The hippocampus is required for several types of memory [1]. It must cooperate with other structures that are involved in learning and memory, and brain rhythms are thought to be important for coordinating these interactions. When large groups of neurons synchronize their electrical activity in a periodic manner, brain rhythms (or oscillations) emerge in local field potential (LFP) recordings. The hippocampus exhibits three main classes of rhythms that are associated with particular behavioral states [2]: theta, gamma, and sharp wave-ripples. Theta rhythms (~5–10 Hz) occur during active behaviors as well as REM sleep [3] and are believed to be important for learning and memory [4]. Gamma oscillations are faster waves (~25–140 Hz) that occur during many behaviors but are largest when theta rhythms are present [5]. Ripples are very fast oscillations (~150–300 Hz) that are superimposed on slow and irregularly occurring ‘sharp waves’ (~1–10 Hz). Sharp-wave ripple complexes emerge during slow-wave sleep and periods of inactivity [6]. Each of these three classes of rhythms is believed to play a unique role in coordinating interactions between the hippocampus and the systems with which it communicates.
