It remains unclear whether similar or different mechanisms trigger theta-modulated slow and fast gamma rhythms. Gamma frequency in rats was found to increase with running speed [33] (but see [34] for different results in mice), as was the frequency of interneuron spiking [33]. These findings suggest that similar mechanisms generate slow and fast gamma in rats, with speed-modulated inputs controlling gamma frequency. However, other results support the hypothesis that different mechanisms generate slow and fast gamma. Gamma in the ~30–50 Hz and ~50–90 Hz ranges was found to be associated with current source density (CSD) analysis-defined sinks in stratum radiatum and stratum lacunosum-moleculare, respectively [30]. These results support the conclusion that slow gamma is driven by CA3 inputs and fast gamma is driven by entorhinal inputs. However, a more recent CSD study by Lastoczi and Klausberger reported that ~30 Hz gamma oscillations were dominant in stratum lacunosum-moleculare, not stratum radiatum [35]. Spikes from projection neurons in layer III of MEC were phase-locked to the gamma activity in stratum lacunosum-moleculare, suggesting that this activity was driven by MEC [35]. The reason
