Rhythms of the hippocampal network

Abstract

The hippocampus shows three main classes of rhythms: theta (∼4–12 Hz), sharp wave–ripples (∼150–200 Hz ripples superimposed on ∼0.01–3 Hz sharp waves) and gamma (∼25–100 Hz). Theta rhythm generation involves a variety of mechanisms, including theta rhythmic firing in septal and hippocampal interneurons, excitatory inputs to hippocampus and intrinsic properties of hippocampal neurons. Theta rhythms are likely to be important for the formation of memories of sequences of events. Sharp wave–ripple complexes are composed of two distinct network patterns: sharp waves (excitatory events that propagate from CA3 to CA1) and ripples (which reflect high frequency firing in hippocampal interneurons). Accumulating evidence suggests that sharp wave–ripples are important for intrinsic hippocampal operations, including offline memory processing, retrieval of previously stored memories and planning of future behaviours. The class of brain rhythms traditionally defined as gamma probably contains at least two different variants of oscillatory activity. Recent findings suggest that slow (∼25–55 Hz) and fast (∼60–100 Hz) variants of gamma have different origins and may have different functions.