Afferent influences on the hippocampal theta rhythm

The hippocampus has been implicated in mechanisms of learning and memory. Theta, a synchronous ESG rhythm, is the prominent electrophysiological characteristic of that structure. The rhythm appears to emanate from the dorsal layer of pyramidal cells (CA1) and is dependent upon the septum. If an electrode is passed through the hippocampus at an angle normal to the plane of the CA1 area an abrupt shift in the phase relation of the theta occurs, resembling a simple dipole. Recent works (Gerbrandt, Lawrence, Fowler & Weyand, 1974) demonstrated that by carefully analyzing the dipole shift through computer analysis, a simple dipole was an insufficient model. This paper is concerned with the properties other afferent sources to the hippocampus besides the septum have in modulating the dipole behaviour of the theta rhythm. The results are discussed in terms of a single dipole with secondary generators contributing phasic properties to the standing dipole. Implications in terms of arousal and memory storage mechanisms are also discussed.