Say pet. Now say pat. Hear the difference? Of course. Now say one halfway between the two. Can't do it? Or can you just not hear that you're doing it?

Since the 1960s, speech scientists have known that although it's possible to produce sounds that are acoustically halfway between two recognized vowels, speakers never hear them halfway-they perceive them as one or the other. Even when people listen to a series of synthetic vowels, progressing in equal acoustic steps between that in, say, pat and pet, they hear a series of pats followed by a series of pets. Now researchers at MIT and elsewhere are building theories to explain this phenomenon, called categorical perception, so they can better understand how we hear speech. Such research could have implications for learning second languages, and might even help computers understand us better.

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