Dec.
4, 2013 — Tongue twisters are not just fun to say; it turns out that these
sound-related slip-ups can also open windows into the brain's speech-planning
processes.
A team from the Massachusetts Institute of Technology (MIT) will
report new insights gleaned from a comparison of two types of tongue twisters
at the 166th meeting of the Acoustical Society of America (ASA), held Dec. 2-6,
2013, in San Francisco, Calif. Stefanie Shattuck-Hufnagel, an MIT psychologist
who will present this work, studies speech errors as a way of understanding
normal brain functions. "When things go wrong, that can tell you something
about how the typical, error-free operation should go," she said.
For
centuries, people have noticed that when certain combinations of sounds are
spoken too quickly, people seem to lose control of their mouths. Often, one
sound seems to replace another:
-"Toy
boat" becomes "toy boyt."
-"Top
cop" becomes "cop cop."
-"The
seething sea ceaseth and thus the seething sea sufficeth us" becomes a
mess of misplaced "s's" and "th's."
But
when scientists recorded the misspoken sounds and analyzed them, they found
that the errors were not always straightforward sound replacements. At least
some of the time, the mistakes didn't seem to be quite one sound or another,
but something in between. And there were different flavors of in between: in
the "top cop" example, sometimes the "t" and the
"c" seemed to arrive almost at the same time (sort of
"tkop") and sometimes there was a delay between the two, with space
for a vowel ("tah-kop"). Linguists refer to these double sound mistakes
as double onsets.
In
their recent study, the team from MIT -- along with their colleagues at Haskins
Laboratories in New Haven, Conn., Ludwig-Maximilians University in Munich,
Wellesley College in Massachusetts, and the University of Southern California
(USC) in Los Angeles -- tried to determine whether they could induce different
types of double onset with different types of tongue twisters.
The
researchers recorded volunteers saying combinations of alternating words that
fell into two categories: simple lists of words, such as the "top
cop" example above, and full-sentence versions of the same sounds with an
inversion, such as "the top cop saw a cop top."
One
particular list of words turned out to be so difficult that the test subjects
couldn't even get through it. The phrase was "pad kid poured curd pulled
cod," and when volunteers tried it, Shattuck-Hufnagel said, some of them
simply stopped talking altogether. "If anyone can say this [phrase] ten
times quickly, they get a prize," she said.
After
recording their volunteers' efforts, the researchers analyzed the sounds to see
what errors people had produced. They found that in the word list tongue
twisters, there was a preponderance of the "t'kop errors." In
contrast, the sentence twisters induced more of the "tah-kop"-type
errors, with the longer delay and the presence of a short vowel after the
consonant.
Though
it is too early to say exactly what is responsible for these differences,
Shattuck-Hufnagel said, one possible factor is the regular rhythm of the word
lists compared to the more irregular timing of the sentences. But the fact that
both types of errors occur for sentences as well as word lists suggests that
there is some overlap between the brain processes used to produce these two
types of speech. "You can get both kinds of errors in both kinds of
planning," she said, but the different proportion of errors indicates key
differences as well.
The
MIT team, working with colleagues at Haskins Laboratories, Ludwig-Maximilians
University in Munich, and USC, has already collected data for the next stage of
this study, in which tiny transducers are placed on people's tongues to measure
their articulation. The researchers hope to use this work to find the ghosts of
double onsets, in which the tongue has tried to make both a "t" and
"k" but only one sound is heard. If these double onsets are indeed
more likely to be produced by word list twisters than by sentence twisters,
Shattuck-Hufnagel said, then this work will help scientists understand how the
brain plans these two types of speech.
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