Vertical topography in EEG microstates: Physiology or artifact manifestation?

The analysis of EEG microstates is a useful method for exploring large-scale networks and brain dynamics. In addition to the often-reported microstates, or so-called 'canonical microstates', another topography has been reported in the literature - topography with a prominent straight line separating positive and negative values that extends from the nasion to the inion (vertical topography - VT). This topography was also revealed in our simultaneous EEG/fMRI and shielded cabin EEG data collected from 77 participants. Following analyses based on human and phantom data, we conclude that VT partially reflects artifacts caused by unspecified movements of the EEG cap and its metallic components. Our conclusion is supported by evaluation of spatiotemporal characteristics of VT estimated from EEG acquired under various conditions, especially by significant correlation between the framewise displacement (obtained from human EEG/fMRI) and the temporal characteristics of VT. We recommend cautious interpretation of VT when revealed in the data. Its very presence as a resulting topography may affect the spatiotemporal parameters of the other microstates and distorts the shapes of the other topographies.