Dynamic graph analysis reveals aging effects on motor network communication

The vast majority of our actions, including their preparation and execution, result from a complex interplay of brain regions. To date our knowledge of aging-associated functional changes in the motor networks, which are known to impact motor performance, remains sketchy. In this study, we generated and analyzed dynamical graphs based on phase-locking of EEG signals recorded from healthy right-handed younger (YS) and older subjects (OS) while they performed a simple finger-tapping task. The network analysis yielded four major results: An underlying coupling structure around movement onset in the low frequencies (2-7 Hz) present in YS and OS. The network in OS, however, contained several additional connections, in particular interhemispheric ones, and showed an overall increased coupling density, which was supported by significantly increased node degrees. Louvain clustering, the calculation of the variance of information, and the node flexibility revealed reduced variability of the subnetworks in OS, particularly during movement preparation. The analysis of hub nodes showed a strong involvement of occipital, parietal, sensorimotor, and central regions in YS. In OS, the first occurrence of sensorimotor hubs was noticeably delayed and preceded by a hub in frontal areas. We were able to unravel the temporal development of specific age-related dynamic network structures, which seem to be a necessary prerequisite for the execution of a motor act. The increased interhemispheric connectivity and the additional inclusion of frontal electrodes converge with but extend previous fMRI data, which report an overactivation, especially in the prefrontal and pre-motor areas, associated with a loss of hemispheric lateralization in OS. All observed network changes, i.e., an increase in frontal nodes and connections and the decrease in flexibility of the established large network, are compatible with a compensatory mechanism to maintain motor function in OS. We further hypothesize that the more extended information processing, suggested by a detour via frontal regions, is related to the longer reaction times observed in OS.