Physiology
The hippocampus shows two major "modes" of activity, each associated with a distinct pattern of neural population activity and waves of electrical activity as measured by an electroencephalogram (EEG). These modes are named after the EEG patterns associated with them: theta and large irregular activity (LIA). The main characteristics described below are for the rat, which is the animal most extensively studied.
The theta mode appears during states of active, alert behavior (especially locomotion), and also during REM (dreaming) sleep. In the theta mode, the EEG is dominated by large regular waves with a frequency range of 6–9 Hz, and the main groups of hippocampal neurons (pyramidal cells and granule cells) show sparse population activity, which means that in any short time interval, the great majority of cells are silent, while the small remaining fraction fire at relatively high rates, up to 50 spikes in one second for the most active of them. An active cell typically stays active for half a second to a few seconds. As the rat behaves, the active cells fall silent and new cells become active, but the overall percentage of active cells remains more or less constant. In many situations, cell activity is determined largely by the spatial location of the animal, but other behavioral variables also clearly influence it.
The LIA mode appears during slow-wave (non-dreaming) sleep, and also during states of waking immobility, such as resting or eating. In the LIA mode, the EEG is dominated by sharp waves, which are randomly timed large deflections of the EEG signal lasting for 200–300 ms. These sharp waves also determine the population neural activity patterns. Between them, pyramidal cells and granule cells are very quiet (but not silent). During a sharp wave, as many as 5–10% of the neural population may emit action potentials during a period of 50 ms; many of these cells emit bursts of several action potentials.
These two hippocampal activity modes can be seen in primates as well as rats, with the exception that it has been difficult to see robust theta rhythmicity in the primate hippocampus. There are, however, qualitatively similar sharp waves, and similar state-dependent changes in neural population activity.
Read more about this topic: Hippocampus
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