On the existence of synchrostates in multichannel EEG signals during face-perception tasks

Phase synchronisation in multichannel electroencephalography (EEG) is known as the manifestation of functional brain connectivity. Traditional phase synchronisation studies are mostly based on time average synchrony measures, and hence do not preserve the temporal evolution of the phase difference. Here we propose a new method to show the existence of a small set of unique phase synchronised patterns or 'states' in multi-channel EEG recordings, each 'state' being stable of the order of ms, from typical and pathological subjects during face perception tasks. The proposed methodology bridges the concepts of EEG microstates and phase synchronisation in the time and frequency domain, respectively. The analysis is reported for four groups of children including typical, autism spectrum disorder, low and high anxiety subjects-a total of 44. In all cases, we observe consistent existence of these states-termed as synchrostates-within specific cognition-related frequency bands (beta and gamma bands), though the topographies of these synchrostates differ for different subject groups with different pathological conditions. The inter-synchrostate switching follows a well-defined sequence capturing the underlying inter-electrode phase relation dynamics in a stimulus- and person-centric manner. Our study is motivated by the well-known EEG microstate exhibiting stable potential maps over the scalp. However, here we report a similar observation of quasi-stable phase synchronised states in multichannel EEG. The existence of the synchrostates coupled with their unique switching sequence characteristics could be considered as a potentially new field over contemporary EEG phase synchronisation studies.