Neurocorrelates of speech-motor planning and execution in adults and children who stutter

From decades of research, one of the most consistent observations we have made about speech is that it is complicated. Our lips, tongue, jaw, vocal folds, and lungs must work together in a seamless dance of sounds, words and sentences. It’s truly amazing that it all works together—and works together so quickly!

One of the ways it is able to work so quickly is because over the course of development our brains learn how to predict what movements are going to be necessary to produce the words we want to say. This prediction is followed up by a system that monitors errors in speech production. These two systems work together to form a highly precise and accurate method of producing speech. However, sometimes speech production breaks down. Developmental stuttering (or stuttering since childhood) is one of these ways speech production can break down.

Developmental stuttering is a communication disorder of childhood characterized by disruptions in the flow of speech. Speech requires movements; as such stuttering is undoubtedly a disorder of movement. However, when we move our brains must first plan the movements then the brain can carry out the movement. It is unclear if stuttering is related to atypical movement planning or atypical movement execution.

One of the ways we can measure this is through measuring brain activity during speech. There have been many attempts to measure brain activity related to stuttering. However, most of the current studies have two major flaws: they collect data from adults only and the tasks utilized did not examine speech-motor planning and speech-motor execution independently. Therefore, from these studies we do not know if the differences observed are a cause or a consequence of stuttering and we don’t know if the differences are related to the planning of speech or the execution of speech.

To answer both of these questions we asked 15 adults and six children who stutter (along with age- and gender matched peers) to perform two different speech tasks while we measured their brain activity. One task identified brain regions that were active when the planning of speech changed, but the execution did not. The second task identified brain regions that were active when speech execution changed but speech planning did not.

One of our main findings was that while adults and children who stutter showed similar differences during the execution of speech, they showed very different results during the planning of speech. Adults who stuttered showed many more differences in the planning of speech compared to the children. This suggests that while the differences in the execution of speech are fairly consistent across age groups, the planning of speech appears to change as children who stutter become adults who stutter.

Our study also revealed that adults and children who stutter may have a difficult time developing the precise predictions necessary for smooth speech movements. The left hemisphere inferior frontal gyrus is essential for these predictive movements, and in our study both adults and children who stutter showed decreased brain activity in this region. The right hemisphere inferior frontal gyrus is essential for the error monitoring. Similar with previous results, our study found that adults and children who stutter show increased activity in this region. This suggests that the increased activity might be due to the need to make more corrections more frequently. When we consider both of these along with the fact that differences in the execution of speech (but not planning) are present from early on, it suggests that the differences in the development of the predictive aspect of speech production are the result of differences in the execution of speech and not the planning of speech.

The left inferior frontal gyrus is one of the important regions for speech production. People who don’t stutter show more activity in the left inferior frontal gyrus during speech planning and speech execution. The results showed us that both adults and children who stutter showed atypical brain activity during speech planning and during speech execution. During speech planning adults and children who stutter demonstrated more activity in the right inferior frontal gyrus. This increased activity is though to be compensating for reduced activity in the left inferior frontal gyrus. During speech execution adults and children who stutter demonstrated decreased activity in the left inferior frontal gyrus.