Given how complicated speech production is, it's a small wonder
that morepeople don't stutter. Speech engages a wide swath of the upper
body, starting with inflation of the lungs and expansion of the chest
cavity and diaphragm. Vocal chords vibrate in the larynx; muscles in !the
lips, jaw and tongue help articulate the sounds; and the ears provide
quality control.
Command central for speech--as well as most everything else--is the
brain. "From a neurological perspective, language takes up more parts of
the brain than any other task, and it doesn't take much of an error to
throw it out of whack," observes Lawrence Molt, Ph.D., a speech
pathologist and neuroscientist at the Auburn University Neuroprocess
Research Laboratory. "If you hold the motion of a muscle 20 milliseconds,
often the system will break down. It will all tumble like a deck of
cards."
Do the brains of stutterers behave differently from those of normal
speakers? Scientists have been trying to figure that out since the 1930s.
In fact, some of the earliest EEG research involved people who stutter.
"Unfortunately, the technology of the time for neurological function
couldn't tap into subtle differences," says Molt.
The next 50 years did bring some discoveries, including the finding
that stutterers did worse than others in distinguishing competing sounds
that were presented to them at the same time. But only since the late
1980s have scientists been able to figure out what's really going on
inside the stutterers' heads, by mapping blood-flow patterns.
Positron Emission Tomography (PET), which creates color scans of
the functioning brain, have provided the clearest picture yet of what
happens when people speak. Gerald Maguire, M.D., an assistant professor
of psychiatry at the University of California, Irvine, has shown that
when stutterers stammer out their words, they have low levels of activity
in two parts of the brain key to speech: Broca's area and the striatum.
These sites also show elevated levels of dopamine, a neurochemical that
regulates the striatum.
Scientists had suspected a link between dopamine and stuttering
even before PET scans were available. As part of experiments in the
1970s, people who stutter were given dopamine-receptor blockers like
haloperidol (Haldol), which depress the neurochemical's activity in the
brain. Though speech improved somewhat, the drugs had intolerable side
effects: participants felt like they were mired in quicksand. "Even
people who experienced benefit said, 'Ah, I'd rather stutter than walk
around like a zombie,'" remembers Molt, who stutters himself and was
involved in the early trials.
In the last five years, scientists have been developing new
medications, particularly risperidone and olanzapine, that block
dopamine's activity by at least 50%, but with fewer of the numbing and
debilitating side effects. Clinical trials are now under way with these
drugs.
So far, according to Maguire, the results are promising. The
medications seem to reduce the frequency of stuttering by about half. "It
doesn't cure the problem," says Maguire, "but it helps treat it." The
drugs are not perfect, however; users complain of lowered sex drive,
missed menstrual periods, weight gain and fatigue. Maguire says the
initial results warrant further testing, but people shouldn't get too
excited, since stuttering appears to be a disorder without a single
cause.
Because so many stutterers also have relatives who stutter, one
area drawing attention is whether stuttering has a genetic component.
Studies of families with multiple stutterers--especially those with
stammering twins--strongly suggest a genetic link. Scientists believe
that many genes play some part in stuttering, but hope to identify one
that is more critical than others.
Still, notes Ehud Yairi, Ph.D., a professor of speech and hearing
science at the University of Illinois at Urbana-Champaign in a recent
issue of Asha, the official publication of the American
Speech-Language-Hearing Association, "the knowledge of how stuttering is
being transmitted does not tell us what is being transmitted. We still do
not know if stuttering evolves from an inheritance of abnormalities in
one or more biochemical pathways involved in speech fluency, or even from
structural difference." Faulty brain processing or motor skills as well
as altered emotional states or any combination could have an
impact.
One of the mysteries of stuttering is why it appears more commonly
in men than women. Researchers offer many possible explanations, covering
everything from sex-linked genetic factors to the effects of testosterone
on the brain. One theory holds that stuttering might be linked to other
apparently brain-related syndromes that appear with greater frequency in
males, including attention deficit hyperactivity disorder (ADHD),
learning disabilities and language development problems. Another popular
theory holds that men's brains deal with language differently than do
women's when it comes to neural organization and function. "That may make
men a bit more susceptible," says Molt.
More research and more sophisticated technology, say scientists,
will yield the ultimate answer to why people stutter.
PHOTO (COLOR): As subject listens and repeats words, blood flows to
brain's speech area (yellow dot, above) as well as listening center
(below)
PHOTO (COLOR): PET scan shows blood flow (yellow and red) to
brain's auditory center as a normal subject hears words spoke
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