Both the appearance of LFP traces and the spectral representation of the rhythmic PFC signal showed essential similarities with HC theta rhythm and was drastically different from wide-band delta activity, limited on one side (~0.5 Hz) by the high-pass filter of AC coupled recording and on the other side (~4 Hz) slowly tapering toward higher frequencies of the delta band. Thus, to avoid confusion of these fundamentally different delta-band activities we use the term “2–5 Hz oscillations” to denote sinusoidal-like delta activity, dominating the PFC signal in “active” states. The two types of delta activity occur in un-anesthetized animals, in different behavioral states. Wide-band delta activity is well known as the dominant EEG pattern of slow wave sleep whereas highly synchronized neuronal firing generating sinusoidal LFP in PFC was reported in awake rats during working memory task, referred to as “4 Hz oscillation” (Fujisawa and Buzsaki 2011).
