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High Frequency Oscillations (HFOs) are oscillations usually defined as being in the frequency range of 80 to 600 Hz present in the electroencephalogram (EEG) of humans and other mammals. Most of the useful content of the EEG has been considered to be below 80 Hz but the gamma band (often defined as being between 30 and 80 to 140 Hz, see gamma wave) has also been extensively studied in the context of cognitive processing.


 * Recording and visualization . Standard EEG recording equipment usually includes an anti-aliasing filter at 70 to 100 Hz and digital sampling at frequencies from 200 to 500 Hz. Such filtering and sampling are not appropriate to record HFOs. The anti-aliasing filter must be at 500 Hz or higher and the sampling at 2000 Hz or higher to record HFOs faithfully. HFOs are of low amplitude compared to activities of lower frequency bands and are hardly visible when the EEG is displayed with its usual gain and time scale. They become clearly visible when the EEG is high-pass filtered at around 80 Hz and the EEG is displayed with a gain of 5 µV/cm, 10 times higher than the usual 50 µV/cm and a time scale of 1 s/screen rather than the usual 10 or 20 s/screen
 * Discovery . HFOs were first reported in microelectrode recordings performed in experimental animals in relation to cognitive processing. They were then seen, also in microelectrode recordings, in animal models of epilepsy and in patients with epilepsy (some patients receive intracerebral electrodes (see stereoencephalography) during the investigation for a possible surgical treatment (see epilepsy surgery), and in some patients microelectrodes are implanted in addition to the much larger clinical electrodes). They were later seen in the standard macroelectrodes used for clinical intracranial investigations and finally it was found that they could also be seen in the scalp EEG.


 * Frequency . HFOs are usually divided in two frequency bands: the ripple band between 80 and 250 Hz and the fast ripple band between 250 and 600 Hz. Many studies indicate that HFOs in the ripple band can be of physiological origin, reflecting healthy brain mechanisms, but can also be of pathological origin, reflecting abnormalities related to epilepsy. HFOs in the fast ripple band, on the other hand, are thought to be primarily of pathological origin, again reflecting abnormal brain tissue related to epilepsy.