Abstract

Motor balance in developmental stuttering (DS) was investigated with Transcranial Magnetic Stimulation (TMS), with the aim to define novel neural markers of persistent DS in adulthood. Eleven DS adult males were evaluated with TMS on tongue primary motor cortex, compared to 15 matched fluent speakers, in a “state” condition (i.e. stutterers vs. fluent speakers, no overt stuttering). Motor and silent period thresholds (SPT), recruitment curves, and silent period durations were acquired by recording tongue motor evoked potentials. Tongue silent period duration was increased in DS, especially in the left hemisphere (P<0.05; Hedge’s g or Cohen’s dunbiased = 1.054, i.e. large effect size), suggesting a “state” condition of higher intracortical inhibition in left motor cortex networks. Differences in motor thresholds (different excitatory/inhibitory ratios in DS) were evident, as well as significant differences in SPT. In fluent speakers, the left hemisphere may be marginally more excitable than the right one in motor thresholds at lower muscular activation, while active motor thresholds and SPT were higher in the left hemisphere of DS with respect to the right one, resulting also in a positive correlation with stuttering severity. Pre-TMS electromyography data gave overlapping evidence. Findings suggest the existence of a complex intracortical balance in DS tongue primary motor cortex, with a particular interplay between excitatory and inhibitory mechanisms, also in neural substrates related to silent periods. Findings are discussed with respect to functional and structural impairments in stuttering, and are also proposed as novel neural markers of a stuttering “state” in persistent DS, helping to define more focused treatments (e.g. neuro-modulation).

Highlights

  • Developmental Stuttering (DS) is a disruption in the normal rhythm of speech: persons are unable to utter a fluent speech

  • DS improves after administration of antidopaminergic, serotoninergic or GABAergic drugs [24,25,26,27,28,29,30], i.e. substances acting on the excitatory/inhibitory ratio of motor networks that rely on basal ganglia

  • Considering that neural abnormalities in DS are present when stutterers are not realizing a speech-specific motor task [38], and/or during rest [31,35,42], in the present work we aimed to investigate the “basic” motor activity of speech muscles, in DS males, by measuring Transcranial magnetic stimulation (TMS) indexes, such as motor thresholds, motor evoked potentials (MEPs), and durations of silent periods, in comparison to fluent speakers

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Summary

Introduction

Developmental Stuttering (DS) is a disruption in the normal rhythm of speech: persons are unable to utter a fluent speech. Speech-related structures show greater activity in the right hemisphere with respect to homologue areas of the left hemisphere [12], and greater neural activation in supplementary motor regions [5] It is not clear if the majority of DS neural abnormalities are a prerequisite for its appearance, or if they are mainly the result of long-term stuttering (comprising attempts to overcome dysfluencies) [14,15]. Considering that neural abnormalities in DS are present when stutterers are not realizing a speech-specific motor task [38], and/or during rest [31,35,42], in the present work we aimed to investigate the “basic” motor activity of speech muscles, in DS males, by measuring TMS indexes, such as motor thresholds, motor evoked potentials (MEPs), and durations of silent periods, in comparison to fluent speakers (some of which will be investigated for the first time in tongue muscles of DS, such as silent period). Data were correlated with stuttering severity and behavioral/cognitive indexes of DS

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