  Self-organized criticality (SOC) refers to the ability of complex systems to
evolve towards a second-order phase transition at which interactions between
system components lead to scale-invariant events beneficial for system
performance. For the last two decades, considerable experimental evidence has
accumulated that the mammalian cortex with its diversity in cell types,
interconnectivity, and plasticity might exhibit SOC. Here we review
experimental findings of isolated, layered cortex preparations to self-organize
towards the four dynamical motifs of up-states, oscillations, neuronal
avalanches, and coherence potentials. During up-states, the synchronization
observed for nested theta/gamma-oscillations embeds scale-invariant neuronal
avalanches identified by robust power law scaling in avalanche sizes with a
slope of -3/2. This precise dynamical coordination can be tracked in negative
transients of the local field potential (nLFP), emerges autonomously in
superficial layers of organotypic cortex cultures, is homeostatically
regulated, exhibits separation of time scales, and reveals unique size vs.
quiet time dependencies. A subclass of avalanches, the coherence potential,
exhibits precise maintenance of temporal precision in propagated local
synchrony. Avalanches emerge in superficial cortex layers under conditions of
strong external driving. The balance of excitation and inhibition (E/I) and
neuromodulators such as dopamine establish powerful control parameters for
avalanche dynamics. This rich dynamical repertoire is not observed in
dissociated cortex cultures, which lack the differentiation into layered
components, suggesting that SOC in cortex requires essential components found
in superficial layers of cortex. The precise interaction between up-states,
nested oscillations and avalanches in layered cortex provides compelling
evidence for SOC in the brain.
