Autonomic and central nervous indices of the orienting response (OR) were investigated in awake human subjects and sleeping rats. Ten acoustic stimuli of 60 dB and two stimuli of 80 dB were presented for 10 s each with a constant interstimulus interval of 50 s. Responses were averaged across subjects for each single trial. An exponential fit to scores of each physiological variable was used to compare OR, habituation and dishabituation between samples and variables. An OR to the first stimulus, habituation of response amplitude, and dishabituation in response to the change in stimulus intensity were observed for event-related EEG desynchronization, a negative-positive evoked potential complex in humans and rats, as well as for skin conductance in humans. While heart rate did not show systematic changes across the 60 dB tones in both samples, a deceleration was observed in response to the first 80 dB tone. Results suggest comparable patterns of orienting to acoustic stimuli and habituation of the OR in the awake human and the sleeping rat, suggesting the possibility of OR as a unitary response. Species differed with respect to speed of habituation but not with respect to sensitivity towards stimulus change.

Cortical arousal, cardiac response, and body movement to auditory stimuli were studied during slow wave sleep (SWS) of the rat. Rats were exposed to sequences of 10 tone pulses of a frequency of 16 kHz, a sound pressure level of 60dB, and n duration of 2 s (Experiment I) or 10 s (Experiment II). Repeated stimulus presentations permitted the evaluation of short‐term habitation. Similar results were found in both experiments, i.e. habitation of the cortical arousal response and of the phasic cardiac deceleration to stimulus onset and offset. With repeated presentations, cardiac acceleration showed a less regular course than cardiac deceleration and cortical arousal. Subsequently presented tone pulses of 80dB did not result in a recovery of the habituated response. Cardiac deceleration and cortical arousal on the one hand, and cardiac acceleration and motor activity on the other hand, were strongly correlated. The results are compared with human studies.

EEG arousal and heart rate responses to change in tone frequency following habituation training were studied during slow wave sleep in the rat. Rats were exposed to sequences of 12 tone pulses. Habituation stimuli (trials 1-7) had a frequency of 16 kHz. The frequency of the following (test) trials was changed to either 5, 12, 14, 22, or 40 kHz. Hearing level was held constant for all frequencies. Test trials were repeated five times to study rehabituation. The results indicate that heart rate deceleration is an indicator of the orienting reflex and its magnitude is a positive function of the amount of frequency change of the stimulus. Large frequency changes in either direction relative to the habituation frequency lead to re-evocation of the habituated EEG arousal and heart rate deceleration, with subsequent rehabituation. Small tone-frequency changes do not result in EEG arousal or heart rate deceleration responses that differ from the responses to the habituated frequency. Heart rate acceleration shows neither short-term habituation nor significant reaction to any of the test frequencies.

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