Age-Related EEG Power Reductions Cannot Be Explained by Changes of the Conductivity Distribution in the Head Due to Brain Atrophy

Abstract

Electroencephalogram (EEG) power reductions in the aging brain have been described by numerous previous studies. However, the underlying mechanism for the observed brain signal power reduction remains unclear. One possible cause for reduced EEG signals in elderly subjects might be the increased distance from the primary neural electrical currents on the cortex to the scalp electrodes as the result of cortical atrophies. While brain shrinkage itself reflects age-related neurological changes, the effects of changes in the distribution of electrical conductivity are often not distinguished from altered neural activity when interpreting EEG power reductions. To address this ambiguity, we employed EEG forward models to investigate whether brain shrinkage is a major factor for the signal attenuation in the aging brain. We simulated brain shrinkage in spherical and realistic brain models and found that changes in the conductor geometry cannot fully account for the EEG power reductions even when the brain was shrunk to unrealistic sizes. Our results quantify the extent of power reductions from brain shrinkage and pave the way for more accurate inferences about deficient neural activity and circuit integrity based on EEG power reductions in the aging population.

Comparison of EEG source localization using simplified and anatomically accurate head models in younger and older adults

“…One potential explanation between our hypothesis and the observed results is that changes in the electrical properties of the head with aging may have small effects on EEG source locations (in millimeters) and may become unlikely to distinguish from the effects of simplified head model (in centimeters). A prior study that performed a forward simulation analysis suggested that a 10% cortical shrinkage only led to EEG power reduction by ~3.7 dB [16]. In comparison, older adults demonstrated up to ~20 dB power reduction relative to younger adults during resting trials [60].…”

“…For example, cerebrospinal fluid volume increases in ventricles and between the cortex and the skull due to overall atrophy with aging. Since the cerebrospinal fluid is more conductive than the skull and the brain, increased cerebrospinal fluid shunts more current and attenuates scalp potentials for older adults compared to younger adults [16], [23], [24]. While these prior studies investigated the effects of age differences in brain structure on EEG forward modeling, further work is needed to examine how these changes may affect EEG source localization.…”

Ageing‐related changes in nap neuroscillatory activity are mediated and moderated by grey matter volume

“…We control for potential overall effects of head size on scalp EEG by including estimated total intracranial volume, a proxy for head size (Buckner et al, 2004), as a covariate in our regression models. Additionally, ageing-related changes in head conductivity as a function of brain shrinkage and cerebrospinal fluid increases are insufficient to explain ageing-related scalp EEG power reductions (He et al, 2021). Further, changes in skull and scalp thickness do not have appreciable contributions to ageing-related changes in scalp-recorded EEG, as skull thickness does not change substantially during healthy ageing and scalp thickness changes with ageing are variable (Albert et al, 2007;He et al, 2021;Lynnerup, 2001).…”

Comparison of EEG Source Localization Using Simplified and Anatomically Accurate Head Models in Younger and Older Adults

“…Because prior head model comparisons have relied exclusively on young adults, it is necessary to evaluate the effect of head models on source localization in older adults. Morphological changes in the aging brain can influence the electrical properties of the head model during forward modeling [ 18 ], [ 19 ], [ 20 ]. Age-related changes include a decrease in total brain mass [ 21 ], cortical thinning [ 22 ], cerebrospinal fluid expansion [ 18 ], ventricles enlargement [ 23 ], and gyral atrophy [ 24 ], [ 25 ].…”

“…For example, cerebrospinal fluid volume increases in ventricles and between the cortex and the skull due to overall atrophy with aging. Since the cerebrospinal fluid is more conductive than the skull and the brain, increased cerebrospinal fluid shunts more current and attenuates scalp potentials for older adults compared to younger adults [ 19 ], [ 26 ], [ 27 ]. While these prior studies investigated the effects of age differences in brain structure on EEG forward modeling, further work is needed to examine how these changes may affect EEG source localization.…”