Hippocampal Electrical Activity in Arousal1

THE CORTEX of the hippocampus and dentate gyrus, together with the pyriform area and the nuclear masses of the amygdala, are traditionally described as portions of the rhinencephalon, with the implication that they are predominantly or exclusively concerned with olfactory sensation. There is evidence, however, that more generalized functions only incidentally related to olfaction may be subserved by these ancient parts of the forebrain. With respect to the hippocampus, anatomical evidence shows that it receives no direct connections from the olfactory bulbs or tracts, although the olfactory pathways may be traced to the olfactory tubercle, amygdala and pre-pyriform cortex (11, 12). In general, these findings harmonize with the results of experiments which show that electrical activity induced by odors or by stimulation of the olfactory bulb can be traced to the same general areas and to the pyriform cortex (22, 1, 2, 6, 49, 14). Except for very slow potentials described by Hasama, such evoked activity has not so far been detected in the hippocampus. Swann (55,56) found olfactory discrimination in rats to be unaffected by lesions of various parts of the rhinencephalon, including the hippocampus and fornix, and Allen (4, 5) found no changes in olfactory conditioned reflexes when the greater part of the hippocampus was removed. While there seems to be little evidence for direct olfactory connections to the hippocampus, there have been a number of reports of “primary” responses and more prolonged alterations in the activity of the hippocampus after various modalities of stimulation. Gerard et aZ. (15) recorded primary responses upon click stimulation or single-shock electrical stimulation of the sciatic and optic nerves, and Liberson and Cadilhac (37) have obtained similar results. Jung and Kornmiiller (29) obtained 5-6/set. discharges from the hippocampus of unanesthetized rabbits after strong nociceptive and occasionally after auditory stimulation, and MacLean et al. (41) obtained faster