What Causes Stuttering

The National Stuttering Association estimates that about 1 percent of the global population stutters. Four times the number of adult men stutter, compared to women. The exact cause has been a mystery, but genetics and neurophysiology have long been believed to be at its core.

Now a new study led by researchers at Children’s Hospital Los Angelesmay finally be moving the ball down the field a bit. They have discovered that regional cerebral blood flow is reduced in the Broca’s area – the region in the frontal lobe of the brain linked to speech production – in people who stutter. The worse someone stutters, the greater the reductions in blood flow to this region.

In addition, a greater abnormality of cerebral blood flow in the posterior language loop, associated with processing words that we hear, correlates with more severe stuttering. This finding suggests that a common pathophysiology throughout the neural “language” loop that connects the frontal and posterior temporal lobe likely contributes to stuttering severity.

Bradley Peterson, MD, director of the Institute for the Developing Mind at CHLA and a professor of the Keck School of Medicine at the University of Southern California, calls all this “a critical mass of evidence.” Such a study of resting blood flow, or perfusion, has never before been conducted in persons who stutter.

A previous study by Peterson's team used proton magnetic resonance spectroscopy to look at brain regions in both adults and children who stutter. Those findings demonstrated links between stuttering and changes in the brain circuits that control speech production, as well as those supporting attention and emotion. The new blood flow study suggests that disturbances in the speech processing areas of the brain are likely of central importance as a cause of stuttering.

According to Peterson, the blood flow study provides scientists with a completely different direction into the brain. The researchers were able to zero in on the Broca’s area as well as related brain networks specifically linked to speech, using regional cerebral blood flow as a measure of brain activity, since blood flow is typically coupled with neural activity.

“When other portions of the brain circuit related to speech were also affected according to our blood flow measurements, we saw more severe stuttering in both children and adults,” said first author Jay Desai, MD, a clinical neurologist at CHLA. “Blood flow was inversely correlated to the degree of stuttering – the more severe the stuttering, the less blood flow to this part of the brain,” said Desai, adding that the study results were “quite striking.”

The research was published in the journal Human Brain Mapping.