Startle reflexes to intense sound bursts are inhibited by weak stimuli that briefly precede their elicitation. In three experiments the startle stimulus (a 110-dB SPL tone burst) was presented 100 ms after the final link in a train of stimuli, the length of the train varying from 1 to 1,000, its repetition rate varying from 1 per s to 10 per s, and its constituents being 40 dB or 50 dB white noise bursts of 25 ms duration. Inhibition was invariant across train length and repetition rate. In a final experiment the startle stimulus was presented a variable interval after the final link, from 40 ms to 1280 ms, with 1 or 100 noise bursts (50 dB) in the train. Inhibition developed more rapidly following the last member of the 100-stimulus train, suggestive of a "priming" or sensitization effect of stimulus repetition, but its overall strength and subsequent rate of decay were not different in the two conditions. The general persistence of inhibition following these extended series of stimuli reveals that reflex inhibition must be the outcome of a fixed and obligatory process associated with sensory input.

The rat's (Long-Evans) acoustic startle reflex to a high-frequency tone burst (10.5 kHz) was depressed by intense high-frequency band-pass noise (8-16 kHz) but enhanced by low frequency noise (1-2 kHz). However, contrary to the hypothesis that the depression of startle in intense background noise is produced by sensory masking, the reflex to a low-frequency tone burst (at 1 kHz) was depressed by both high- and low-frequency band-pass noise. Two additional hypotheses are offered to supplement sensory masking in order to explain the asymmetry in these data. The first is that the intratympanic reflex, which acts as a high pass filter on acoustic input, is elicited in intense backgrounds. The second is that acoustic startle reflexes elicited by intense low-frequency tones are in part elicited by their high-frequency distortion products and that these distortion products are then masked by high-frequency background noise.

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