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.

It's well known that our ability to discriminate between different versions of the same vowel isn't uniform for all gradations of that vowel. Some are harder to distinguish from their neighbors than others. Researchers think that variations in our sensitivity to small acoustic differences between speech sounds may help us categorize and interpret the sounds of our native languages.

Louis Braida, a professor in the Sensory Communication Group at MIT's Research Laboratory of Electronics (RLE), set out to map native speakers' sensitivity to English vowels. When he asked subjects whether they could distinguish between slightly different vowels, he found that discrimination ability is highest not between the most perfect example of each vowel and its immediate neighbors, but between examples near the category boundaries, where one vowel is on the verge of being perceived as a different vowel. In earlier research, Braida and his colleague Nathaniel Durlach had discovered a similar pattern in people's sensitivity to variations in loudness. Subjects found it harder to discriminate pairs of faint tones than pairs of loud tones, as expected. But they got a boost in sensitivity when comparing subtly different examples at the extremes of a set of tones, around the loudest or the softest examples. They had been the least sensitive to variations square in the middle of the range.

Braida theorizes that people categorize sounds by estimating how far each is, acoustically, from what he calls "perceptual anchors"-memorable stimuli located at the edges of a range of examples. To discern where a particular vowel sound falls in relation to the extremes, he says, "you gauge the difference from each anchor with a 'perceptual ruler' that measures in units of a just-noticeable difference." The farther the vowel is from an anchor, however, the blurrier the ruler becomes and the less accurately the sound is perceived. For Braida, the imperfection of the ruler corresponds to the limitations of our basic auditory resolution ability.