TMS–EEG Co-Registration in Patients with Mild Cognitive Impairment, Alzheimer’s Disease and Other Dementias: A Systematic Review

This review explores how combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) can help understand brain connectivity in patients with dementia. TMS–EEG co-registration enables us to explore the interactions between distinct neural areas during cognitive processes, the causal role of specific brain area in behaviour, as well as the connectivity and relationship between activities in distinct brain regions. The technique allows researchers to observe how stimulating one brain area with TMS affects other connected areas, offering insights into how Alzheimer's disease (AD) and other dementias disrupt brain networks. By means of combined TMS–EEG, the causal role can be established in the interactions between cortical areas and neuronal networks. Studies using this approach have revealed altered brain responses and connectivity patterns in dementia patients, potentially aiding in diagnosis and treatment monitoring. The combination of TMS with EEG may offer a direct non-invasive method to explore cortical reactivity and functional connectivity in physiological and pathological conditions because the TMS-evoked cortical response extends to anatomically distant but connected areas.

• 1
  TMS-EEG studies have identified a reduced TMS-evoked P30 response in AD patients, particularly in the temporo-parietal and fronto-central areas, indicating sensorimotor system rearrangement. The authors found in the AD patients a significantly decreased TMS-evoked P30 (time-locked response 30 ms after the TMS) in the temporo-parietal cortex ipsilateral to stimulation side and in the contralateral fronto-central area corresponding to the sensori-motor network
• 2
  Alterations in dorsolateral prefrontal cortex (DLPFC) connectivity and an inverse correlation between prefrontal-to-parietal connectivity and cognitive impairment were observed, suggesting maladaptive pathogenic mechanisms. These results suggest that advancing disease severity might be associated with effective connectivity enhancement involving long-distance fronto-parietal connections and may reflect maladaptive pathogenic mechanisms that arise from a damaged balance between the excitatory and inhibitory activity in anterior and posterior regions.
• 3
  rTMS applied over PC lead to a significant improvement in episodic memory, but not in other cognitive domains.rTMS of the precuneus (PC) can increase neural activity and modulate brain connectivity within the default mode network (DMN), potentially improving episodic memory in early AD patients.Analysis of TMS–EEG signal in patients’ PC showed an increase in neural activity and of brain oscillations in the beta band as well as changes in the functional interactions between the PC and medial frontal areas within the default mode network (DMN)