Communication through coherence with inter-areal delays

•CTC considered delayed uni-directional and zero-phase bi-directional communication.•Recent studies show that inter-areal gamma coherence has a non-zero phase lag.•Therefore, feedforward and feedback communication likely use CTC separately.•Feedforward-input receiving and feedback-output sending neurons are separate.•Gamma dominates feedforward, beta dominates feedback communication.

The communication-through-coherence (CTC) hypothesis proposes that anatomical connections are dynamically rendered effective or ineffective through the presence or absence of rhythmic synchronization, in particular in the gamma and beta bands. The original CTC statement proposed that uni-directional communication is due to rhythmic entrainment with an inter-areal delay and a resulting non-zero phase relation, whereas bi-directional communication is due to zero-phase synchronization. Recent studies found that inter-areal gamma-band synchronization entails a non-zero phase lag. We therefore modify the CTC hypothesis and propose that bi-directional cortical communication is realized separately for the two directions by uni-directional CTC mechanisms entailing delays in both directions. We review evidence suggesting that inter-areal influences in the feedforward and feedback directions are segregated both anatomically and spectrally.