Abstract
The study of the healthy brain in elders, especially age-associated alterations in cognition, is important to understand the deficits created by Alzheimer's disease (AD), which imposes a tremendous burden on individuals, families, and society. Although, the changes in synaptic connectivity and reorganization of brain networks that accompany aging are gradually becoming understood, little is known about how normal aging affects brain inter-regional synchronization and functional networks when items are held in working memory (WM). According to the classic Sternberg WM paradigm, we recorded multichannel electroencephalography (EEG) from healthy adults (young and senior) in three different conditions, i.e., the resting state, 0-back (control) task, and 2-back task. The phase lag index (PLI) between EEG channels was computed and then weighted and undirected network was constructed based on the PLI matrix. The effects of aging on network topology were examined using a brain connectivity toolbox. The results showed that age-related alteration was more prominent when the 2-back task was engaged, especially in the theta band. For the younger adults, the WM task evoked a significant increase in the clustering coefficient of the beta-band functional connectivity network, which was absent in the older adults. Furthermore, significant correlations were observed between the behavioral performance of WM and EEG metrics in the theta and gamma bands, suggesting the potential use of those measures as biomarkers for the evaluation of cognitive training, for instance. Taken together, our findings shed further light on the underlying mechanism of WM in physiological aging and suggest that different EEG frequencies appear to have distinct functional correlates in cognitive aging. Analysis of inter-regional synchronization and topological characteristics based on graph theory is thus an appropriate way to explore natural age-related changes in the human brain.
Highlights
Memory decline is typically the first symptom noticed in patients with Alzheimer’s disease (AD)
MWU represent for the Mann-Whitney non-parametric U test to assess the age-related difference of behavioral performance in each n-back task
(2) For the young group, task-induced changes were observed in the beta band between the 2-back and resting conditions; while for the senior group, the 0-back task evoked significant alterations in network topology in both the beta and gamma band
Summary
Memory decline is typically the first symptom noticed in patients with Alzheimer’s disease (AD). Working memory (WM) involves the ability to maintain and manipulate information over short periods of time. It can be subdivided into two parts: the initial encoding of information, and maintenance and retrieval of WM items (Roux and Uhlhaas, 2014). One of the most popular experimental paradigms for WM studies has been the n-back task. In this task, participants are asked to monitor the identity or location of a series of verbal or nonverbal stimuli, and to indicate whether the currently presented stimulus is the same as the one presented previously (Owen et al, 2005). Much effort has been made to elucidate the agerelated alterations of brain characteristics during n-back tasks and to uncover their underlying mechanisms using functional neuroimaging techniques such as functional magnetic resonance imaging (fMRI) (Lamar et al, 2004; Mattay et al, 2006; Schulze et al, 2011; Heinzel et al, 2014; Li et al, 2015; Dev et al, 2017; Jacobs et al, 2017) and electroencephalography (EEG) (Missonnier et al, 2004, 2011; Pesonen et al, 2007; Ho et al, 2012; Saliasi et al, 2013; Barr et al, 2014; Gajewski and Falkenstein, 2014; Dong et al, 2015; Padgaonkar et al, 2017)
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