CTC requires two rhythms with a phase relation that is (partly) consistent across time (or multiple observation epochs). The consistency of phase relations is precisely what is quantified by coherence. Crucially, coherence Hydroxychloroquine ic50 entails that the phase estimates of the two signals do
not reflect noise, because with a pure noise signal on either one of the sides, phase relations would be random and there would be no coherence. Thereby, coherence in itself demonstrates (1) the presence of two meaningful rhythms on the two sides and (2) the presence of synchronization. As exemplified in the above scenarios, coherence does not require that two sites show rhythms with the same or similar peak frequency. And we note also that rhythms with the same or similar peak frequency are not sufficient for coherence. If, e.g., the two visual hemispheres are separated by cutting the corpus callosum, then the gamma rhythms in the two hemispheres of a given animal are essentially identical, but there is no coherence (Engel et al., 1991a). We found that Granger-causal influences in the gamma band were substantially stronger in the bottom-up V1-to-V4 direction than vice versa. Granger analyses alone can ultimately not prove or disprove one particular network organization. Yet, the strong bottom-up directedness of the V1-V4 gamma GC influence combines with two additional pieces 3-MA supplier of evidence: (1) both in
V1 and V4, neuronal spiking is gamma synchronized almost exclusively in the superficial layers, while neuronal spiking in infragranular layers lacks gamma synchronization (Buffalo et al., 2011), and (2) V1 neurons projecting to V4 are located almost exclusively in supragranular layers, while V4 neurons projecting to V1 are located almost exclusively in infragranular layers (Barone et al., 2000).
These three pieces of evidence together suggest that (1) in V1, gamma synchronization emerges in supragranular layers, and the behaviorally relevant V1 gamma influences V4 through feedforward projections with their 17-DMAG (Alvespimycin) HCl respective delay; (2) in V4, gamma synchronization also emerges in supragranular layers and primarily influences areas further downstream of V4; and (3) the top-down influence from V4 to V1 originates from deep V4 layers and is therefore mediated to a much lesser extent through the gamma band. A direct test of these predictions will require laminar recordings in both areas simultaneously. Most importantly, we demonstrate strong interareal gamma-band synchronization that links V4 dynamically to the relevant part of V1, precisely as predicted by the CTC hypothesis. The CTC hypothesis states that a local neuronal rhythm modulates input gain rhythmically, that input is therefore most effective if it is consistently timed to moments of maximal gain, and that thereby the synchronization between input and target modulates effective connectivity (Fries, 2005, 2009; Schoffelen et al., 2005, 2011; Womelsdorf et al., 2007; van Elswijk et al., 2010).