On the Functional Significance of the Hippocampal θ–Rhythm

Abstract

Publisher Summary This chapter focuses on the functional significance of the hippocampal θ-rhythm. The appearance of the θ-rhythm in the hippocampogram depends on a moderate level of reticular activation. In this condition the hippocampal output elicits a rhythmic firing of subcortical neurones. Via the anterior thalamic nuclei and the gyrus cinguli a hippocampo-hippocampal circuit is closed, and reactivation of the hippocampus at the same rate is quite possible. This circuit as well as the other thalamo-neocortical circuits of the nuclei lateralis dorsalis and in part medialis dorsalis are not consistently affected by the ascending influences of the activating reticular formation. In conditions of moderate arousal the hippocampal output during the appearance of the θ-rhythm has enough driving power to exert definite control on the bioelectrical activity of subcortical and neocortical structures. It is evident that the preceding considerations cannot be directly extended to higher mammals. The depression or suppression of the hippocampal θ-rhythm does not necessarily mean that the hippocampus has lost or basically changed its functional significance.