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.
