Age related differences in functional synchronization of EEG activity as evaluated by means of TMS-EEG coregistrations

It was recently demonstrated that the characteristics of EEG rhythms preceding a transcranial magnetic stimulation (TMS) of the motor cortex (M1) influence the motor-evoked potential (MEP) amplitude with a peculiar pattern, thus reflecting the M1 functional state. As physiological aging is related to a decrease in motor performance and changes in excitability and connectivity strength within cerebral sensorimotor circuits, we aimed to explore whether aging affects EEG-MEP interactions. Using MRI-navigated TMS and multichannel EEG, we compared the EEG-MEP interactions observed in healthy aged subjects with those observed in young volunteers. We divided the MEPs amplitude into two different subgroups consisting of "high" and "low" MEPs, based on the 50th percentile of their amplitude distribution. Then we analysed the characteristics of the pre-stimulus EEG from M1 and correlated areas separately for the "high" and "low" MEPs, comparing the two conditions. In both young and old subjects, significantly larger MEPs were evoked when the stimulated M1 was coupled in the beta-2 band with the homolateral prefrontal cortex. Conversely, only in young participants was the MEP size modulated when the M1 and homolateral parieto-occipital cortices were coupled in the delta band. The elderly didn't show this kind of pattern. Importantly, this coupling was significantly higher in elderly brains than in young brains, both for high and low MEPs. Our results suggest an age-related significant influence of time-varying coupling of spatially patterned EEG rhythms on motor cortex excitability in response to TMS.