Speech rate association with cerebellar white-matter diffusivity in adults with persistent developmental stuttering.

Speech rate is a basic characteristic of language production, which affects the speaker's intelligibility and communication efficiency. Various speech disorders, including persistent developmental stuttering, present altered speech rate. Specifically, adults who stutter (AWS) typically exhibit a slower speech rate compared to fluent speakers. Evidence from imaging studies suggests that the cerebellum contributes to the paced production of speech. People who stutter show structural and functional abnormalities in the cerebellum. However, the involvement of the cerebellar pathways in controlling speech rate remains unexplored. Here, we assess the association of the cerebellar peduncles with speech rate in AWS and control speakers. Diffusion MRI and speech-rate data were collected in 42 participants (23 AWS, 19 controls). We used deterministic tractography with Automatic Fiber segmentation and Quantification (AFQ) to identify the superior, middle, and inferior cerebellar peduncles (SCP, MCP, ICP) bilaterally, and quantified fractional anisotropy (FA) and mean diffusivity (MD) along each tract. No significant differences were observed between AWS and controls in the diffusivity values of the cerebellar peduncles. However, AWS demonstrated a significant negative association between speech rate and FA within the left ICP, a major cerebellar pathway that transmits sensory feedback signals from the olivary nucleus into the cerebellum. The involvement of the ICP in controlling speech production in AWS is compatible with the view that stuttering stems from hyperactive speech monitoring, where even minor deviations from the speech plan are considered as errors. In conclusion, our findings suggest a plausible neural mechanism for speech rate reduction observed in AWS.

White matter pathways in persistent developmental stuttering: Lessons from tractography.

PURPOSE: Fluent speech production relies on the coordinated processing of multiple brain regions. This highlights the role of neural pathways that connect distinct brain regions in producing fluent speech. Here, we aim to investigate the role of the white matter pathways in persistent developmental stuttering (PDS), where speech fluency is disrupted. METHODS: We use diffusion weighted imaging and tractography to compare the white matter properties between adults who do and do not stutter. We compare the diffusion properties along 18 major cerebral white matter pathways. We complement the analysis with an overview of the methodology and a roadmap of the pathways implicated in PDS according to the existing literature. RESULTS: We report differences in the microstructural properties of the anterior callosum, the right inferior longitudinal fasciculus and the right cingulum in people who stutter compared with fluent controls. CONCLUSIONS: Persistent developmental stuttering is consistently associated with differences in bilateral distributed networks. We review evidence showing that PDS involves differences in bilateral dorsal fronto-temporal and fronto-parietal pathways, in callosal pathways, in several motor pathways and in basal ganglia connections. This entails an important role for long range white matter pathways in this disorder. Using a wide-lens analysis, we demonstrate differences in additional, right hemispheric pathways, which go beyond the replicable findings in the literature. This suggests that the affected circuits may extend beyond the known language and motor pathways.

Beyond production: Brain responses during speech perception in adults who stutter.

Developmental stuttering is a speech disorder that disrupts the ability to produce speech fluently. While stuttering is typically diagnosed based on one's behavior during speech production, some models suggest that it involves more central representations of language, and thus may affect language perception as well. Here we tested the hypothesis that developmental stuttering implicates neural systems involved in language perception, in a task that manipulates comprehensibility without an overt speech production component. We used functional magnetic resonance imaging to measure blood oxygenation level dependent (BOLD) signals in adults who do and do not stutter, while they were engaged in an incidental speech perception task. We found that speech perception evokes stronger activation in adults who stutter (AWS) compared to controls, specifically in the right inferior frontal gyrus (RIFG) and in left Heschl's gyrus (LHG). Significant differences were additionally found in the lateralization of response in the inferior frontal cortex: AWS showed bilateral inferior frontal activity, while controls showed a left lateralized pattern of activation. These findings suggest that developmental stuttering is associated with an imbalanced neural network for speech processing, which is not limited to speech production, but also affects cortical responses during speech perception.

Dorsal and ventral language pathways in persistent developmental stuttering.

Persistent developmental stuttering is a speech disorder that affects an individual's ability to fluently produce speech. While the disorder mainly manifests in situations that require language production, it is still unclear whether persistent developmental stuttering is indeed a language impairment, and if so, which language stream is implicated in people who stutter. In this study, we take a neuroanatomical approach to this question by examining the structural properties of the dorsal and ventral language pathways in adults who stutter (AWS) and fluent controls. We use diffusion magnetic resonance imaging and individualized tract identification to extract white matter volumes and diffusion properties of these tracts in samples of adults who do and do not stutter. We further quantify diffusion properties at multiple points along the tract and examine group differences within these diffusivity profiles. Our results show differences in the dorsal, but not in the ventral, language-related tracts. Specifically, AWS show reduced volume of the left dorsal stream, as well as lower anisotropy in the right dorsal stream. These data provide neuroanatomical support for the view that stuttering involves an impairment in the bidirectional mapping between auditory and articulatory cortices supported by the dorsal pathways, not in lexical access and semantic aspects of language processing which are thought to rely more heavily on the left ventral pathways.

The frontal aslant tract underlies speech fluency in persistent developmental stuttering.

The frontal aslant tract (FAT) is a pathway that connects the inferior frontal gyrus with the supplementary motor area (SMA) and pre-SMA. The FAT was recently identified and introduced as part of a "motor stream" that plays an important role in speech production. In this study, we use diffusion imaging to examine the hypothesis that the FAT underlies speech fluency, by studying its properties in individuals with persistent developmental stuttering, a speech disorder that disrupts the production of fluent speech. We use tractography to quantify the volume and diffusion properties of the FAT in a group of adults who stutter (AWS) and fluent controls. Additionally, we use tractography to extract these measures from the corticospinal tract (CST), a well-known component of the motor system. We compute diffusion measures in multiple points along the tracts, and examine the correlation between these diffusion measures and behavioral measures of speech fluency. Our data show increased mean diffusivity in bilateral FAT of AWS compared with controls. In addition, the results show regions within the left FAT and the left CST where diffusivity values are increased in AWS compared with controls. Last, we report that in AWS, diffusivity values measured within sub-regions of the left FAT negatively correlate with speech fluency. Our findings are the first to relate the FAT with fluent speech production in stuttering, thus adding to the current knowledge of the functional role that this tract plays in speech production and to the literature of the etiology of persistent developmental stuttering.

Reduced fractional anisotropy in the anterior corpus callosum is associated with reduced speech fluency in persistent developmental stuttering.

Developmental stuttering is a speech disorder that severely limits one's ability to communicate. White matter anomalies were reported in stuttering, but their functional significance is unclear. We analyzed the relation between white matter properties and speech fluency in adults who stutter (AWS). We used diffusion tensor imaging with tract-based spatial statistics, and examined group differences as well as correlations with behavioral fluency measures. We detected a region in the anterior corpus callosum with significantly lower fractional anisotropy in AWS relative to controls. Within the AWS group, reduced anisotropy in that region is associated with reduced fluency. A statistically significant interaction was found between group and age in two additional regions: the left Rolandic operculum and the left posterior corpus callosum. Our findings suggest that anterior callosal anomaly in stuttering may represent a maladaptive reduction in interhemispheric inhibition, possibly leading to a disadvantageous recruitment of right frontal cortex in speech production.

Accelerated maturation of white matter in young children with autism: a high b value DWI study.

The goal of this work was to study white matter maturation in young children with autism following previous reports of increased cerebral volume during early development, as well as arguments for abnormal neural growth patterns and regulation at this critical developmental period. We applied diffusion tensor imaging (DTI) and high b value diffusion-weighted imaging (DWI) to young children diagnosed with autism and to a typically developing (TD) control group. Fractional anisotropy (FA), probability and displacement were measured in overall analysis as well as in regions of interest (ROI). Individual data points of children with autism were compared to the developmental curves obtained from typically developing children. Increased restriction, reflected in significantly increased FA and probability along with reduced displacement values, was detected in overall analysis as well as in several brain regions. Increased restriction, suggesting an early and accelerated abnormal maturation of white matter, was more dominant in the left hemisphere and was mainly detected in the frontal lobe. No changes were detected in the occipital lobes. These results support previous claims of abnormal brain overgrowth in young children with autism and are in contrast to the decreased restricted diffusion reported in previous studies in adolescent with autism.