Stimulus-induced gamma sources reduce in power but not in spatial extent
with healthy aging in human EEG
Abstract
Aging alters brain structure and function, and studying such changes may
help understand the neural basis underlying aging and devise
interventions to detect deviations from healthy progression.
Electroencephalogram (EEG) offers an effective way to study healthy
aging owing to its high temporal resolution and affordability. Recent
studies have shown that narrow-band stimulus-induced gamma oscillations
(20-70 Hz) in EEG, induced with cartesian gratings in a fixation task
paradigm, weaken with healthy aging and onset of Alzheimer’s Disease
(AD) while remaining highly reproducible for a given subject, and thus
hold promise as potential biomarkers. However, functional connectivity
(FC) sometimes changes in a different way compared to sensor power with
aging. This difference could be potentially addressed by studying how
underlying gamma sources change with aging, since either a reduction in
source power or a shrinkage of the sources (or both) could reduce the
power in the sensors but may have different effects on other measures
such as FC. We therefore reconstructed EEG gamma sources through a
linear inverse method called exact Low-resolution Tomography Analysis
(eLORETA) on a large (N=217) cohort of healthy elderly subjects
(>50 years). We further characterized gamma distribution in
cortical space as an exponential fall-off from a seed voxel with maximal
gamma source power to delineate a reduction in magnitude versus
shrinkage. We found significant reduction in magnitude but not shrinkage
with healthy aging. Overall, our results shed light on changes in EEG
gamma source distribution with healthy aging, which could provide clues
about underlying neural mechanisms.