۱- Introduction

۲- Materials and methods

۳- Results

۴- Discussion

۵- Conclusions and future directions

References

Abstract

Previous studies regarding developmental stuttering (DS) suggest that motor neural networks are strongly affected. Transcranial magnetic stimulation (TMS) was used to investigate neural activation of the primary motor cortex in DS during movement execution, and the influence of muscle representations involved in movements on “surrounding” ones. TMS was applied over the contralateral abductor digiti minimi (ADM) motor representation, at rest and during the movement of homologue first dorsal interosseous muscles (tonic contraction, phasic movements cued by acoustic signalling, and “self-paced” movements). Results highlighted a lower cortico-spinal excitability of ADM in the left hemisphere of stutterers, and an enhanced intracortical inhibition in their right motor cortex (in comparison to fluent speakers). Abnormal intracortical functioning was especially evident during phasic contractions cued by “external” acoustic signals. An exaggerated inhibition of muscles not directly involved in intended movements, in stuttering, may be useful to obtain more efficient motor control. This was stronger during contractions cued by “external” signals, highlighting mechanisms likely used by stutterers during fluency-evoking conditions.

Introduction

Developmental stuttering (DS) is a disturbance in which the rhythm of speech and its normal flow is impaired. It is characterized by symptoms such as blocks and/or repetitions, especially at the start of words and sentences, but also by secondary symptoms such as associated jerks and/or grimaces of the oro-facial muscular districts. DS usually appears during childhood; it may spontaneously recover, but may also persist in adulthood. The causal factors are not fully clarified, but it is now evident that DS is a multi-factorial disorder involving genetic factors as well as neurological impairments (Chang, Garnett, Etchell, & Chow, 2019; Etchell, Civier, Ballard, & Sowman, 2018). More specifically, previous studies have demonstrated that DS is characterized by broad impairments of neural networks (especially in the left hemisphere) involving brain regions such as the left inferior frontal cortex, the associative and the primary motor cortices, and temporo-parietal regions (Chang et al., 2018, 2019; Etchell et al., 2018). Impairments are evident at a structural (e.g. abnormal grey and white matter connections) and functional level (e.g. impaired/lower excitability of the sensorimotor networks of the left hemisphere vs. augmented activity of homologue regions of the right one; Chang et al., 2019; Etchell et al., 2018). In this context, DS should be more appropriately considered as a “dynamic” disorder of motor control, where an impaired/delayed exchange of information is evident in neural networks, thus easily resulting in motor disruptions and stuttering symptoms (e.g. Busan et al., 2019; Ludlow & Loucks, 2003; Salmelin, Schnitzler, Schmitz, & Freund, 2000).