  Phase synchronisation in multichannel EEG is known as the manifestation of
functional brain connectivity. Traditional phase synchronisation studies are
mostly based on time average synchrony measures 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 time and frequency domain respectively. The analysis
is reported for four groups of children including typical, Autism Spectrum
Disorder (ASD), low and high anxiety subjects - a total of 44 subjects. 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 stimulus- and
person-centric manner. Our study is motivated from 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.
