Inter-stimulus interval and the rate of behavioural habituation

A series of 10 tone stimuli was presented to rats at intervals of 1/4, 1/2, 1, 1 1/2, 2, 2 1/2, or 3 min or 24 h. The next day a test tone was presented and the distraction to this measured. There was less habituation (i.e. greater distraction) at shorter inter-stimulus intervals, but this effect asymptoted at 2 min and thereafter there was no further increase in habituation, even with inter-stimulus intervals of 24 h. A second independent variable was the stimulus duration which did not affect the rate of habituation, nor did it interact in any way with the inter-stimulus interval.