(Anderson et al. 2010; Benchenane et al. 2010; O'Neill et al. 2013; Sauseng et al. 2010; Siapas et al. 2005; Jones and Wilson 2005; Hyman et al. 2005). Theta rhythm is the most prominent oscillatory signal in the rodent brain generated by the septo-hippocampal network (Buzsaki 2002; Vertes and Kocsis 1997), and thus theta coupling tacitly assumes driving by the HC. Since however PFC is thought to play a key role integrating cognitive function, a functional network should also exist to allow the PFC to communicate back to the HC and other structures to modify behavior (Miller and Cohen 2001). It was indeed proposed recently that HC-PFC coupling also involves a second rhythm of PFC origin that co-occurs with HC theta and can mediate driving by PFC (Fujisawa and Buzsaki 2011). The frequency of this oscillation varies between 2–5 Hz in the upper delta range but its regularity, with sharp spectral peak, clearly distinguishes this signal from wide-band (1–4 Hz) delta activity mostly recorded in sleep.
