Neuroimaging and transcranial magnetic stimulation provide insights into the neuronal mechanisms underlying speech disfluencies in chronic persistent stuttering. In this paper, the authors sought to highlight robust findings, and conducted a meta-analysis of diffusion tensor imaging studies which have recently implicated disrupted white matter connectivity in stuttering. A reduction of fractional anisotropy in persistent stuttering has been reported at several different loci. The authors’ meta-analysis revealed consistent deficits in the left dorsal stream and in the interhemispheric connections between the sensorimotor cortices. In addition, recent fMRI meta-analyses link stuttering to reduced left fronto-parieto-temporal activation while greater fluency is associated with boosted co-activations of right fronto-parieto-temporal areas. However, the physiological foundation of these irregularities is not accessible with MRI. Complementary, transcranial magnetic stimulation (TMS) reveals local excitatory and inhibitory regulation of cortical dynamics. Applied to a speech motor area, TMS revealed reduced speech-planning-related neuronal dynamics at the level of the primary motor cortex in stuttering. The authors noted this review provides a focused view of the neurobiology of stuttering to date and may guide the rational design of future research in accounting for the perpetual dynamic interactions between auditory, somatosensory, and speech motor circuits that shape fluent speech.
Neef, Nocole E., Anwander, Alfred, and Friederici, Angela D., (2015). The Neurobiological Grounding of Persistent Stuttering: from Structure to Function. Current Neurology and Neuroscience Reports. doi: 10.1007/s11910-015-0579-4