I've spent a couple hours researching this. High dopamine levels in the brain affect the basal ganglia, a region of the brain responsible for voluntary motor actions, including the production of speech. This in turn blocks the "go" signals normally sent from the basal ganglia to Broca's area, the part of the cortex that handles speech. Here are the links I used: https://www.stutteringhelp.org/stuttering-and-basal-ganglia https://www.sciencedirect.com/science/article/pii/104457659290015T Excessive levels of dopamine may also explain why some stutterers, myself included, have tics.