Hippocampal output and sensory mechanisms

Abstract

This study was performed with the aim of clarifying the mechanisms of the influence that the hippocampal output during theta rhythm exerts on sensory processes. The hippocampal output was simulated by low-frequency stimulation (single trains of 5 rectangular pulses: 5/sec, 0.5 msec, 3–9 V) of the dorsal part of the hippocampal formation. The changes induced by such output in the responsiveness of primary auditory areas to clicks were studied in curarized cats. Primary auditory responses ipsilateral and contralateral with respect to the hippocampus stimulated were modified when the interval between the last stimulus of the train and clicks ranged from 20 to 45 msec and from 30 to 55 msec respectively. The changes in the contralateral auditory response depended on the fact that stimulation of one hippocampus also involved the commissural activation of the other. Signs of hippocampal facilitation were not observed in medial geniculate responses to clicks. Coagulation of the fornix or of non-specific thalamic nuclei strongly depressed or suppressed the hippocampal influence on auditory responses. Laminar analysis of the small surface positive-negative responses elicited in primary auditory areas by train stimulation of the hippocampus alone showed that the hippocampal output activated the superficial layers of the neocortex.