way to produce exactly the same misperception. How does this happen? After all, we do not teach little children the "rules" of perception (or misperception). How does a child, for example, "learn" to judge colour intensity or hue on the basis of light and shadow? In the well known colour illusion, humans of all ages see the same colour as being of different hues depending on whether that colour is interpreted as being in light or shadow. No one is ever taught how to do this, or even that it should be done. Yet it happens unfailingly and spontaneously.
This is very similar to the way humans develop the ability to judge the correctness or incorrectness of linguistic utterances and acquire the capacity to produce what they believe to be correct utterances. Chomsky attributes this seemingly remarkable ability to what he calls a "language organ" (or Language Acquisition Device) in the brain, or to an inherent property of language, to which he gives the name "Universal Grammar." Such a leap of the imagination is the linguistic equivalent of attributing the human susceptibility to optical illusions to an "optical illusion organ" or an "Optical Illusion Acquisition Device" in the brain. Of course, the very suggestion of an "optical illusion organ" in the brain strikes us as ludicrous. But Chomsky and the Universal Grammarists appear not to be struck by a similar sense of absurdity when it comes to positing the existence of a "language organ."
The fact is that both the capacity for language and the susceptibility to optical illusions arise from "lower-level," more fundamental processes and phenomena in the brain, processes that are of a rather general nature and are associated broadly with information processing, built-in mechanisms such as pattern recognition, induction, and analogy formation. Many of the processes and phenomena are the same in both cases, and have nothing specifically to do with either language or optical illusions.
