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The communication between brain areas is subserved by rhythmic neuronal synchronization. Invasive animal studies have found inter-areal rhythmic synchronization to be particularly prominent in the alpha/beta and the gamma band. In visual cortex, local gamma-band synchronization predominates in superficial layers, and alpha/beta band synchronization in deep layers. Layer-wise anatomical connections differentiate between feedforward and feedback directions: The feedforward (feedback) output of an area originates primarily in superficial (deep) layers, and this preference is stronger for projections traversing more hierarchical levels. Therefore, we investigated whether feedforward and feedback signalling are subserved by distinct frequency bands in the human visual cortex. We correlated frequency resolved Granger-causal (GC) influences between 7 human visual areas with the retrograde anatomical tracing data between the homologous macaque visual areas. This correlation revealed two significant frequency bands corresponding closely to the typical alpha/beta and gamma bands. The sign of the correlation demonstrated that inter-areal gamma-band synchronization subserves feedforward communication, and inter-areal alpha/beta-band synchronization subserves feedback communication. Based on these functional markers of feedforward and feedback streams we examined the hierarchy of an extended set of 26 human visual areas and we found that it is in close agreement with the respective macaque anatomical hierarchy.
