Abstract
Electroencephalogram (EEG) microstates that represent quasi‐stable, global neuronal activity are considered as the building blocks of brain dynamics. Therefore, the analysis of microstate sequences is a promising approach to understand fast brain dynamics that underlie various mental processes. Recent studies suggest that EEG microstate sequences are non‐Markovian and nonstationary, highlighting the importance of the sequential flow of information between different brain states. These findings inspired us to model these sequences using Recurrent Neural Networks (RNNs) consisting of long‐short‐term‐memory (LSTM) units to capture the complex temporal dependencies. Using an LSTM‐based auto encoder framework and different encoding schemes, we modeled the microstate sequences at multiple time scales (200–2,000 ms) aiming to capture stably recurring microstate patterns within and across subjects. We show that RNNs can learn underlying microstate patterns with high accuracy and that the microstate trajectories are subject invariant at shorter time scales (≤400 ms) and reproducible across sessions. Significant drop in the reconstruction accuracy was observed for longer sequence lengths of 2,000 ms. These findings indirectly corroborate earlier studies which indicated that EEG microstate sequences exhibit long‐range dependencies with finite memory content. Furthermore, we find that the latent representations learned by the RNNs are sensitive to external stimulation such as stress while the conventional univariate microstate measures (e.g., occurrence, mean duration, etc.) fail to capture such changes in brain dynamics. While RNNs cannot be configured to identify the specific discriminating patterns, they have the potential for learning the underlying temporal dynamics and are sensitive to sequence aberrations characterized by changes in metal processes. Empowered with the macroscopic understanding of the temporal dynamics that extends beyond short‐term interactions, RNNs offer a reliable alternative for exploring system level brain dynamics using EEG microstate sequences.
Highlights
Four quasi-stable states explain consistently around 80% of total topographic variance in spontaneous electroencephalography (EEG)
Empowered with the macroscopic understanding of the temporal dynamics that extends beyond short-term interactions, Recurrent Neural Networks (RNNs) offer a reliable alternative for exploring system level brain dynamics using EEG microstate sequences
Microstates denote the quasi-stable topography of scalp-EEG that remain constant for approximately 80 ms and are believed to be the building blocks of adaptive chain of neuro-cognitive states
Summary
Four quasi-stable states explain consistently around 80% of total topographic variance in spontaneous electroencephalography (EEG). Modeling techniques based on hidden Markov models (Gschwind, Michel, & Van De Ville, 2015), random walk (von Wegner, Tagliazucchi, Brodbeck, & Laufs, 2016), and stochastic process (von Wegner, Tagliazucchi, & Laufs, 2017) are gaining momentum to investigate the transition properties of microstates Such approaches are limited in terms of their dynamical richness (Gschwind et al, 2015). Instead of trying to explicitly model the temporal dynamics of EEG microstates, we ask if there are any temporal patterns in the sequence of EEG microstates that can be reliably and reproducibly detected This question is best addressed using recurrent neural networks (RNNs) that are known to be a rich and flexible methodology to learn complex temporal dependencies without making any assumption on the temporal characteristics of the signal. Low prediction accuracies beyond a few milliseconds corroborates the nonstationary nature of the resting state microstate sequences due to irregular structure of microstate durations
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