Slow Oscillations Help Spread Epileptic Fast Oscillations Across Brain Regions
In their eNeuro publication, Sheybani and colleagues explore how slow oscillations in an epileptic hippocampus can influence epileptic activity in distant brain regions. The authors used intrahippocampal kainate injections to induce mesial temporal lobe epilepsy in mice, then monitored broadband (16,000 Hz) electrophysiological recordings with multielectrode arrays in both hippocampi and the frontal cortex. The goal was to elucidate some of the network behaviors underlying epileptic activity, as technology is now available that allows analysis on broad temporal and spatial scales. In this experiment, the authors were able to identify a wide range of brain rhythms, from slow oscillations (2-5 Hz) all the way to high-frequency oscillations known as “fast ripples” (> 200 Hz) and multiunits (~1000 Hz), recorded simultaneously from the hippocampus and the frontal lobes. Using phase-amplitude coupling analysis, they were able to find slow oscillations that occurred first within the hippocampus and then in the frontal lobes, and in each location, there were fast ripples locked to specific phases of the slow oscillations. Importantly, the authors were able to show the multiunit activity was phase locked in both of these regions, providing evidence that the fast ripples were produced by fast neuronal firing. The results are not easily translatable to humans: the scale of the mouse brain is vastly different, and it is unclear if the frontal lobes are truly “normal” in the kainate model. Nevertheless, the co-occurrence of epileptic activity in two different regions is an important unsolved problem in epilepsy research, and these data provide intriguing results. Past literature has shown that low-frequency activity is capable of synchronizing faster activity related to normal cognition, but this work is the first to show this effect with epileptic waveforms. The results suggest that 3-5 Hz slow oscillations synchronize, and possibly help propagate, epileptic waveforms across brain regions. This work shows the great potential of using broadband, high-resolution technology with network analysis to start understanding the complex network activities of epilepsy.
Read the full article:Large-Scale 3–5 Hz Oscillation Constrains the Expression of Neocortical Fast Ripples in a Mouse Model of Mesial Temporal Lobe Epilepsy
Laurent Sheybani, Pieter van Mierlo, Gwénaël Birot, Christoph M. Michel and Charles Quairiaux