Objective: A core feature of persistent developmental stuttering is the paroxysmal, transient, involuntary loss of speech motor control. However, the neurophysiology of stuttering remains largely unexplored. The objective of this study was to explore speech-dependent effects of intracortical excitability modulation on the ventral primary motor cortex (vM1) in persistent developmental stuttering.

Background: In stuttering, previous studies demonstrated a lack of state-dependent facilitation of vM1 during resting-state and during speaking, suggesting defective facilitatory cortical inputs, particularly for speech motor execution. Yet, it is unclear whether local intracortical regulations of excitability are defective during the state of speaking. Here, we studied short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) in the speech-dominant vM1 during speech preparation. We hypothesized that adults who stutter (AWS) would exhibit a reduced intracortical facilitation as compared to adults who do not stutter (ANS).

Method: Conditioning-test paired-pulse transcranial magnetic stimulation was applied to the left vM1 in 14 AWS ( mean age ± SD, 30.4 ± 13.4) and 14 ANS (29.5 ± 9.7) during rest, and while participants uttered an invariable prefix (“auf”) of compound German verbs, and in the 50 msec time window before they started to speak the subsequent verb stem. At each of these time points, we recorded motor evoked potentials from the right tongue side and normalized conditioned amplitudes to the amplitudes of the corresponding unconditioned potentials.

Results: Active motor threshold did not differ between groups. We succeeded in recording ICI and ICF in all participants. AWS showed similar levels of ICI and ICF for rest and speaking. By contrast, in the control group speaking induced a reduction of ICI and a trend for an increase of ICF compared to rest. A repeated-measures ANOVA with ‘group’ (ANS, AWS) as between-subjects factor and ‘time’ (rest, during prefix, before verb stem) as within-subjects factor yielded an interaction of ‘time’ by ‘group’ (F(1, 26), 2.79, p=0.02).

Conclusion: We conclude that the insufficient motor cortex preparation to forthcoming speech is related to an impaired modulation of both, inhibitory and facilitatory, motor intracortical circuitries.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/motor-intracortical-excitability-before-speech-onset-in-adults-who-stutter/