No evidence for widespread synchronized networks in binocular rivalry: MEG frequency tagging entrains primarily early visual cortex

We investigated the spatio-temporal dynamics of the steady-state-visual-evoked field during perceptual switches in binocular rivalry using MEG. Several authors have previously used frequency tagging in MEG studies on binocular rivalry and have claimed to have found a widespread network of synchronized areas that are entrained by the stimulus, reaching up to frontal regions. Moreover, it has been claimed that the results prove that becoming aware of a stimulus is established by increased intra- and interhemispheric synchronization of brain areas, separated by large distances. Our results dovetailed nicely with previous findings such as power and coherence modulations as a function of perceptual state. However, while we also found stimulus-entrained activity across the entire scalp, a phase analysis revealed that the spatially extended nature of the frequency tag was produced by a limited set of occipital sources. Furthermore, we provide evidence that the coherence results from earlier studies are ambiguous in that they likely measured coherence between different sensors whose signals were dominated by the same sources. We conclude that the claims about widespread synchronized networks to consciously perceive flicker stimuli are currently unconvincing.