Abstract
The effects of septal and dorsal hippocampal lesions on the cardiac component of the rat's orienting response (OR) to auditory stimulation were examined. Control, septal and hippocampal rats all showed deceleratory responses, maximal at 2 sec following onset, to a 1000 Hz tone. With repeated presentations of the tone the control rat's deceleratory OR's disappeared, and a longer latency acceleratory response followed stimulus onset. Septal and hippocampal rats failed to show this evidence of habituation to the 1000 Hz tone. Subsequent presentations of a 1375 Hz tone resulted in deceleratory OR's in all groups. The control rats' OR's again habituated with repeated presentations of the stimulus, being replaced by a longer latency acceleration; while septal rats continued to make OR's throughout the series. The hippocampal rats' deceleratory OR's to the second tone did habituate; but neither they nor the septal rats showed the longer latency acceleratory response characteristic of the later stages of habituation. Resting cardiac rate differences between septal and control rats were shown to be a function of the severity of imposed restraint.
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