Neuronal Firing in Human Epileptic Cortex: The Ins and Outs of Synchrony During Seizures

Epileptic seizures propagate extensively across the brain. The associated electrophysiological mass phenomena can be cap-tured by scalp or intracranial electroencephalography. Recent evidence shows the initiating mechanisms of seizures operate at the smaller scale of neuronal ensembles, producing phenomena such as high-frequency oscillations (1, 2) and microseizures (3, 4) that are best detected at a tighter spatial resolution of local field potentials recorded by microelectrodes. More recently, researchers have started using microelectrodes to analyze the activity of large numbers of individual neurons during seizures in neocortex (5) and hippocampus (6) and to relate this activity to the corresponding macroscopic EEG recordings. The main focus of this commentary is the most recent of these studies by Schevon et al., which provides potential solutions to puzzles arising from previous neocortical work.The idea that seizures represent “hypersynchrony” goes back at least to the 1950s (7). However, until recently, record-ings from people with epilepsy suggested that neuronal firing is not hypersynchronous, despite the presence of large rhythmic—arguably hypersynchronous—field potentials. Early studies with single microelectrodes (8, 9) found no evidence for extensive or synchronous neuronal firing during seizures. The advent of dense multi-electrode arrays allows sampling of the activity of many neurons within local regions in the brain. Schevon et al. and Trucculo et al. both used microelec-trode (Utah) arrays in the neocortex of epileptic patients. The arrays sampled 96 sites at a depth of 1 mm within a 4 mm × 4 mm area. Closely spaced penetrating electrodes are likely to