Abstract
Electroencephalography (EEG) and magnetoencephalography (MEG) are widely interpreted as direct, non-invasive measures of neural population activity, assuming surrounding tissue and cerebrospinal fluid (CSF) act as passive volume conductors that distorts but do not contribute to signal generation. Using conductive-fluid phantom experiments, structural magnetic resonance imaging, and in vivo EEG and MEG recordings, the present report challenges this assumption. Mechanically driven conductive fluid generated structured, oscillatory, source-localizable electromagnetic signals resembling M/EEG in the absence of neural tissue, and interindividual differences in CSF volume systematically moderated M/EEG signal amplitude. These findings suggest that, at the scales accessed by invasive and non-invasive electrophysiology, electromagnetic and mechanical processes form a coupled system in which contributions of neural currents and fluid motion cannot readily be uniquely separated.