Evoked activity of single units and neural populations in the hippocampus of the cat.

Abstract The evoked activity of single units and neural populations in the hippocampus of cats was studied in relation to a proposed feedback circuit. The forward branch of the feedback circuit consisted of pyramidal cells which send collaterals to interneurons in a feedback branch. The pyramidal cell excitation of an interneuron pool, followed by inhibition of the pyramidal cell population, is analogous to a circuit within the prepyriform cortex. On the basis of the neural network, oscillatory wave forms were predicted under the additional assumptions that a sustained level of background activity biased the neural network and that fornix stimulation perturbed only slightly the mean activity of the cell population. The prediction was tested by recording a series of evoked potentials. Oscillatory PST histograms were recorded from 40 pyramidal cells, and oscillatory potential fields were mapped on a coronal plane through the dorsal and ventral hippocampus. A delay of a quarter of a cycle of averaged evoked potentials from cell populations with respect to poststimulus time histograms was observed following fornix stimulation. These oscillatory wave forms were consistent with the proposed feedback circuit.