Synchronization during an internally directed cognitive state in healthy aging and mild cognitive impairment: a MEG study

Mild Cognitive Impairment (MCI) is a stage between healthy aging and dementia. Functional connectivity is widely used to study the brain activity during resting state or cognitive tasks. We aimed to determine the functional connectivity changes required to deal with an Internally Directed Cognitive State (IDICS) in healthy aging and MCI. This task differs from the most commonly employed in Magnetoencephalograpy (MEG)/ Electroencephalography (EEG), since inhibition from external stimuli is needed, and it allows the study of this control mechanism in healthy and pathological aging. To this end, MEG signals were acquired from 32 healthy individuals and 38 MCI patients, both in resting state and while performing a subtraction task of two levels of difficulty. Phase Locking Value (PLV) was calculated for five frequency bands: delta, theta, alpha, beta and gamma. Synchronization patterns changed in both groups while performing the task. MCI patients presented higher connectivity changes than those in the control group, and this was related to a lower cognitive performance. In particular, in MCIs a hypersynchronization in delta, theta, beta and gamma bands was found, which reveals an abnormal functioning in this group. Contrary to controls, MCIs presented a lack of synchronization in the alpha band which may denote an inhibition deficit. Additionally, the magnitude of connectivity changes rose with the task difficulty in controls but not in MCIs, in line with the CRUNCH model (Compensation-Related Utilization of Neural Circuits Hypothesis)

MEG spectral analysis in subtypes of mild cognitive impairment

Mild cognitive impairment (MCI) has been described as an intermediate stage between normal aging and dementia. Previous studies characterized the alterations of brain oscillatory activity at this stage, but little is known about the differences between single and multidomain amnestic MCI patients. In order to study the patterns of oscillatory magnetic activity in amnestic MCI subtypes, a total of 105 subjects underwent an eyes-closed resting-state magnetoencephalographic recording: 36 healthy controls, 33 amnestic single domain MCIs (a-sd-MCI), and 36 amnestic multidomain MCIs (a-md-MCI). Relative power values were calculated and compared among groups. Subsequently, relative power values were correlated with neuropsychological tests scores and hippocampal volumes. Both MCI groups showed an increase in relative power in lower frequency bands (delta and theta frequency ranges) and a decrease in power values in higher frequency bands (alpha and beta frequency ranges), as compared with the control group. More importantly, clear differences emerged from the comparison between the two amnestic MCI subtypes. The a-md-MCI group showed a significant power increase within delta and theta ranges and reduced relative power within alpha and beta ranges. Such pattern correlated with the neuropsychological performance, indicating that the a-md-MCI subtype is associated not only with a "slowing" of the spectrum but also with a poorer cognitive status. These results suggest that a-md-MCI patients are characterized by a brain activity profile that is closer to that observed in Alzheimer disease. Therefore, it might be hypothesized that the likelihood of conversion to dementia would be higher within this subtype