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Abstract

A hallmark of cortical organization is the coexistence of serial feedforward with reentrant processing. The latter is based on feedback projections from higher to lower processing levels and massive reciprocal excitatory projections which link neurons located within the same cortical areas as well as cortical areas occupying the same level in the processing hierarchy. These reentrant connections, together with local negative feedback loops, give rise to exceedingly complex dynamics that are characterized by oscillations in a broad range of frequencies, synchronization of discharges, and cross-frequency coupling. Evidence is reviewed which suggests that these dynamic properties support specific computations: the flexible binding of distributed neurons into functionally coherent assemblies, the attention-dependent selection of sensory signals, the conversion of semantic relations into temporal relations, the comparison of stored priors with sensory evidence, the selective routing of signals in densely interconnected networks, the definition of relations in the context of learning, and the dynamic formation of functional networks. Arguments challenging a functional role of oscillations and synchrony, due to their volatile nature, are discussed in relation to recent evidence that highlights the advantages of volatility.