Hippocampal volume and cortical sources of EEG alpha rhythms in mild cognitive impairment and Alzheimer disease

Atrophy of hippocampus and alteration of resting eyes-closed electroencephalographic (EEG) rhythms represent important features of mild cognitive impairment (MCI) and Alzheimer's disease (AD). Here we evaluated linear and non-linear aspects of the relationship between these features in the continuum along MCI and AD conditions, as a reflection of neurodegenerative processes. Eyes-closed resting EEG data were recorded in 60 healthy elderly (Nold), 88 MCI, and 35 Alzheimer's disease (AD) patients. Hippocampal volume was measured in magnetic resonance imaging of the MCI and AD subjects. Based on the normalized hippocampal volume, selected MCI subjects could be divided into two demographically paired sub-groups: those with larger hippocampal volume (MCI + h; N = 40; mini mental state evaluation - MMSE - score = 27.5 ± 0.26 SE) and those with smaller hippocampal volume (MCI - h; N = 40; h; MMSE = 26.5 ± 0.34 SE); the normalized hippocampal volume was statistically greater in the MCI + h than in the MCI - h and AD subjects (p <0.0001). EEG rhythms of interest were delta (2-4 Hz), theta (4-8 Hz), alpha 1 (8-10.5 Hz), alpha 2 (10.5-13 Hz), beta 1 (13-20 Hz), and beta 2 (20-30 Hz). Cortical EEG generators were estimated by LORETA software. Results showed that the power of occipital, parietal, and temporal alpha 1 sources was maximum in MCI + h, intermediate in MCI - h, and low in AD patients. Furthermore, the power of these sources was linearly and non-linearly correlated with the normalized hippocampal volume. These 3 EEG sources were given as input for evaluating correlations (linear, exponential, logarithmic and power) with hippocampal volume. When subjects were considered as a unique group, there was a significant linear correlation of hippocampal volume with the magnitude of alpha 1 sources in the parietal, occipital and temporal areas. In general, the EEG sources showing significant linear correlation with hippocampal volume also supported a non-linear correlation with hippocampal volume strongly for the logarithmic one. The present results suggest that progressive atrophy of hippocampus correlates with decreased cortical alpha power, as estimated by using LORETA source modeling, in the continuum along MCI and AD conditions.