High-frequency oscillations in epileptic brain

Neuronal oscillations recorded from the human brain span a range of spatial and temporal scales that extend far beyond traditional clinical electroencephalography (EEG). Extracellular local field potentials (LFPs) recorded directly from human cortex range from near DC to ultra-fast oscillations (~0 to 1000 Hz) (Figure 21.1). The mechanisms generating these oscillations are varied, and the correlation with mechanism, function, or pathology is challenging. Despite the lack of mechanistic specificity for neuronal oscillations, EEG research has a rich, successful history of identifying neural correlates of physiological and pathological brain function. Until recently, human EEG was primarily focused on neuronal oscillations within the Berger bands (1 to 25 Hz), named in honor of Hans Berger, who first reported on human EEG (Berger, 1929; Gloor, 1969, 1975). This relatively narrow clinical EEG bandwidth is largely a technical legacy and at least partly associated with the challenge of recording and storing wide-bandwidth EEG data. In addition to the technical challenge, the EEG spectral power is dominated by activity in the Berger frequency bands. At high frequency, the EEG power has been reported to fall off rapidly, as either f -1 (Linkenkaer-Hansen et al., 2001; Worrell et al., 2002) or f -2 (Bédard et al., 2006; Milstein et al., 2009). This is most dramatic when recording from the scalp but is also evident from intracranial EEG (iEEG). Recent studies, however, have begun to establish the importance of neural activity occurring outside the Berger bands in animal and human electrophysiology (Bragin et al., 1999a, 2002a, b; Curio, 2000a; Jacobs et al., 2008; Jirsch et al., 2006; Traub et al., 2001; Vanhatalo et al., 2005; Worrell et al., 2004, 2008) (Figure 21.1).