Acoustic prepulses can facilitate the startle reflex in rats: discrepancy between rat and human data resolved

Small changes in the sensory environment, called prepulses, prior to a startle-eliciting stimulus can inhibit or facilitate the startle reaction. Previous studies reported that at large intervals between prepulse and startle stimulus the prepulse facilitates the startle reaction, possibly by means of an orienting response. This was, however, only observed in humans and not in rats, and was consequently proposed to measure unique brain functions. In the present study with rats, the prepulse intensity was decreased from 85 to 81 dB, which resulted in decreased prepulse inhibition. Prepulse facilitation was now observed at large intervals (> 800 ms), which suggests that this facilitation was masked by inhibition in previous studies. These results solve the discrepancy between rat and human data and indicate that similar mechanisms are involved.