Neural substrates of speech disorders in Parkinson's disease: A narrative review of the evidence from task-based and resting-state fMRI studies.

Speech disorders are highly prevalent among patients with Parkinson's disease (PD). Although insights into the neural substrates of these disorders have been achieved since the introduction of functional magnetic resonance imaging (fMRI), the evidence has never been analyzed altogether. This review aims to summarize and discuss the findings from the two most common approaches of fMRI: task-based and resting-state. We grouped the evidence into four statements: (i) speech disorders in patients with PD are linked with functional changes in the speech production network (SPN), (ii) PD alters the SPN even before the onset of speech disorders, (iii) levodopa have a discrete effect on the SPN functioning, and (iv) speech therapy in patients with PD can induce changes on the activity of the SPN. Limitations of current evidence and future directions in the field are also discussed.