Dementia and Oral Function
The Official Journal of Japan Research Society for Dementia and Oral Function

Review

Neurophysiology in mild cognitive impairment: focusing on the default-mode network

Hiroshi Yoshimura

DOI: https://doi.org/10.63587/dof.1-1.p22

Abstract：
Cognitive ability tends to decline with age and in some cases becomes severe cognitive impairment. The intermediate transition state is named “mild cognitive impairment” (MCI), identifying the onset of MCI is difficult. This difficulty is caused by the fact that current knowledge of MCI is limited. Amyloid beta (Aß) and tau are widely recognized as causative agents of Alzheimer’s disease (AD). Abnormal pathophysiological changes lie below the threshold of detection for AD-related biomarkers, such as Aß42 and tau protein in cerebrospinal fluid. The Aß-tau interaction is initially generated in the brainstem and parahippocampal gyrus before the onset of MCI, and Aß and tau propagate into a default-mode-network (DMN) that is involved in endogenously mediated, self-referential mental activity. The DMN is frequently found to be abnormal due to the progression of AD. Aß-tau interactions affect not only functional connectivity, but also local synaptic activities, resulting in a lowered oscillation frequency by disturbing the balance of activity between excitatory and inhibitory networks (E/I balance). These changes reflect electroencephalogram (EEG) rhythms. The EEG alpha rhythm observed during the resting state tends to decrease, but theta rhythm increases. Before the emergence of clear symptoms of cognitive decline, a lowered frequency of EEG in the resting state becomes apparent. Thus, in the process of transitioning from normal to cognitive impairment, Aß and tau accumulate in the DMN, and Aß-tau interactions disturb the E/I balance. This lowers EEG frequency in the resting state, which may provide a sign of the onset of MCI.

Key words： Mild cognitive impairment, Default-mode-network, Electroencephalogram, E/I balance, Aß-tau interaction