Figure 1.
Left, Historical definition of medial temporal lobe oscillatory bands. For each manuscript, a bar spans the range defined for a specific oscillation. In the instance that a range is not defined, but an oscillation is noted (Lopes-Dos-Santos et al., 2018; Sheremet et al., 2019a), a marker covering a short frequency range was used. Furthermore, some ranges were inferred (Sirota et al., 2008 describe a high 32- to 40-Hz harmonic, indicative that the 16- and 24-Hz subdivisions also exist). Note that, before the subdivision of γ into multiple bands, multiple harmonics of θ were reported. For example, Leung et al. (1982) verified the presence of harmonics through bicoherence analysis. Following the observation of multiple γ bands, few manuscripts account for the harmonics of θ. The notable exceptions are Schomburg et al. (2014), cautioning that pyramidal neuron spike modulation in the 20- to 30-Hz band may be related to the third harmonic of θ and not slow γ, and Scheffer-Teixeira and Tort (2016) cautioned that θ wave asymmetry may erroneously contribute to cross-frequency coupling. Cowen et al. (2018) observed up to the fifth harmonic (40+Hz) in both old and young rats which overlapped with their slow γ range, but did not clarify the difference between harmonics and slow γ. Zheng et al. (2015) also increased the lower bound of their slow γ definition in the medial entorhinal cortex in an attempt to avoid the lower order θ harmonics. The recent implementation of bicoherence analysis, however, reveals that the harmonics of θ can extend as high as 48 Hz (Sheremet et al., 2016, 2019a). Note that before the discovery of slow γ, only θ harmonic were reported in the 25- to 50-Hz range. Following the slow γ discovery, reports of harmonics became rare. Right top, An examination of the most extensive defined ranges of all oscillations. Note that θ harmonics spill into the lowest range of traditional and (s)low γ. Therefore, it is evident that harmonics potentially contribute to the γ band up to 50 Hz. No attempt has been made to decipher if the medium γ (Belluscio et al., 2012) was equivalent to fast/high γ across manuscripts. Right bottom, Perhaps more concerning is that when examining the lowest defined ranges (defined by identifying the lowest high pass and the lowest low pass ranges), the (s)low γ band does not overlap across studies, demonstrating inconsistency. Critically, a meaningful definition of what is a fundamental rhythm is missing. Black: θ and θ harmonics; gray: “fast oscillation” and non-subdivided γ; yellow: β; red: (s)low γ; green: medium γ; blue: fast/high γ (Petsche and Stumpf, 1960; Stumpf, 1965; Harper, 1971; Coenen, 1975; Leung and Vanderwolf, 1980; Leung, 1982, 1984, 1985; Buzsáki et al., 1983, 1985, 2003; Leung and Buzsáki, 1983; Bullock et al., 1990; Ning and Bronzino, 1993; Lee et al., 1994; Bragin et al., 1995; Rezvova et al., 1995; Breen and Morzorati, 1996; Traub et al., 1996; Chrobak and Buzsáki, 1998; Penttonen et al., 1998; Czurkó et al., 1999; Buhl et al., 2003; Csicsvari et al., 2003; Leung et al., 2005; Terrazas et al., 2005; Robbe et al., 2006; Montgomery and Buzsáki, 2007; Montgomery et al., 2008; Senior et al., 2008; Sirota et al., 2008; Cappaert et al., 2009; Colgin et al., 2009; Sabolek et al., 2009; Tort et al., 2009; Wulff et al., 2009; Chen et al., 2011; Jackson et al., 2011; Sullivan et al., 2011; Ahmed and Mehta, 2012; Belluscio et al., 2012; Buzsáki and Wang, 2012; Carr et al., 2012; Insel et al., 2012; Scheffer-Teixeira et al., 2012; Jacobson et al., 2013; Kemere et al., 2013; Bieri et al., 2014; Cabral et al., 2014; Lasztóczi and Klausberger, 2014, 2016; Schomburg et al., 2014; Trimper et al., 2014; Rangel et al., 2015; Zheng et al., 2015, 2016; Lansink et al., 2016; Scheffer-Teixeira and Tort, 2016; Sheremet et al., 2016, 2019a; Fernández-Ruiz et al., 2017; Zhong et al., 2017; Cowen et al., 2018; Dvorak et al., 2018; Lopes-Dos-Santos et al., 2018).