Stratification of Motor Cortex Excitability to Transcranial Stimulation Uncovers Functional Network Differences in Healthy Older Adults as Revealed by Resting State EEG Functional Coupling in Brain Network

Background: Understanding the neural organization underlying motor function is essential for explaining individual differences in motor performance and the impact of aging. Methods: We examined 87 healthy older adults who underwent, in different sessions, resting state electroencephalography (EEG) recordings and Transcranial Magnetic Stimulation (TMS) applied to the hand muscles representation area in the primary motor cortex to elicit Motor Evoked Potentials (MEPs). Furthermore, Mini-Mental State Examination (MMSE) and handgrip strength evaluations were carried out on subjects. Subjects were split into two groups, Low MEP (L-MEP) and High MEP (H-MEP) groups, based on individual MEP amplitude, and age, sex, and education matched. Functional connectivity was analyzed through Magnitude-Squared Coherence (MSCoh) and Total Coherence (TotCoh) in different frequency bands and brain regions of interest. Findings: The L-MEP presented decreased MSCoh in the Alpha 2 and Beta 1 bands, and decreased TotCoh in the Alpha 2 band within the Temporal region as well as in the Beta 1 band across Parietal, Occipital, and Temporal regions. No significant difference in grip strength was found while the MMSE score of L-MEP group was significantly lower compared to the H-MEP one. These findings indicate that reduced motor cortex excitability reflects decreased network integration, particularly in regions associated with cognitive and sensorimotor processing. These findings may reflect early neurophysiological vulnerability. Conclusions: These results underscore the resting state EEG as a non-invasive, highly sensitive tool monitoring subtle alterations in brain functional networks that may precede clinical symptoms, offering a powerful tool for monitoring and individualized intervention.