Put on a pair of headphones and turn up the volume so that you can’t even hear yourself speak. For those who stutter, this is when the magic happens. Without the ability to hear their own voice, people with this speech impediment no longer stumble over their words—as was recently portrayed in the movie The King’s Speech. This simple trick works because of the unusual way the brain of people who stutter is organized—a neural setup that affects other actions besides speech, according to a new study.
Normal speech requires the brain to control movement of the mouth and vocal chords using the sound of the speaker’s own voice as a guide. This integration of movement and hearing typically happens in the brain’s left hemisphere, in a region of the brain known as the premotor cortex. In those who stutter, however, the process occurs in the right hemisphere—probably because of a slight defect on the left side, according to past brain-imaging studies. Singing requires a similar integration of aural input and motor control, but the processing typically occurs in the right hemisphere, which may explain why those who stutter can sing as well as anyone else. (In a related vein, The King’s Speech also mentioned the common belief that people who stutter are often left-handed, but studies have found
no such link.)
In the new study, published in the September issue of Cortex, researchers found that the unusual neural organization underlying a stutter also includes motor tasks completely unrelated to speech. A group of 30 adults, half of whom stuttered and half of whom did not, tapped a finger in time to a metronome. When the scientists interfered with the function of their left hemisphere using transcranial magnetic stimulation, a noninvasive technique that temporarily dampens brain activity, nonstutterers found themselves unable to tap in time—but those who stuttered were unaffected. When the researchers interfered with the right hemisphere, the results were reversed: the stuttering group was impaired, and the nonstutterers were fine.
According to lead author Martin Sommer, a neuroscientist at the University of Göttingen in Germany,the results suggest that the left-hemisphere defect underlying a stutter causes trouble with sensory integration in general, rather than specifically speech-related problems as was historically thought. “Like in stroke patients, the right side seems to jump in and compensate,” Sommer explains. But that part of the brain did not evolve to handle those tasks, so problems—such as a stutter—can emerge.