Scale-free dynamics in cortex after perinatal hypoxia

Following perinatal hypoxia, neonatal cortex follows a stereotypical recovery sequence that includes a period of "burst suppression", during which the EEG exhibits sudden, irregular fluctuations of highly variable size and shape. Clinical outcome depends critically on this phase, ranging from complete recovery to permanent cognitive or motor disability and even death. Here we analyze the statistical properties of burst suppression in neonatal EEG recordings. We show that fluctuations in burst size exhibit long-tailed power law distributions
up to a remarkable six orders of magnitude. Despite this immense variability, their average shape at all temporal scales can be rescaled to a near universal template. Deviations from universality include a flattening of fluctuation shapes at long time scales and the expression of leftward or rightward asymmetry. These features are consistent with the phenomenon of crackling noise that arises in disparate physical systems such as crumpling paper,
ferromagnetic materials subject to a slowly increasing external field, and earthquakes, all of which exhibit scale-free bursty events. Here, as in studies of crackling noise, the average shapes shed light on the underlying mechanisms, which are still poorly understood during burst suppression. Using simple phenomenological models, we show how changes to the average shapes arise from different forms of state-dependent damping. Statistical analysis of
the variability and average shapes of bursts holds promise for new diagnostic opportunities in this critical clinical window and will inform future biologically-detailed models.