  Neuronal avalanches, measured in vitro and in vivo, exhibit a robust critical
behaviour. Their temporal organization hides the presence of correlations. Here
we present experimental measurements of the waiting time distribution between
successive avalanches in the rat cortex in vitro. This exhibits a non-monotonic
behaviour, not usually found in other natural processes. Numerical simulations
provide evidence that this behaviour is a consequence of the alternation
between states of high and low activity, named up and down states, leading to a
balance between excitation and inhibition controlled by a single parameter.
During these periods both the single neuron state and the network excitability
level, keeping memory of past activity, are tuned by homeostatic mechanisms.
