Arousal threshold in the rat determined by “meaningful” stimuli

It was found that the threshold for behavioral arousal was highest during PS, lower during SWS, and lowest during waking, and the tendency to react adaptively once awakened was greater for PS than for SWS arousals.

Abstract

Paradoxical sleep may be a stage of sleep which functions to increase sensitivity to environmental danger or by providing periodic arousals during sleep so that an animal can monitor its environment. The present study investigated the first aspect of this question. Five rats were trained to shuttle back and forth in a shuttle box at the onset of a tone in order to avoid a 3.5-mA foot shock. They learned this response to a stable level of 90–100% avoidances. Thus, the tone became "meaningful" as a cue for the onset of intense shock. After training the rats were implanted with electrodes for ECoG, EKG, and EMG recording. Psychophysical arousal tests were then made using the tone in differing intensities as an arousing stimulus. It was found that the threshold for behavioral arousal was highest during PS, lower during SWS, and lowest during waking. Paradoxically, the tendency to react adaptively once awakened was greater for PS than for SWS arousals. Control studies showed the tone to be an ineffective stimulus unless it had been made meaningful by avoidance conditioning. These data argue against a sentinel function for PS so far as a learned meaningful stimulus is concerned. However, the rat is a species whose behavior may be more determined by innate rather than learned factors. Further tests using innate releasing stimuli in wild species must now be made.