Clinical EEG provides us with diagnostic information of epileptogenicity by epileptiform discharges, i.e., spikes, sharp waves, which reflect the paroxysmal depolarization shifts (PDS) in the epileptic neurons. Currently advanced technology has enabled us to record wide-band EEG: direct current (DC) shifts and high frequency oscillation (HFO). The both conditions could widen the neurophysiological definition of epileptgenicity.
Ictal DC shifts was recorded by using a DC amplifier in 1960s with technical difficulty, but recently is by applying very small low frequency filter (0.016 Hz) of an AC amplifier which has the large input impedance more than 200 Mohm without difficulty in patients with invasive electrodes (Ikeda et al., 1996, 1999, 2008). It could reflect the massive, synchronized paroxysmal depolarization of the neurons in the epileptogenic area, and thus also represents associated depolarization of the glia. It could be regarded as the surrogate markers of the core epileptogenicity in human epilepsy regardless of the etiology.
Once HFO is thought to highly reflect epileptogenicity in human epilepsy, we have investigated both ictal DC shifts and HFO simultaneously in patients with intractable partial epilepsy by means of subdural electrodes (Imamura et al., 2011; Kanazawa et al., 2015). We observed that 1) both occurred together as early as electrodecremental pattern occurred or earlier than conventional ECoG changes, 2) both occurred in the same electrodes, 3) ictal DC shifts were more often observed than HFO, and 4) ictal DC shifts could occur earlier than HFO. It could suggest more active role of glia in not only generating DC shifts but also presumably in ictogenesis.

1. In intracranial recording of epilepsy patients, ictal direct current (DC) shifts are clinically defined as sustained mainly negative (sometimes positive) potentials longer than 3 sec. They should be essentially better delineated with TC of 10 sec than that of 0.1 sec. They must be reproducible in location, waveform, duration and amplitude within each patient. In intractable partial epilepsy, intracranial electrodes are chronically placed to record electrocorticogram in order to delineate epileptogenic focus. Ictal DC shifts can be recorded with both higher sensitivity and specificity without galvanic skin response and with less motion artifacts in intracranial recording than in scalp recording. DC shift (or slow potential) is well recorded even with alternate current (AC) amplifier once low frequency filter (LFF) is opened to 0.016 Hz (=TC 10 sec). Input impedance should be sufficiently high. Although reversible electrodes more suitable, platinum electrodes are used in intracranial recording due to no toxicity as opposed to silver-silver chloride electrodes. Electrodes with larger recording surface are more suitable for slow potential recording. As system reference electrode, scalp electrodes made of platinum, i.e., the same metal as intracranial electrodes, are to be placed on the skin over the mastoid process contralateral to the surgery side.
2. The display settings for ictal DC shifts on EEG review software clinically used are as follows: (1) Reference montage is most useful. Initially, intracranial electrodes are referenced to the scalp electrodes placed on the skin over the mastoid process. If it is not suitable, one of the intracranial electrodes rather silent of epileptic activity is chosen as the display reference. If it still fails, the averaged reference from all the intracranial electrodes is chosen. (2) Time window 60 sec. (3) TC 10 sec. (4) High frequency filter (HFF) 15 Hz. Besides on-line analysis, off-line signal analysis software is also used for the display and analysis of the data. The reproducibility of the data needs to be confirmed in order to differentiate ictal DC shifts from any type of artifacts. Ictal DC shifts need to be reproducibly recorded at the same electrodes in multiple seizures within each patient.
3. It is possible to analyze wide-band EEG by EEG review software clinically. LFF, HFF, AC filter, waveform color, and sensitivity can be set for each of conventional EEG, slow shifts, and as for high frequency oscillations (HFOs), ripples and fast ripples. Finally, in the analysis result window, it shows ictal DC shifts, conventional ictal EEG changes, and the whole spectrogram including HFOs.
4. In conclusion, DC shifts can be recorded not only with DC amplifier, but also well with AC amplifier with long TC such as 10 sec. The more convenient on-line analysis of ictal DC shifts and HFOs is possible on EEG review software clinically.

High-frequency oscillations (HFOs) are defined as electroencephalographic activity at a frequency above 80 Hz. Activity of this type has drawn attention as a surrogate marker of the epileptogenic zone, and its use as such may help improve seizure outcome after epilepsy surgery. The methods by which HFOs are recorded and interpreted varies significantly among epilepsy centers, however. For recording HFOs, a sampling frequency at or over 2 kHz and an antialiasing filter at or over 600 Hz should be used. The use of intracranial electrodes with a surface contact area of 0.2 to 5 mm² is recommended. For appropriate display of HFOs using the viewing software by Nihon Kohden Co., the use of a monitor with at least 1,920 pixels in the horizontal direction is recommended. The high-cut filter should be turned off, a time constant of 0.001 to 0.003 s (corresponding to the low-cut filter at 53 to 160 Hz) should be used, the sensitivity should be set to 1 to 5 μV/mm, and the time scale should be set to 0.5 to 1 s/page. Computerized spectral analysis and automated detection of HFOs are also described.