Over the last 30 years, much of the research in stuttering has focused on understanding neural processes that may play a role in speech disfluency. A lot has been learned but still much remains to be understood. My objective in this article is to provide a concise overview of current research focused on the role of brain processes in developmental stuttering. It is based on a presentation I gave at the annual meeting of the Canadian Stuttering Association in Kingston in 2025.
While it is impossible within this space to provide a comprehensive and detailed overview of this often complex topic, my hope is that this brief summary will succeed in providing you a better understanding of current research, and maybe form a basis for further reading for those of you interested in learning more. Most of what I discuss here is based on research in our lab as well as work by other investigators. While I will refrain from including specific references in the text itself, I will provide a list of some key articles that have served as the basis for this paper is based and that readers can access if they want to learn more.
Early history of stuttering research
The early history of stuttering is replete with examples of people speculating about physical causes of stuttering. These speculations were often focused on deficits of the tongue and other articulators. However, one could argue that the true scientific study of stuttering started with the work of Dr. Lee Travis in the early decades of the 20th century and, in particular, his theory that stuttering is caused by atypical lateralization of speech processes in the brain. According to Travis, people who stutter lacked the typical left hemisphere dominance for speech, resulting in neural signals to speech muscles that were often in conflict with each other. What is important in Travis’s work is that he tried to find experimental proof for his theory using tools, such as electromyography, available to him at that time. It is this effort to find experimental evidence, despite his later decision to abandon his theory due to lack of perceived experimental support, that formed the start of scientific research into what causes stuttering.
Struggle and avoidance behaviours and stuttering
The decades following Travis’s pioneering work saw a move toward a more psychodynamic or sociocultural explanation for stuttering, the latter largely fueled by the work of Dr. Wendell Johnson in the middle of the 20th century. Johnson argued that stuttering is not genetic or biological but the result of negative reactions by listeners. According to this historical view, speech disfluencies are a normal part of a child’s language learning process, but are labeled as abnormal and ‘stuttering’ by people in the child’s environment. The negative reactions, Johnson argued, leads a child to develop struggle behaviours in an attempt to overcome and avoid these disfluencies. Within this ‘diagnosogenic’ theory of stuttering, it is not the speech disfluencies but the struggle and avoidance behaviours that constitute the actual stuttering. Johnson’s approach to stuttering and its treatment, while now primarily of historical significance, had a significant influence on his contemporaries and, in part, resulted in a significant decrease in research focused on neural, physiological and genetic factors in stuttering.
Tools illustrating differences in brain anatomy and function
This changed around 1960 when interest in the role of brain processes in the onset and development of stuttering once again experienced a significant upswing. This was also motivated by the increased availability of more sophisticated electrophysiological brain imaging technology, as well as later on various brain imaging instruments such as positron emission tomography, magnetic resonance imaging and more recently functional near-infrared spectroscopy and magnetoencephalography. These tools allow researchers to visualize differences in brain anatomy and function between people who stutter and nonstuttering individuals and, thereby, develop increasingly detailed mapping of the neural connections between various parts of the brain. This all resulted in an exponential increase in our understanding of brain influences on stuttering.
Understanding of brain processes and areas underlying speech production
Interest in the neurology underlying stuttering was also fueled by our increased understanding of brain processes underlying speech production. In 1861, a French physician by the name of Paul Broca, reported on a patient who had lost his ability to speak, but not to understand language, and who was found to have a lesion in the frontal cortex. This area later became known as Broca’s area. For many years, it was believed that this area was responsible for speech production. Since then, while Broca’s area continues to be seen as a critical region for speech production, it has become clear that speech production is not limited to one brain region but rather is controlled by many distributed but closely interconnected brain areas, both at the cortical and the subcortical level. In addition to Broca’s area, which is located in the lateral frontal cortex, some of the other main areas for speech include the temporal cortex, important for understanding and formulating language, the premotor and motor cortex, involved in planning and executing speech articulation, the parietal cortex for processing sensory information, the basal ganglia important for timing and sequencing of speech movements, and the cerebellum which has an important role in learning new movements, including those for speech. Together, these brain regions orchestrate a network of intricate and very well coordinated neural connections that allow us not only to produce fluent speech but also to anticipate, detect, and correct errors in our speech.
Brain imaging research discovers structural and functional differences
So, what happens when people stutter? Researchers do not yet fully understand how stuttering happens, but the insights that have emerged over the last few decades provide an increasingly compelling picture of what happens in the brain when people stutter. Brain imaging research has revealed both structural (i.e., the anatomy of the brain) and functional (i.e., how the brain works) differences between people who stutter and people who do not. At the outset, it is important to point out that differences do not necessarily mean deficiencies, as there is a significant level of variation between individuals in how their brain develops and functions, in part depending on genetic and developmental experiences. Nevertheless, the differences that have been found to be associated with stuttering may help researchers to better understand what leads to the onset and development of stuttering, as well as what leads to its persistence or recovery.
Anatomical differences
Research to date has found anatomical differences in several brain regions that are part of the speech network. These differences include increased or decreased areas of gray matter, which are areas that contain the nerve cells that process neural signals. Differences have also been observed in the white matter tracts which are the communication highways within the speech network and are critical for coordination and sending neural signals between the various nodes in the network. Similarly, functional brain imaging studies have reported differences between people who stutter and those who do not in how the brain processes speech. These findings suggest that stuttering may be related to differences in speech planning and execution, even in the absence of observable differences in brain anatomy. The precise timing of the neural signals is a critical factor in speech coordination across that neural network. Research has provided evidence that the speed and accuracy of neural timing, critical for speech planning and execution, may be affected in people who stutter. If so, this may help us explain the presence of disfluencies in speech.
Possible causal factors of stuttering in adults
Two other important observations are worth mentioning. First, these anatomical and functional differences are not limited to the left hemisphere, which is the speech dominant hemisphere in most people, but also may affect regions of the right hemisphere. Second, while some imaging findings have revealed similar differences in children and adults, this is not so in all studies. Indeed, some research has reported differences that may be age dependent. The latter finding suggests that while differences that can be observed in both children and adults may represent core causal factors for stuttering, differences that are age dependent may reflect mechanisms that develop over time, possibly as a result of learned compensatory behaviours. One example of this is the observation, especially in studies of adults, of overactivation of certain brain areas during speech production. While it has been suggested that this overactivation may be one of the causal factors of stuttering, it is also possible instead that it is more a reflection of the increased effort when a person who stutters speaks. Indeed, studies have shown that increased effort may result in overactivation of the speech network, even in nonstuttering individuals.
Basal ganglia and speech motor control
One part of the neural speech network that has received increased attention in the last years is a subcortical set of nuclei known as the basal ganglia. It is well known that these nuclei have an important role in movement initiation and coordination, including speech motor control. An increasing number of brain imaging studies have found functional differences at the level of the basal ganglia between people who stutter and those who do not. A recent study compared several brain imaging studies of adults who acquired stuttering later in life, as a result of some neurological condition such as stroke, with a group of adults who experienced developmental stuttering since childhood. The main finding of this study was that there were striking similarities in the observed differences in the neural speech network in individuals with acquired compared to persistent developmental stuttering and that these functional differences were centered on the putamen, one of the nuclei that makes up the basal ganglia . Importantly, the putamen is closely linked to other speech motor areas in the cortex and has a significant role in (speech) motor control.
Future research required
While our understanding of neural mechanism associated with stuttering has grown significantly over the last few decades, many questions remain and require future research. Crucial among these are questions about the precise causal factors that contributed to the onset of developmental stuttering. As technology becomes more sophisticated, facilitating fine-grained brain imaging studies with very young children, answers to these questions will continue to emerge. In addition, studies that link genetics, especially epigenetics (the study of how gene expression is affected by behavioural and environmental factors), with brain imaging studies hold significant promise to provide at least some of the answers needed. Increased insight in early neural mechanisms associated with stuttering onset also may help in early detection and, possibly, prevention. Finally, a better understanding of how stuttering is caused may help dispel some of the myths that still surround stuttering in society.
Multidimensional approach required to understand stuttering
In all of this, it is important to remember that stuttering is not unidimensional and that proper understanding of this complex condition requires a multidimensional approach. Such an approach should combine brain research with a greater appreciation of significant contributions from environmental, psychological, cognitive and learned factors. Involving children and adults who stutter, not only as research participants but, importantly, as knowledge users with learned experiences who can help shape future research will be critical. Additionally, because of the multidimensional nature of stuttering, it is clear that basic research will not in itself provide a pathway for the most effective interventions. As our knowledge of brain processes in stuttering grows, researchers are starting to address how various brain stimulation interventions can be used to modify these processes. We will review some of this research in a future contribution. But speech modification approaches may not be appropriate for all individuals. Indeed, a number of advocates in clinical and knowledge user communities have argued that interventions should focus more on awareness and acceptance of stuttering rather than increased speech fluency. Ultimately, in my view, it is the individual who stutters who will determine what goals they want to set for their speech, goals that may change over time. These goals may include participating more freely in conversations, reducing struggle during speech, or feeling more comfortable communicating, alongside or independent of changes in speech fluency. It is up to us as a professional community to make sure that we develop the skills and evidence-based tools that are effective and that will help them in reaching the goals they set for themselves.
Further reading:
Bloodstein, O., Ratner, N. B., & Brundage, S. B. (2021). A handbook on stuttering. Plural Publishing.
Chang, S. E., & Guenther, F. H. (2019). Involvement of the cortico-basal ganglia-thalamocortical loop in developmental stuttering. Frontiers in Psychology 10.
Franken, M. C., Oonk, L. C., Bast, B. J., Bouwen, J., & De Nil, L. (2024). Erasmus clinical model of the onset and development of stuttering 2.0. Journal of Fluency Disorders, 80.
Mersov, A. M., Jobst, C., Cheyne, D. O., & De Nil, L. (2016). Sensorimotor oscillations prior to speech onset reflect altered motor networks in adults who stutter. Frontiers in human neuroscience, 10.
Smits-Bandstra, S., & De Nil, L. F. (2007). Sequence skill learning in persons who stutter: implications for cortico-striato-thalamo-cortical dysfunction. Journal of fluency disorders, 32(4).
Theys, Catherine, Elina Jaakkola, Tracy R. Melzer, Luc F. De Nil, Frank H. Guenther, Alexander L. Cohen, Michael D. Fox, and Juho Joutsa. (2024) "Localization of stuttering based on causal brain lesions." Brain 147, no. 6.
Professor Luc De Nil is a Full Professor in the Department of Speech-Language Pathology at the University of Toronto. His primary research is focused on investigating the neural bases of developmental stuttering using behavioural and brain imaging technology. He also has published several papers and book chapters on acquired neurogenic stuttering. Dr. De Nil has been named a Fellow of the American Speech-Language-Hearing Association in recognition of his international research and service contributions to the field of communication disorders. He has served as the President of the International Fluency Association and currently serves as Co-Editor-in-Chief of the Journal of Fluency Disorders, the leading peer-reviewed journal publishing papers on basic and clinical research on stuttering and cluttering.