Both ongoing alpha and visually-induced gamma oscillations show reliable diversity in their across-site phase-relations

Neural oscillations have emerged as one of the major electrophysiological phenomena investigated in cognitive and systems neuroscience. These oscillations are typically studied with regard to their amplitude, phase and/or phase coupling. Here we demonstrate the existence of another property that is intrinsic to neural oscillations, but that has hitherto remained largely unexplored in cognitive and systems neuroscience. This pertains to the notion that these oscillations show reliable diversity in their phase relations between neighboring recording sites (phase-relation diversity). In contrast to most previous work, we demonstrate that this diversity is neither restricted to low-frequency oscillations, nor to periods outside of sensory stimulation. On the basis of magnetoencephalographic (MEG) recordings in humans, we show that this diversity is prominent not only for ongoing alpha oscillations (8-12 Hz), but also for gamma oscillations (50-70 Hz) that are induced by sustained visual stimulation. We further show that this diversity provides a dimension within electrophysiological data that, provided a sufficiently high signal-to-noise ratio, does not co-vary with changes in amplitude. These observations place phase-relation diversity on the map as a prominent and general property of neural oscillations that, moreover, can be studied using non-invasive methods in healthy human volunteers. This opens important new avenues for investigating how neural oscillations contribute to the neural implementation of cognition and behavior