Ten- to 12-Hz EEG oscillation in the rat hippocampus and rhinal cortex that is modulated by environmental familiarity.

A characteristic feature of the electroencephalogram (EEG) of the hippocampus and rhinal (entorhinal and perirhinal) cortex of the freely moving rat is theta rhythm, a prominent oscillation of approximately 8 Hz. Here we demonstrate that a novel rhythm that occurs at the border between the theta and alpha range of frequencies (10-12 Hz) can also be recorded from these structures. This rhythm (referred to here as "flutter") appears to be of non-theta origin as it can occur simultaneously with theta and it does not display the phase inversion across the hippocampus that characterizes theta activity. Flutter is observed in locomoting rats that are foraging for food reward in a familiar environment. Flutter disappears when rats are placed into a novel (although visually identical) environment, even though their foraging behavior does not appear to be altered. It is, at the present time, unclear what function flutter subserves. The presence of flutter may relate to a particular motivational state of the animal or to a particular type of information processing.