As a result of studying stroke patients who cannot vocally produce certain letters, Alfonso Caramazza (left) and Doriana Chialant have found that the brain perceives and processes vowels and consonants as two distinct types of objects. Photo by Kris Snibbe/Lettering by William Reimann.

One of the grand mysteries of language concerns the nature of vowels and consonants. Are they really different entities processed separately by the brain, or just artificial labels developed for convenience?

Researchers at Harvard University and their collaborators in Rome believe they have found the answer.

Vowels are more sonorous than consonants, that is, they require greater resonance, or vibration of the vocal cords to produce. “O” is more sonorous than “r,” for example. Consonants themselves vary in sonority; for example “b” is more sonorous than “p.” You can, in fact, arrange all vowels and consonants on a continuum of sonority.

Are vowels and consonants, then, classified as different objects simply because they are at opposite ends of this continuum, or are they actually different types of knowledge handled separately by the brain?

Alfonso Caramazza, a professor of psychology, and Doriana Chialant, a research associate, both at Harvard, decided they could find an answer by looking at whether consonants and vowels are independently affected by brain damage.

While working with their colleagues in Rome, Rita Capasso and Gabriele Miceli, they serendipitously met two women with intriguing speech deficits. As the result of a stroke, Alicia (not her real name), could not reproduce the sounds of vowels properly. Another patient, call her Irena, experienced the same trouble with consonants.

During speech testing, Alicia, 41, failed to correctly produce vowels three times more often than consonants. Irena, 52, had the opposite problem; she failed three times more often on consonants. For example, instead of saying “leggere” (“to read” in Italian), Irena might say “seppere” and Alicia “loggiare.” Both words are meaningless.

If the distinction between vowels and consonants only reflects an arbitrary grouping of more versus less sonorous speech, then Irena, who had few problems with sonorous vowels, should have less trouble with more sonorous consonants than with the less sonorous ones. Similarly, if Alicia’s problems with vowels represented a difficulty with sonority, then she would have more trouble with the most sonorous consonants than with the least sonorous ones.

Neither turned out to be the case. Therefore, the researchers assumed that difficulties with sonority did not explain their speech impairments. Neither did other aspects of speech difficulties that were also tested. So Caramazza’s team concludes that the difference between vowels and consonants must be real, not arbitrary

” It’s a happy ending to many years of work,” says Chialant. “We’re excited about answering a question that has been around for such a long time, excited about reaching a definite conclusion that people possess separate mechanisms to process vowels and consonants. Such mechanisms must be in the brain. Where else could they be?”

Jumping Word Boundaries

The researchers will leave it to others to determine where in the brain. Alicia suffered stroke damage on both the left and right sides of her brain. Irena had injuries to her left side.

“Even if we can use scanning to obtain clear, detailed images of the damaged areas, we cannot conclude that they represent the ‘seat’ of vowel and consonant processing,” Chialant notes. “The damaged regions may well exert effects on other parts of the brain, so there’s not necessarily a one-to-one correlation between the injured place and the speech impairment.”

Caramazza and Chialant prefer to concentrate further research on the basic function of consonants and vowels in language. Chialant believes vowels give speech a pattern the way harmony structures music.

Together with consonants, vowels organize spoken language into a syllable structure. Such a structure is put together “on-line,” that is, from the patterns of consonants and vowels produced as a person speaks.

As a consequence, syllable boundaries often straddle word boundaries, eliminating word breaks. For example, a phrase read as “understand it” is often spoken as “un-der-stan-dit.”

There’s practical importance to such basic understanding, according to the researchers. “The more we know about language processing, the more sophisticated will be our diagnoses of speech problems. The more you know about what’s wrong, the more help you can be to patients.”