By Wynne Parry, LiveScience Senior Writer
Aug. 16, 2011
The brains of people who have stuttered since childhood show evidence of rewiring, with the right side taking on tasks generally handled by the left. A new study, in which participants tapped their fingers in time with sounds, shows that this rewiring extends beyond speech.
Research so far indicates that stutterers have problems linking what they hear with what they say, according to Martin Sommer, a study researcher and neurologist at the Department of Clinical Neurophysiology, Georg-August- University of Goettingen, Germany in Germany. He compared stuttering speech to music from a disorganized orchestra.
"The question is not single elements themselves, not the instruments. They all know their parts. The question is how to activate them in a coordinated and well-timed fashion," Sommer said.
The musicians know when it's time to begin playing their instruments based on what they hear around them. So they fine-tune their actions in response to sound. Likewise, the part of the brain that controls the movement that creates speech must fine-tune its instructions based on what the person hears, including his or her own voice.
Fluent speakers predominantly use the left side of their brain to integrate what they've heard with speech, while people who stutter shift the workload to the brain's right side. This appears to happen because a defect on the left side prevents the brain's motor cortex — the part of the brain that controls movement — from generating a good set of instructions for the muscles of the throat and mouth.
Researchers believe that with the left side not functioning properly, the right side of the brain attempts to compensate. Even so, the result is that a person with a stutter struggles to get the words out, even though he or she knows what to say and has the equipment to do it.
Sommer and his colleagues wanted to build on previous research indicating that this shift in location involves more than speech.
They asked participants who stuttered and those who didn't to tap their index fingers in time to clicks they heard through headphones. The researchers stimulated electrical currents in the brain to temporarily interfere with it, including the areas believed responsible for sending faulty messages to the motor cortex in stutterers. They stimulated the right and left sides separately.
The two groups responded differently. Fluent speakers became inaccurate tappers after the left side was interfered with, but their tapping was unaffected when the right side was stimulated. For participants who stuttered, the effect was reversed. They were unaffected when the left sides of their brains were stimulated. However, when they lost the use of their right side, their accuracy suffered.
This shows that the brains of stuttering people handle an issue not related to speech differently than other people, according to the researchers.
Sommer noted that the baseline tapping performance of stuttering subjects was normal, so the shift to the right does not necessarily affect low-level task performance. However, if the demands increased, the stuttering subjects' performance could be affected, he wrote in an email to LiveScience.
"We are understanding better and better what goes wrong in stuttering and that this has probably a neurological or brain-related cause," he said.
Even so, therapy to address stuttering must also take into account its psychological aspects, Sommer added.
"It is strongly influenced by how you feel about your speech and how you feel about yourself," he said.
Sommer recommends that people who want to address a problem with stuttering seek information through the Stuttering Foundation of America.
The research appears in the September issue of the journal Cortex.