Zero-phase-delay connectivity increases the reliability, concordance with structure, and prognostic ability of functional connectivity metrics
Zero-phase-delay synchrony between the activity of distant neural populations has been robustly observed. Nevertheless, contemporary electroencephalography and magnetencephalography functional connectivity analyses typically exclude zero-phase-delay functional connections, assuming that they are predominantly artefactual. However, the effects of excluding them on the performance of functional connectivity metrics as potential biomarkers are unknown. Here, we showed that most cortico-cortical functional connections occur with zero- or near-zero phase-delay, even where such connectivity was unlikely to be artefactual. Including, rather than excluding, zero-phase-delay connectivity increased the reliability, concordance with structural connectivity, and predictive validity (for longitudinal changes in cognition) of functional connectivity metrics. We found that excluding zero-phase-delay connections penalised functional connectivity strength between the strongest structurally connected regions: stronger structural connections led to functional connections with phase-delays closer to zero, mediated by a shorter signal propagation time. Our findings challenge generally accepted assumptions that zero-phase-exclusive methods are superior to zero-phase-inclusive methods.