In total, 921 40-min LFP traces from 80 recording sessions were included for analysis. Recording locations were localized histologically to ventral striatum (nucleus accumbens core and ventral caudate-putamen, Figure 3). Consistent with previous reports in rats (Berke, 2005; Berke et al., 2004; Kalenscher et al., 2008) and humans (Cohen et al., 2009) oscillations in the gamma range were prominently apparent. Inspection of individual traces and spectrograms (see Figure 4A for an example), as well as the average PSD (Figure 4B) revealed the presence of distinct power bands in the “low gamma” range (gamma-50, a relatively narrow peak around 50 Hz) and the “high gamma” range (gamma-80, a broader increase in power between 60 and 100 Hz). There was a tendency for power in these two frequency bands to alternate, rather than co-occur, as suggested by the structure of the spectrogram in Figure 4A. To obtain an overall quantification of this effect, we calculated the average cross-frequency self-coherence (Masimore et al., 2004). This method computes the correlation of power over time (taken from multitaper spectrograms computed with the Chronux toolbox, Mitra
