Vertical Topography in <scp>EEG</scp> Microstates: Physiology or Artifact Manifestation?

The analysis of EEG microstates offers a valuable approach for investigating large-scale brain networks and dynamics. Beyond the commonly reported "canonical microstates," prior literature has identified another distinct topography: the vertical topography (VT). This VT is characterized by a prominent straight line dividing positive and negative values, extending from the nasion to the inion. Notably, our own simultaneous EEG/fMRI and shielded cabin EEG data, collected from 77 participants, also revealed the presence of this topography. Based on our subsequent analyses of both human and phantom data, we conclude that VT partly represents artifacts arising from unspecified movements of the EEG cap and its metallic components. This conclusion is strongly supported by our evaluation of VT's spatiotemporal characteristics, derived from EEG recorded under diverse conditions. Specifically, we found a significant correlation between framewise displacement (obtained from human EEG/fMRI) and VT's temporal characteristics. Therefore, we advocate for a prudent interpretation of VT when it appears in data. Its mere existence as a resulting topography can impact the spatiotemporal parameters of other microstates and even distort the shapes of the other topographies.