Does anyone have any experience? The discovery that electroencephalography (EEG) contains useful information at frequencies above the traditional 80Hz limit has had a profound impact on our understanding of brain function. In epilepsy, high-frequency oscillations (HFOs, >80Hz) have proven particularly important and useful. But can I register the HFO routinely?
HFO EEG recording is a new advance in epileptogenesis mapping. It is being adopted more widely as time goes.
The Montreal study
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3786932/
A recent commentry from AES.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3316367/
Several challenges exist to seeing HFOs with standard equipment and software. First, the sampling rate should be at least 1 kHz to resolve the frequencies (though the HFO detection sensitivity is quite low at 1 kHz. 2 kHz is OK, but it is best to have 5 kHz). Secondly, the anti-aliasing filter position of the equipment must be set higher than the frequencies of interest--i.e., >500 Hz so that you can resolve the shape of the HFO. Third, the viewing software often puts another anti-aliasing filter in place, as they assume that the screen resolution will be too low to see the higher frequencies anyhow. Fourth, even with all the rest in place, you have to zoom in (in both time and amplitude) or they will not be observable due to screen resolution. For more info about screen resolution, see Schevon, et al., (2004) "Inadequacy of Standard Screen Resolution for Localization of Seizures Recorded from Intracranial Electrodes".
The questions you did not ask are that even if you could see them, how will you detect them? And once you detect them, how will you interpret them clinically? There are many methods to detect HFOs, and there is not an accepted method by the HFO community to prospectively interpret the HFO data. Note many physiologic processes also produce HFOs (both ripples and fast ripples). See, for example, Blanco, et al., (2011) "Data mining neocortical high-frequency oscillations in epilepsy and controls" where HFOs were recorded in control patients without epilepsy. In short, HFOs are a promising biomarker, but are not yet ready for clinical translation.
It may also be important to make a distinction between ictal and interictal high frequency oscillations (HFO). Ictal HFO are excellent markers of epileptogenicity and often occur in a frequency range
....and what I forgot: we always use a Nicolet Recording System (sampling rates at either 512 or 1024 Hz). For research purposes we run a Neurolynx system in parallel if we record from microwires.
dear Pau; I fully agree with Stephen's answer;
we have developped a tool for better visualization of HFOs (which is indeed a diffficult task) ; we use it with great results on intracerebral data sampled at 2000 Hz; at the scalp level it seems much more difficult.
Roehri N, Lina JM, Mosher JC, Bartolomei F, and Benar CG, (2016). Time-frequency strategies for increasing high frequency oscillation detectability in intracerebral EEG. IEEE Transactions on Biomedical Engineering PP: 1-1.
in any case, be very careful of the 'ringing' artefacts that can come from filtering sharp data (such as epileptic spikes....), see:
Bénar CG, Chauvière L, Bartolomei F, Wendling F. Pitfalls of high-pass filtering for detecting epileptic oscillations: a technical note on "false" ripples. Clin Neurophysiol. 2010 Mar;121(3):301-10.
I can send you the reprints if you want
good luck and best regards
Christian
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