The hippocampus is one of the earliest cortical areas to appear in evolution. Actually it is at the top of the limbic system and at the bottom of the cerebral hemispheres, folded deep inside. It receives connections from the posterior perceptual areas and then sends out connections to the frontal areas involved in planning. I read about it first in the late 70s and found it fascinating because it detects novelty and then organizes around that new salient perception. More recent research cited by Jaak Panksepp confirms the view that hippocampal theta indicates active processing of salient stimuli. Theta waves are longer, slower waves.
Delta are the slowest and signal sleep. Theta are slow rhythmic waves indicating a new response set. Once started they slowly increase in frequency and become choppier (alpha), much like an ocean swell does when the winds and currents intensify. I interpret this to mean that the initial orienting gestalt is filled in with the particulars and the new becomes old and useful in planning. Hippocampal theta appears in some birds when their heads bob, e.g., chickens when they walk or stand still move their head and each move brings about a new orientation. Rats show this theta when they walk. Cats show this theta when they orient, e.g., they hear or see something interesting. Apes do not show this theta that often but do when they expect something which does not happen, e.g., they push the lever for food and nothing appears. Humans do not show this theta much after early childhood. These last two examples suggest that the primate hippocampus is more involved in processing mental events than perceptual ones, that we are subjective creatures not originally but now primarily.