Increased high-frequency oscillations precede in vitro low-Mg²⁺ seizures

Purpose: High-frequency oscillations (HFOs) in the range of ≥80 Hz have been recorded in neocortical and hippocampal brain structures in vitro and in vivo and have been associated with physiologic and epileptiform neuronal population activity. Frequencies in the fast-ripple range (>200 Hz) are believed to be exclusive to epileptiform activity and have been recorded in vitro, in vivo, and in epilepsy patients. Although the presence of HFOs is well characterized, their temporal evolution in the context of transition to seizure activity is not well understood. Methods: With an in vitro low-magnesium model of spontaneous seizures, we obtained extracellular field recordings (hippocampal regions CA1 and CA3) of interictal, preictal, and ictal activity. Recordings were subjected to power-frequency analysis, in time, by using a local multiscale Fourier transform. The power spectrum was computed continuously and was quantified for each epileptiform discharge into four frequency ranges spanning subripple, ripple, and two fast-ripple frequency bands. Results: A statistically significant increasing trend was observed in the subripple (0-100 Hz), ripple (100-200 Hz), and fast-ripple 1 (200-300 Hz) frequency bands during the epoch corresponding to the transition to seizure (preictal to ictal). Conclusions: Temporal patterns of HFOs during epileptiform activity are indicative of dynamic changes in network behavior, and their characterization may offer insights into pathophysiologic processes underlying seizure initiation.