Brain scans show inner side of stuttering.

People who stutter often come to dread talking to others because of the embarrassing disruptions that break up their speech. These include repetitions of syllables at the start of some words and prolonging of the initial sound in many others. Yet in a fascinating and poorly understood twist, stuttering often vanishes temporarily when the process of speaking is somehow altered, such as by reading aloud in unison with a group, singing, or whispering. The curious curative powers of group reading have now given scientists an opening through which, with the help of brain-scanning technology, they have glimpsed the cerebral foundations of this condition.

"Stuttering is a disorder affecting the multiple neural systems used for speaking," contends a team of researchers directed by Peter T. Fox, a neuroscientist at the University of Texas Health Science Center at San Antonio.

Most prominently, stuttering induces widespread hyperactivity in motor areas throughout the brain, particularly in the right hemisphere, Fox and his coworkers assert. The cerebellum, a structure at the base of the brain, shows especially strong activity during stuttering, they note.

In contrast, stuttering is associated with the nearly complete shutdown of activity in interconnected parts of the brain's outer layer, or cortex, that are thought to regulate the conscious monitoring of one's own speech, the investigators contend.

Related cortical areas implicated in the ability to string words together fluently also remained unusually inactive during stuttering, they report in the July 11 Nature.

This particular mix of excessive and insufficient brain activity largely cleared up when stutterers spoke fluently as they participated in a group reading of a written passage.

Fox's team studied 10 men, ages 21 to 46, who had stuttered since childhood, as well as 10 men in approximately the same age range who had never exhibited a speech or language disorder.

Positron emission tomography (PET) scanners measured changes in brain activity, indicated by alterations in blood flow, on three occasions-as each volunteer took a solo turn reading a paragraph aloud, during a group reading of a paragraph, and while the men rested with their eyes closed. Each stutterer displayed his usual speech problems during solo reading but spoke flawlessly while taking part in a group reading.

All of the stutterers exhibited a remarkably consistent pattern of rises and falls in brain activity as they read the paragraph alone, the researchers maintain. This consistency occurred despite a wide range in the severity of the participants' stuttering. The frequency with which each man stuttered also varied widely throughout the course of a day.

The new PET findings lend support to several theories about what causes stuttering, Fox and his colleagues hold. For instance, some researchers speculate that hyperactivity of the brain's right hemisphere disturbs speech production in the left hemisphere. Others suspect that stuttering reflects hyperactivity in specific motor structures that facilitate speech sounds. Several alternative theories of stuttering posit primary disturbances in either auditory or speech production areas of the brain.

"Current theories of stuttering each emphasize one or another individual component of what we believe to be a dysfunctional [brain] system or systems," the investigators argue. "Our results strongly indicate the need for a unifying theory of sufficient scope to accommodate the full complexity of the observed actions and interactions of the neural systems."