Neuropathological mechanisms of acquired apraxia of speech investigated with multimodal transcranial magnetic stimulation and electroencephalography.

Acquired apraxia of speech (AOS) is a debilitating motor speech disorder that frequently co-occurs with post-stroke aphasia. However, its distinct neurophysiological underpinnings remain poorly characterized. This study used combined transcranial magnetic stimulation and electroencephalography (TMS-EEG) to investigate cortical excitability and network dynamics in 30 patients with left-hemisphere stroke (14 with aphasia accompanied by AOS and 16 with aphasia alone) and 10 healthy controls. By stimulating the left inferior frontal region, we analyzed TMS-evoked potentials (TEPs), time-frequency oscillations, and time-varying functional connectivity. While TEP components remained stable across groups, time-frequency analysis demonstrated significantly reduced theta and alpha band power over left frontoparietal regions in patients with AOS compared to those with aphasia alone. Additionally, time-varying network analysis revealed significantly impaired interhemispheric connectivity between left sensorimotor and right motor regions during the late post-stimulus window in patients with AOS, distinct from the other groups. These findings provide novel electrophysiological evidence that comorbid AOS post-stroke is characterized by specific deficits in left hemisphere oscillatory dynamics and interhemispheric sensorimotor integration, offering potential biomarkers for differential diagnosis and targets for neuromodulatory interventions.