Electroencephalographic Upper/Low Alpha Frequency Power Ratio Relates to Cortex Thinning in Mild Cognitive Impairment

Abstract

Objective: Temporoparietal cortex thinning is associated with mild cognitive impairment (MCI) due to Alzheimer disease (AD). The increase in EEG upper/low α frequency power ratio has been associated with AD converter MCI subjects. We investigated the association of the EEG upper/low α frequency power ratio with patterns of cortical thickness in MCI. Methods: 74 adult subjects with MCI underwent clinical and neuropsychological evaluation, electroencephalography (EEG) recording and high-resolution 3-dimensional magnetic resonance imaging (MRI). The EEG upper/low α frequency power ratio as well as cortical thickness were computed for each subject. Three MCI groups were detected according to increasing tertile values of EEG upper/low α frequency power ratios, and the difference of cortical thickness among the groups was estimated. Results: The EEG high upper/low α frequency power ratio group had a total cortical grey matter volume reduction of 471 mm², greater than that of the EEG low upper/low α frequency power ratio group (p < 0.001). The EEG high upper/low α frequency power ratio group showed a similar but less marked pattern (160 mm²) of cortical thinning when compared to the EEG middle upper/low α frequency power ratio group (p < 0.001). Moreover, the EEG high upper/low α frequency power ratio group had wider cortical thinning than other groups, mapped to the supramarginal gyrus and precuneus bilaterally. No significant regional cortical thickness differences were found between middle and low EEG upper/low α frequency power ratio groups. Conclusion: A high EEG upper/low α frequency power ratio was associated with temporoparietal cortical thinning in MCI subjects. The combination of upper/low α frequency power ratio and cortical thickness measurement could be useful for identifying individuals at risk for progression to AD dementia and may be of value in the clinical context.

Electroencephalography reveals lower regional blood perfusion and atrophy of the temporoparietal network associated with memory deficits and hippocampal volume reduction in mild cognitive impairment due to Alzheimer&rsquo;s disease

“…This pattern of atrophy was clearly expressed in the MCI subjects with a higher α3/α2 power ratio. This finding fits well with the results of a recent study that investigated the functional connectivity of the human precuneus by resting state functional MRI 75. The authors of that study found that there is a preferential pathway for connectivity between the dorsal precuneus and the supramarginal gyrus, parietal cortex, superior temporal gyrus, and insula.…”

“…Our results are in line with these previous influential studies. A possible integrative view of all the results could be as follows: the higher neuronal activity in the hub regions starts from dysfunctional cellular inhibition; the consequent disinhibition drives the neural network toward oversynchronization; this oversynchronization is specific to hub regions with a higher amyloid burden; these overactivated regions are prone to degeneration and atrophy; a possible neurophysiological sign of this oversynchronization is the higher α3/α2 power ratio we have found in typical hub regions 70–75. It is of great interest that there is overlap between the brain regions associated with an increased EEG α3/α2 power ratio (hypersynchronization of upper alpha) in our study and the regions associated with higher amyloid burden related to memory processes 70,71.…”

“…The deleterious role of synchronization has recently been demonstrated by an interesting study addressing the intriguing relationship between functional and structural degeneration in AD 67. The authors detected some hub (heteromodal associative) regions that are selectively vulnerable in AD pathology, caused by damage to inhibitory interneurons, providing a loss of inhibition at the cellular level 70–75. According to the authors, this disinhibition provokes an increasing amount of neural activity at the network level, ultimately resulting in a hypersynchronization of brain areas.…”

“…As a consequence, the atrophy detected in the MCI group with a higher α3/α2 power ratio could be hypothesized to reflect loss of gray matter in an entire anatomofunctional network rather than atrophy of isolated brain areas. Of note, it is widely accepted that AD is the result of cortical network impairment more than atrophy of single cortical areas 75. In subjects with a low or middle α3/α2 power ratio, the cognitive impairment is possibly due to cerebrovascular impairment or a non-AD degenerative process.…”

“…This could be due to the particular nature of the memory domain, underpinning a large number of brain areas. On the other hand, MCI subjects at higher risk of converting to AD may show impairment in other cognitive domains, such as visuospatial attention or in execution and preparation of spatially guided behavior 75. Of note, the cortical network encompassing the precuneus and inferior parietal cortex is deeply involved in visuospatial abilities 75.…”