The main aim of this study was to investigate potential neuroanatomical variations in individuals with PDS compared to control group, with the goal of identifying neural correlates associated with stuttering. Structural MRI scans were conducted on adults with PDS and age, sex, hand preference, and education-matched controls. By utilizing the whole brain voxel-based morphometry technique, we compared the WM and GM volume differences between adults with PDS and healthy controls. Our findings revealed multiple regional abnormalities in GM volume across the speech production network in individuals with PDS, while fewer WMV abnormalities were observed in the cerebellum compared to the fluent control group.
In our VBM results, adults with PDS showed increased GMV concentration in two distinct regions: the right SFG and the left MTG. In contrast, a decrease in GMV was observed in the left SFG, paracentral lobule (both left and right), the right cuneus (a region situated in the occipital lobe responsible for visual processing), and the right cerebellum.
Certainly, investigations conducted on children and adults who stutter have indeed provided evidence of abnormalities in the volume of grey matter in the frontal and temporal gyri, which are regions known to be involved in speech production. These abnormalities have been observed bilaterally, indicating alterations in both hemispheres of the brain [14] [15].
One of our notable findings is a reduction in GMV in the left SFG, accompanied by an increase in GMV in the right SFG in adults with PDS compared to the control group. Indeed, the SFG, particularly Broca's area, is crucial for the production and comprehension of speech and plays a significant role in various language-related functions. It contributes to language comprehension, grammar processing, syntactic analysis, and the integration of meaning in sentences [16] [17]. As suggested in previous studies, increased volume in the right frontal and prefrontal regions can be considered as a compensatory mechanism in adults with PDS as a compensatory mechanism [18]. This can explain the differential SFG volume changes in our group of adults with PDS (i.e. decrease left SFG and increase right SFG volume) compared to the control group.
Another significant finding of our study indicates that individuals with PDS exhibit an increase in GM volume in left MTG. The primary auditory cortex, located in the temporal lobe, is responsible for processing the meaning of both spoken words and visual stimuli in humans. Wernicke's area, positioned between the temporal and parietal lobes, plays a crucial role in understanding speech, working together with Broca's area in the frontal lobe. It's worth noting that the functions of the left temporal lobe go beyond basic perception, encompassing comprehension, naming, and verbal memory [19].
While the reported results in the literature regarding the directionality of findings are inconsistent, there exists a substantial body of evidence that supports our own findings [20]. Notably, previous researches have demonstrated that adults who stutter exhibit higher GMV in the left inferior frontal gyrus, as well as the bilateral pre- and post-central gyri, superior temporal gyri, and MTG [21][22]. Conversely, regional decreases in GMV have also been observed in PDS, specifically in the left SFG, left inferior frontal gyrus, and bilateral middle frontal gyrus [22] [23].
Regarding our finding related to decreased GMV in the paracentral lobe, we could not find any supporting evidence. The paracentral lobe plays a crucial role in motor control and coordination, particularly in the lower limbs and pelvic region. In relation to stuttering, the involvement of the paracentral lobe has been a topic of interest in neuroimaging studies that suggested a potential dysfunction in motor planning and execution processes associated with stuttering [24]. However, there is limited research specifically linking GM volume impairments in the paracentral lobe to stuttering [25]. While some studies have reported alterations in GMV in brain regions associated with motor control and coordination, including the paracentral lobe, other studies have not found significant differences in GMV in this specific region among individuals who stutter.
The findings regarding GM/WM volume in the cerebellum and stuttering are not yet fully conclusive and require further investigation [24]. The cerebellum is a brain region involved in motor coordination and timing, and it has been implicated in speech production. We observed that adults with PDS have decreased GM and WM volume in the right and left cerebellum, respectively; however, additional research using larger sample sizes and more refined methodologies is needed to gain a better understanding of the potential role of cerebellum alterations in stuttering.
It is important to acknowledge a limitation of this study, which is the small number of participants. While several MRI studies with similar sample sizes have been published in the literature, the small sample size in our study may introduce higher variability in the data, potentially reducing the reliability of the findings.
Interestingly, it is worth noting that not all studies have observed differences in GM/ WM among adults who stutter [26]. This suggests that the relationship between GMV and stuttering may be complex and influenced by various factors, such as sample characteristics, methodology, and other variables. Further research is needed to better understand the neurobiological underpinnings of stuttering and the potential involvement of structural alterations.
It is crucial to recognize that our understanding of the underlying mechanisms of stuttering is still in its early stages. As research progresses in investigating the neural and genetic foundations of stuttering, scientists may eventually discover an objective biomarker for PDS disorder and identify brain changes associated with effective rehabilitation from this condition. These future advancements would significantly improve clinical assessment methods and bring researchers closer to pinpointing targets for successful treatment. As our understanding of the underlying causes of stuttering deepens, we move closer to the prospect of discovering a lasting cure for this speech condition.