Figure 6.

Different rhythms allow for directionality in functional connectivity. (A) Cross-correlogram showing a highly stable phase relationship between association cortex (S2) and primary auditory cortex (A1) when both generate beta2 frequency (c. 25 Hz) oscillations driven by kainate in the presence of cholinergic neuromodulation. (B) While the beta2 frequency rhythms in each area are spectrally the same, the mechanisms underlying them at the local network level are not. S2 beta2 rhythms are almost completely resistant to blockade of fast glutamatergic (SYM = SYM2206, 20 μM) or GABAergic (gabazine, 500 nM) synaptic transmission, whereas the same frequency generated by A1 requires both these components of synaptic transmission. The resulting combination of a rhythm dependent on chemical synaptic activity and one resistant to it results in highly significant partial directed coherence estimates (Granger scores) within the beta2 frequency band: The relatively synaptically inert S2 beta2 rhythm is able to dominate over the synaptically labile A1 beta2 rhythm. Note that in both regions, the accompanying superficial layer gamma rhythm is abolished by synaptic blockade and that, for the gamma rhythm, interactions are bi-directional. Figure from Roopun et al. (2010).