New Research Provides Insight into the Underlying Neural Mechanisms of Stuttering

A recent study published in the journal Neuropsychologia has found that adults who stutter exhibit lower accuracy in repeating nonword syllable sequences compared to typically fluent speakers. This difference in performance is linked to distinct patterns of brain activity, particularly in regions associated with sensorimotor and auditory processing. The findings shed light on the underlying mechanisms of stuttering.

Developmental stuttering is a condition that affects speech fluency, impacting millions of people worldwide. It often begins in early childhood and, while many children naturally recover, those who do not can experience significant psychosocial challenges throughout their lives. Understanding the neural mechanisms behind stuttering can help develop better therapeutic strategies.

Researchers have long suspected that stuttering involves anomalies in brain regions responsible for speech production and working memory. However, the exact mechanisms remain unclear. This study aimed to explore these mechanisms by examining brain activity patterns in adults who stutter during a task that challenges their phonological working memory.

“Most kids who stutter recover naturally and one of the tasks that predicts recovery or persistence is a nonword repetition task. I want to work toward why that task is predictive and I started with adults,” said study author Andrew Bowers, an associate professor at the University of Arkansas and a speech-language pathologist.

The researchers recruited 23 adults who stutter and 23 typically fluent speakers, matched for age, sex, and handedness. Participants were asked to repeat nonword syllable sequences presented to them, a task known to be challenging for individuals who stutter. The sequences included two and four syllable combinations of simple syllables like “ba,” “ma,” “pa,” and “wa.”

Participants performed the repetition task in a controlled environment, seated comfortably in a sound-attenuated room to minimize external distractions. The task was designed to isolate different phases of processing—encoding, maintenance, and execution—by including a delay period between hearing the syllables and being cued to repeat them.

This setup allowed researchers to pinpoint when and where in the brain differences in processing occurred. The EEG data were analyzed to identify patterns of event-related desynchronization and event-related synchronization in specific frequency bands, focusing on alpha, beta, and theta oscillations.

Event-related desynchronization and event-related synchronization are measures of changes in brain wave activity in response to specific events or stimuli. Event-related desynchronization refers to a decrease in the power of specific frequency bands, indicating active processing or engagement of neural resources in a particular task. Event-related synchronization, on the other hand, refers to an increase in the power of specific frequency bands, which is often associated with a state of relaxation or disengagement of certain neural circuits.

The researchers found that adults who stutter showed lower accuracy in repeating both two-syllable and four-syllable sequences compared to typically fluent speakers. This difference was more pronounced in the four-syllable sequences, which demand higher cognitive load. The EEG data revealed that these performance differences were linked to distinct patterns of brain activity.

In typically fluent speakers, the EEG data showed significant event-related desynchronization in the alpha and beta frequency bands over the left temporal lobe during the encoding and maintenance phases of the task. This desynchronization indicates active processing of auditory and phonological information. In contrast, adults who stutter showed reduced event-related desynchronization in these regions, suggesting less efficient auditory-phonological processing.

“Most of the findings were predictable from theory and other findings but I was a little surprised at how well auditory alpha rhythms predicted syllable repetition accuracy in adults who stutter,” Bowers told PsyPost.

Additionally, typically fluent speakers exhibited greater event-related desynchronization in the left sensorimotor cortex during the task, particularly during the execution phase, reflecting robust sensorimotor integration. Adults who stutter, however, had reduced event-related desynchronization in this region, indicating difficulties in integrating sensory and motor information for speech production.

The study also found that in typically fluent speakers, theta oscillations in the left temporal alpha component were strongly correlated with task accuracy during the maintenance phase. This suggests that these oscillations play an important role in maintaining syllable sequences in short-term memory. In adults who stutter, however, task accuracy was more related to higher frequency theta/alpha oscillations during the execution phase. This indicates that these individuals may rely on compensatory mechanisms during speech production, possibly due to earlier inefficiencies in processing.

The findings provide evidence that “the demands of syllable sequencing and holding syllables in short-term memory are related to some differences in sensorimotor integration processes in adults who stutter,” Bowers explained.

But the study has some limitations. The sample size was relatively small, and the study focused on a specific task that may not fully capture the complexity of stuttering in natural speech contexts. Additionally, the study did not examine trial-by-trial variations in brain activity, which could provide further insights into individual differences in performance.

Future research should explore larger and more diverse populations, including children who stutter, to understand how these neural mechanisms develop over time. Longitudinal studies could also help determine whether the observed brain activity patterns are a cause or a consequence of stuttering. Investigating how these patterns interact with other cognitive processes, such as attention and lexical access, could offer a more comprehensive understanding of stuttering.

“I’d like to build on these findings toward more naturalistic tasks and to explore nonword repetition task demands in preschool age children who stutter,” Bowers said.

The study, “Lower nonword syllable sequence repetition accuracy in adults who stutter is related to differences in audio-motor oscillations,” was authored by Andrew Bowers and Daniel Hudock.