The effect of age and experience on inhibition of the acoustic startle response by gaps in background noise

The acoustic startle response (ASR) is inhibited when the eliciting stimulus is preceded by a brief gap in background noise. The present study examined the ontogeny of ASR gap inhibition in the rat and the role of experience on its development. Independent groups of Long-Evans rats were tested for 3 consecutive days, at one of eight ages ranging from Postnatal Day (PND) 14–16 to PND 98–100. Gaps (0, 2,4, 8,16, 32, or 64 msec) in broad-band background noise (80 dB on/35 dB off) were presented 190 msec prior to the eliciting stimulus (a 13-kHz, 120-dB, 40–msec tone with 2.5-msec rise/decay). The results show that longer gaps (16–64 msec) inhibited the ASR at all ages; however, the magnitude of inhibition increased with age and repeated testing. The efficacy of shorter gaps (4–8 msec) in the inhibition of the ASR improved until PND 35–37, when the adult pattern was achieved. These findings indicate that both the magnitude of ASR gap inhibition and the efficacy of gaps of less than 16 msec are dependent on the age and experience of the animal.