As described previously (Vertes and Kocsis 1997), HC activity in urethane anesthetized rats spontaneously alternated between two states characterized by large amplitude irregular activity and theta rhythm (Fig. 1A–C). PFC also showed this pattern of similar irregular activity alternating with a dominant rhythm that appeared however at lower frequencies (~ 2 Hz) in the delta band (Kiss et al. 2011a; Kiss et al. 2011d), co-occurring with HC theta. Frequently, a strong oscillatory component also appeared mixed within the background of irregular delta activity in the “passive” state (see e.g. sharp spectral peak on top of wide-band delta in Fig. 1D). Furthermore, this rhythm was also present, along with theta, in LFP recordings in the nRE as well as in HC (Fig. 1E).
