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In rats, theta wave rhythmicity is easily observed in the hippocampus, but can also be detected in numerous other cortical and subcortical brain structures. Hippocampal theta waves, with a frequency range of 6–10 Hz, appear when a rat is engaged in active motor behavior such as walking or exploratory sniffing, and also during REM sleep. [3]
Example of a one-second EEG theta wave. The underlying currents producing the theta wave are generated mainly by densely packed neural layers of the entorhinal cortex, CA3, and the dendrites of pyramidal cells. The theta wave is one of the largest signals seen on EEG, and is known as the hippocampal theta rhythm. [112]
The finding that theta wave phase precession is also a property of grid cells in the entorhinal cortex demonstrated that the phenomenon exists in other parts of the brain that also mediate information about movement. [11] Theta wave phase precession in the hippocampus also plays a role in some brain functions that are unrelated to spatial location.
Ripples are only present when theta waves are relatively absent during sharp waves, whereas fast gamma waves occur during theta waves and sharp waves. [11] The magnitude and frequency of both ripples and fast gamma patterns are dependent on the magnitude of hippocampal sharp waves.
It has been proposed that the trisynaptic circuit is responsible for the generation of hippocampal theta waves. These waves are responsible for the synchronization of different brain regions, especially the limbic system. [9] In rats, theta waves range between 3–8 Hz and their amplitudes range from 50 to 100 μV.
When eating, grooming, drowsy, or sleeping in slow-wave sleep (SWS), there has been observed in hippocampal EEG, the non-rhythmic pattern of large irregular activity,(LIA). LIA has the predominant pattern of large amplitude slow waves that contain some fluctuations of sharp spikes (sharp waves) of 50–100 ms duration.
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The hippocampal theta waves, as well as the gamma waves, that influence place cell firing, for example through phase precession, are also affected. [81] Aging