The mysterious origins of stuttering have resulted in some bizarre treatments before the rise of modern medicine. Francis Bacon, the scientist credited with developing the scientific method, suggested drinking warm wine to loosen up a stiff tongue. Johann Frederick Dieffenbach, an infamous Austrian surgeon, tried cutting tongues down in size until his method was outlawed because too many patients died under his knife.
Much about stuttering still remains uncertain, but modern researchers have ruled out older, misguided theories that blamed bad parents or overactive muscles in the lips, jaw and tongue. Now brain-imaging tools and genetic sequencing have finally begun to reveal deep-seated biological signs in the brains and family histories of stutterers.
"We now understand that stuttering is a highly heritable disorder," said Nan Ratner, a psycholinguist at the University of Maryland in College Park. [The Science and Myths of Stuttering in "The King's Speech"]
Nature rather than nurture appears to play the bigger role in the origins of stuttering. Studies that compared twins found 50 to 70 percent heritability level for stuttering, which is considered moderate to moderately highly heritable. For comparison, the heritability for cholesterol levels is about 33 percent.
On the flip side, children adopted by stuttering parents don't show any signs of stuttering at a rate higher than the general population. About 1 percent of people worldwide are afflicted by stuttering, but as many as 4 percent may have a childhood history of stuttering. There is no known cure for the condition beyond 2-5 years of age, when about 80 percent of stuttering children can recover either spontaneously or with therapy in early childhood.
"I think it's fair to say [from at least one adoption study] that there's really no evidence that stuttering is learned," said Dennis Drayna, a chief molecular geneticist at the National Institutes on Deafness and Other Communication Disorders in Rockville, Md.
Both Drayna and Ratner spoke during a panel session on Feb. 20 called "From Freud to fMRI: Untangling the Mystery of Stuttering" at the American Association for the Advancement of Science (AAAS) conference in Washington, D.C.
How stuttering works
By measuring stutterers' brain waves and facial movements, researchers have ruled out the idea that overactive muscles lie behind the condition. Instead, they point to a problem of timing in the commands sent from the brain to the muscles that control speaking – an issue influenced by linguistic, cognitive, psycho-social and genetic factors.
"We found out that adults who stutter have differences in speech and nonspeech movement patterns and muscular patterns even during fluent speech," said panel member Anne Smith, a neurophysiologist at Purdue University in West Lafayette, Ind.
But researchers must study kids to discover exactly how stuttering develops and works, Smith said. Her group confirmed timing problems among stuttering kids by tracking the movements of the upper and lower lips and jaw while speaking. Young stutterers already show higher signs of unstable, less well-controlled muscle movements compared with nonstutterers.
Smith's lab also asked stuttering kids around the ages of 4 or 5 to clap in time with a beat, as one of many ongoing tests done once a year over five years. The researchers found that 60 percent already had timing problems beyond the average for that age; and brain-imaging studies have shown that speech areas activate during such timing tests.
The mind of a stutterer
Brain-imaging devices can also reveal differences in the brains of stutterers, according to Luc de Nil, a speech-language pathologist at the University of Toronto in Canada. The brains of stutterers become over-activated in regions such as the motor cortex, which directs muscle movements, and the anterior cingulate cortex, which deals with attention processes.
"You have that huge over-activation compared to normal speakers, and that has been observed over and over again," de Nil explained.
But researchers can only use brain scanners to study kids as young as about 7 because of requirements that the subject keep still for long periods of time. That means brain-imaging studies can't yet examine the development of stuttering during the crucial window between 2 and 5 years of age.
Researchers urgently want to find out whether they can predict which kids will recover by themselves and which have a greater chance of becoming lifelong stutterers, so that they can begin early therapy on the latter to minimize the effects of stuttering. That's when knowing the family history of stuttering and genetics can come into play.
Tracing the family history
Several studies have begun to try to find genetic links among stuttering in families, which can vary by region. One study found no common gene related to stuttering among North Americans, but researchers had better success looking at Pakistan.
"While stuttering clearly has some significant genetic contribution, exactly how those genes contribute to the disorder remains really very murky," said Dennis Drayna, a molecular geneticist at the National Institutes on Deafness and Other Communication Disorders in Rockville, Md.
Researchers took advantage of the well-recorded family lineages in Pakistan where 70 percent of marriages took place between first or second cousins. They eventually found a common mutation among the Pakistani family stutterers for the gene GNPTAB. That gene normally codes for an enzyme that acts as a "recycling bin" for cells in all higher animals, Drayna said.
More work established that the gene mutation went back at least 14,000 years in the human lineage of that region. Drayna and his colleagues confirmed more mutations involving the same gene among Asians, Europeans and Africans.
The group also discovered two other genes with mutations that similarly show up among stutterers. Together, the three genes may account for 5 to 10 percent of family stuttering worldwide, according to Drayna.
The researchers hope to find even more genes related to stuttering, despite some logistical challenges. One of Drayna's graduate students who had gone to Pakistan to visit his family was blocked from returning to the United States despite having a valid visa. After six weeks, he called Drayna and complained of boredom.
"Go out and find more stuttering families," Drayna advised.
