The θ Mode of Hippocampal Function
The θ rhythm is an approximately sinusoidal electrical signal that can be recorded from the hippocampus, diencephalon, and neocortex of a large number of mammalian species. It was first investigated systematically in 1954 by Green and Arduini, who reported the presence of regular, high-amplitude slow waves at frequencies of 3–7 Hz in the hippocampus of both the curarized and the freely moving rabbit. In the cu-rarized preparation, the waves appeared following natural sensory stimulation or electrical stimulation of the brain stem reticular formation or other subcortical structures. In the freely moving animal, the waves appeared when the animal was judged to be alert and interested in its surroundings. The electrical activity was most pronounced when the animal was presented with a new phenomenon and tended to disappear with repeated stimuli. They termed this activity “θ rhythm” in accordance with the designation that had previously been established for the components of the human EEG in this frequency range (Walter and Walter, 1953). In view of their observations, it was quite natural for them to associate the θ rhythm with a state of arousal in the animal.
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