Abstract
Zero-lag synchrony is generally discarded from functional connectivity studies to eliminate the confounding effect of volume conduction. Demonstrating genuine and significant unlagged synchronization between distant brain regions would indicate that most electroencephalography (EEG) connectivity studies neglect an important mechanism for neuronal communication. We previously demonstrated that local field potentials recorded intracranially tend to synchronize with no lag between homotopic brain regions. This synchrony occurs most frequently in antiphase, potentially supporting corpus callosal inhibition and interhemispheric rivalry. We are now extending our investigation to EEG. By comparing the coherency in a recorded and a surrogate dataset, we confirm the presence of a significant proportion of genuine zero-lag synchrony unlikely to be due to volume conduction or to recording reference artifacts. These results stress the necessity for integrating zero-lag synchrony in our understanding of neural communication and for disentangling volume conduction and zero-lag synchrony when estimating EEG sources and their functional connectivity.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
↵2 Strictly speaking, these recordings are still done during a game protocol and some background stimulation that are not time-locked — like a moving starfield — is still present. Also, although the epochs considered cover a period starting 1 s after the end of the last stimulus period, some residual activation may still be present. However, we do not see this as a limitation since the goal of this study is not to associate EEG zero-lag connectivity with a specific paradigm (e.g., event-related or resting-state) but to demonstrate that such instantaneous activity exists and that it is a significant source of functional connectivity in EEG.
↵3 To allow comparison, we use the same participant for examples across the paper.
↵4 The name current source density is arguably more often used than scalp current density, but we prefer the later name in this context since “current source” can easily be confused with the cortical sources estimated by inverse modeling.
