Effect of muscular activation on surrounding motor networks in developmental stuttering: A TMS study

Psychedelics in developmental stuttering to modulate brain functioning: a new therapeutic perspective?

Developmental stuttering (DS) is a neurodevelopmental speech-motor disorder characterized by symptoms such as blocks, repetitions, and prolongations. Persistent DS often has a significant negative impact on quality of life, and interventions for it have limited efficacy. Herein, we briefly review existing research on the neurophysiological underpinnings of DS -specifically, brain metabolic and default mode/social-cognitive networks (DMN/SCN) anomalies- arguing that psychedelic compounds might be considered and investigated (e.g., in randomized clinical trials) for treatment of DS. The neural background of DS is likely to be heterogeneous, and some contribution from genetically determinants of metabolic deficiencies in the basal ganglia and speech-motor cortical regions are thought to play a role in appearance of DS symptoms, which possibly results in a cascade of events contributing to impairments in speech-motor execution. In persistent DS, the difficulties of speech are often linked to a series of associated aspects such as social anxiety and social avoidance. In this context, the SCN and DMN (also influencing a series of fronto-parietal, somato-motor, and attentional networks) may have a role in worsening dysfluencies. Interestingly, brain metabolism and SCN/DMN connectivity can be modified by psychedelics, which have been shown to improve clinical evidence of some psychiatric conditions (e.g., depression, post-traumatic stress disorder, etc.) associated with psychological constructs such as rumination and social anxiety, which also tend to be present in persistent DS. To date, while there have been no controlled trials on the effects of psychedelics in DS, anecdotal evidence suggests that these agents may have beneficial effects on stuttering and its associated characteristics. We suggest that psychedelics warrant investigation in DS.

Reduced duration of stuttering-like disfluencies and consistent anticipatory slowing during an adaptation task

PURPOSE

The adaptation effect in stuttering, traditionally described as the reduction of stuttering moments over repeated readings, provides a context to investigate fluency facilitation as well as a relatively controlled means of comparing fluent speech in the immediate vicinity of words that were stuttered versus fluently produced. Acoustic studies have documented decreased duration of fluent speech during adaptation but rarely address changes in disfluencies or the speech preceding or following the disfluencies. This study addresses this gap in the research by documenting frequency and duration changes in both fluent and stuttered syllables.

METHOD

Fifteen people who stutter read passages aloud five times in succession. Frequency and duration of fluent syllables, pauses, stuttering-like disfluencies (SLDs) and other disfluencies (ODs) were compared across the five readings. In addition, durations for syllables before and after pauses and SLDs were compared to determine if there were anticipation or carryover effects of SLDs on surrounding syllables.

RESULTS

Durations measured for more than 22 000 fluent syllables, 1531 pauses, 128 ODs and 1752 SLDs. For most of the 15 participants, significant decreases in both frequency and duration of SLDs over the five readings were observed. In addition, lengthening of fluent syllables immediately preceding the disfluent syllables was observed: this pre-SLD lengthening did not change over the five readings.

CONCLUSIONS

Decreased duration of SLDs across readings supports the motor practice hypothesis, which assumes that successive reading of the same text increases the efficiency of the speech motor plans resulting in less stuttering and decreased durations of the stuttering that persists. Pre-SLD lengthening merits further study, because it informs our knowledge of the time course of stuttered events and may be associated with conscious or unconscious anticipation of upcoming SLDs that does not decrease with motor practice.

WHAT THIS PAPER ADDS

What is already known on this subject The frequency of stuttering-like disfluencies (SLDs) can be reduced using a variety of fluency-enhancing strategies. For example, the adaptation effect, in which a reduction of stuttered events occurs over repeated readings of the same material, has been widely studied. Previous studies have shown that durations of fluent syllables decrease during adaptation, supporting the hypothesis that repeated practice of the motor plan leads to increased fluency. However, temporal changes in disfluent syllables and syllables preceding and following SLDs have rarely been studied, so our understanding of the effect of motor practice on stuttering reduction is incomplete. What this study adds This study has two significant findings. First, stuttered disfluencies that persisted after the initial reading of the adaptation task tended to become shorter in duration. Fluently produced syllables and those that were stuttered, both of which are speech events related to motor control of articulators, were affected in a similar manner by the motor practice associated with adaptation. Second, lengthening of fluent syllables immediately preceding stuttered syllables was observed. This pre-stuttering lengthening, however, did not decrease in duration over the five readings: the mechanism that drives this anticipatory behaviour is not affected by repeated practice. What are the clinical implications of this work? People who stutter have neural differences that lead to speech motor planning and/or execution that is less efficient than that of typical speakers. The finding that stuttering is reduced and that persisting SLDs become shorter in duration over repeated readings provides evidence that motor practice can influence the manifestation of stuttering by temporarily making those specific motor plans more efficient. This may inform treatments for stuttering. The observation that fluent syllables immediately before SLDs were lengthened, and that this lengthening was not influenced by repeated practice, extends our understanding of the time course of stuttering events and may be useful in understanding anticipation and listener reactions to stuttering.

Stuttering-Like Dysfluencies as a Consequence of Long COVID-19

PURPOSE

We present two patients who developed neurogenic stuttering after long COVID-19 related to SARS-CoV-2 infection.

METHODS AND RESULTS

Both patients experienced both physical (e.g., fatigue) and cognitive difficulties, which led to impaired function of attention, lexical retrieval, and memory consolidation. Both patients had new-onset stuttering-like speech dysfluencies: Blocks and repetitions were especially evident at the initial part of words and sentences, sometimes accompanied by effortful and associated movements (e.g., facial grimaces and oro-facial movements). Neuropsychological evaluations confirmed the presence of difficulties in cognitive tasks, while neurophysiological evaluations (i.e., electroencephalography) suggested the presence of "slowed" patterns of brain activity. Neurogenic stuttering and cognitive difficulties were evident for 4-5 months after negativization of SARS-CoV-2 nasopharyngeal swab, with gradual improvement and near-to-complete recovery.

CONCLUSIONS

It is now evident that SARS-CoV-2 infection may significantly involve the central nervous system, also resulting in severe and long-term consequences, even if the precise mechanisms are still unknown. In the present report, long COVID-19 resulted in neurogenic stuttering, as the likely consequence of a "slowed" metabolism of (pre)frontal and sensorimotor brain regions (as suggested by the present and previous clinical evidence). As a consequence, the pathophysiological mechanisms related to the appearance of neurogenic stuttering have been hypothesized, which help to better understand the broader and possible neurological consequences of COVID-19.

Inefficient speech-motor control affects predictive speech comprehension: atypical electrophysiological correlates in stuttering

Listeners predict upcoming information during language comprehension. However, how this ability is implemented is still largely unknown. Here, we tested the hypothesis proposing that language production mechanisms have a role in prediction. We studied 2 electroencephalographic correlates of predictability during speech comprehension-pre-target alpha-beta (8-30 Hz) power decrease and the post-target N400 event-related potential effect-in a population with impaired speech-motor control, i.e. adults who stutter (AWS), compared to typically fluent adults (TFA). Participants listened to sentences that could either constrain towards a target word or not, modulating its predictability. As a complementary task, participants also performed context-driven word production. Compared to TFA, AWS not only displayed atypical neural responses in production, but, critically, they showed a different pattern also in comprehension. Specifically, while TFA showed the expected pre-target power decrease, AWS showed a power increase in frontal regions, associated with speech-motor control. In addition, the post-target N400 effect was reduced for AWS with respect to TFA. Finally, we found that production and comprehension power changes were positively correlated in TFA, but not in AWS. Overall, the results support the idea that processes and neural structures prominently devoted to speech planning also support prediction during speech comprehension.

Weak Vestibular Response in Persistent Developmental Stuttering

Vibrational energy created at the larynx during speech will deflect vestibular mechanoreceptors in humans (Todd et al., 2008; Curthoys, 2017; Curthoys et al., 2019). Vestibular-evoked myogenic potential (VEMP), an indirect measure of vestibular function, was assessed in 15 participants who stutter, with a non-stutter control group of 15 participants paired on age and sex. VEMP amplitude was 8.5 dB smaller in the stutter group than the non-stutter group (p = 0.035, 95% CI [−0.9, −16.1], t = −2.1, d = −0.8, conditional R² = 0.88). The finding is subclinical as regards gravitoinertial function, and is interpreted with regard to speech-motor function in stuttering. There is overlap between brain areas receiving vestibular innervation, and brain areas identified as important in studies of persistent developmental stuttering. These include the auditory brainstem, cerebellar vermis, and the temporo-parietal junction. The finding supports the disruptive rhythm hypothesis (Howell et al., 1983; Howell, 2004) in which sensory inputs additional to own speech audition are fluency-enhancing when they coordinate with ongoing speech.

Speech Fluency Improvement in Developmental Stuttering Using Non-invasive Brain Stimulation: Insights From Available Evidence

Developmental stuttering (DS) is a disturbance of the normal rhythm of speech that may be interpreted as very debilitating in the most affected cases. Interventions for DS are historically based on the behavioral modifications of speech patterns (e.g., through speech therapy), which are useful to regain a better speech fluency. However, a great variability in intervention outcomes is normally observed, and no definitive evidence is currently available to resolve stuttering, especially in the case of its persistence in adulthood. In the last few decades, DS has been increasingly considered as a functional disturbance, affecting the correct programming of complex motor sequences such as speech. Compatibly, understanding of the neurophysiological bases of DS has dramatically improved, thanks to neuroimaging, and techniques able to interact with neural tissue functioning [e.g., non-invasive brain stimulation (NIBS)]. In this context, the dysfunctional activity of the cortico-basal-thalamo-cortical networks, as well as the defective patterns of connectivity, seems to play a key role, especially in sensorimotor networks. As a consequence, a direct action on the functionality of "defective" or "impaired" brain circuits may help people who stutter to manage dysfluencies in a better way. This may also "potentiate" available interventions, thus favoring more stable outcomes of speech fluency. Attempts aiming at modulating (and improving) brain functioning of people who stutter, realized by using NIBS, are quickly increasing. Here, we will review these recent advancements being applied to the treatment of DS. Insights will be useful not only to assess whether the speech fluency of people who stutter may be ameliorated by acting directly on brain functioning but also will provide further suggestions about the complex and dynamic pathophysiology of DS, where causal effects and "adaptive''/''maladaptive" compensation mechanisms may be strongly overlapped. In conclusion, this review focuses future research toward more specific, targeted, and effective interventions for DS, based on neuromodulation of brain functioning.