High-frequency oscillations: What is normal and what is not?
Article first published online: 4 DEC 2008
Wiley Periodicals, Inc. © 2008 International League Against Epilepsy
Volume 50, Issue 4, pages 598–604, April 2009
How to Cite
Engel Jr, J., Bragin, A., Staba, R. and Mody, I. (2009), High-frequency oscillations: What is normal and what is not?. Epilepsia, 50: 598–604. doi: 10.1111/j.1528-1167.2008.01917.x
- Issue published online: 3 APR 2009
- Article first published online: 4 DEC 2008
- Accepted September 18, 2008; Early View publication December 4, 2008.
- High-frequency oscillations;
- Fast ripples;
High-frequency oscillations (HFOs) in the 80–200 Hz range can be recorded from normal hippocampus and parahippocampal structures of humans and animals. They are believed to reflect inhibitory field potentials, which facilitate information transfer by synchronizing neuronal activity over long distances. HFOs in the range of 250–600 Hz (fast ripples, FRs) are pathologic and are readily recorded from hippocampus and parahippocampal structures of patients with mesial temporal lobe epilepsy, as well as rodent models of this disorder. These oscillations, and similar HFOs recorded from neocortex of patients, appear to identify brain tissue capable of spontaneous ictogenesis and are believed to reflect the neuronal substrates of epileptogenesis and epileptogenicity. The distinction between normal and pathologic HFOs (pHFOs), however, cannot be made on the basis of frequency alone, as oscillations in the FR frequency range can be recorded from some areas of normal neocortex, whereas oscillations in the ripple frequency range are present in epileptic dentate gyrus where normal ripples never occur and, therefore, appear to be pathologic. The suggestion that FRs may be harmonics of normal ripples is unlikely, because of their spatially distinct generators, and evidence that FRs reflect synchronized firing of abnormally bursting neurons rather than inhibitory field potentials. These synchronous population spikes, however, can fire at ripple frequencies, and their harmonics appear to give rise to FRs. Investigations into the fundamental neuronal processes responsible for pHFOs could provide insights into basic mechanisms of epilepsy. The potential for pHFOs to act as biomarkers for epileptogenesis and epileptogenicity is also discussed.