Stuttering as a matter of delay in neural activation: A combined TMS/EEG study

Objective: Brain dynamics in developmental stuttering (DS) are not well understood. The supplementary motor area (SMA) plays a crucial role, since it communicates with regions related to planning/execution of movements, and with sub-cortical regions involved in paced/voluntary acts (such as speech). We used TMS combined with EEG to shed light on connections in DS, stimulating the SMA. Methods: TMS/EEG was recorded in adult DS and fluent speakers (FS), stimulating the SMA during rest. TMS-evoked potentials and source distribution were evaluated. Results: Compared to FS, stutterers showed lower activity of neural sources in early time windows: 66– 82 ms in SMA, and 91–102 ms in the left inferior frontal cortex and left inferior parietal lobule. Stutterers, however, showed higher activations in later time windows (i.e. from 260–460 ms), in temporal/premotor regions of the right hemisphere. Conclusions: These findings represent the functional counterpart to known white matter and cortico- basal-thalamo-cortical abnormalities in DS. They also explain how white matter abnormalities and cortico-basal-thalamo-cortical dysfunctions may be associated in DS. Finally, a mechanism is proposed in which compensatory activity of the non-dominant (right) hemisphere is recruited. Significance: DS may be a disorder of neural timing that appears to be delayed compared to FS; new mechanisms that support stuttering symptoms are inferred; the SMA may be a promising target for neuro-rehabilitation.