  Epileptic seizures are characterized by abnormal and excessive neural
activity, where cortical network dynamics seem to become unstable. However,
most of the time, during seizure-free periods, cortex of epilepsy patients
shows perfectly stable dynamics. This raises the question of how recurring
instability can arise in the light of this stable default state. In this work,
we examine two potential scenarios of seizure generation: (i) epileptic
cortical areas might generally operate closer to instability, which would make
epilepsy patients generally more susceptible to seizures, or (ii) epileptic
cortical areas might drift systematically towards instability before seizure
onset. We analyzed single-unit spike recordings from both the epileptogenic
(focal) and the nonfocal cortical hemispheres of 20 epilepsy patients. We
quantified the distance to instability in the framework of criticality, using a
novel estimator, which enables an unbiased inference from a small set of
recorded neurons. Surprisingly, we found no evidence for either scenario:
Neither did focal areas generally operate closer to instability, nor were
seizures preceded by a drift towards instability. In fact, our results from
both pre-seizure and seizure-free intervals suggest that despite epilepsy,
human cortex operates in the stable, slightly subcritical regime, just like
cortex of other healthy mammalians.
