Twenty•four rats were presented with 10 tones at a 30-sec interstimulus intervaI, once an hour, for 48 h. Startle likelihood showed a pronounced diurnal rhythm with higher responding at night. The period was 24 hand relatively sinusoidal, with no detectable harmonies. Startle rhythmicity was not simply due to diurnal changes in activity, since activity changes in a control group, which was not exposed to tones, were too small to account for the startle differences. Substantial habituation over the 48-h session was also seen and the results were discussed in terms of their relevance to the design of habituation experiments.
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