The influence of stimulus repetition on the acoustic startle response and on responses of inferior colliculus neurons of mice.

The acoustic startle response (ASR) and discharges of single units in the inferior colliculus (IC) were evaluated as a function of the repetition rate and trial (stimulus 1-10 at each rate) of intense noise bursts in mice. The amplitude of the ASR was inversely proportional to both rate and trial. The mean number of discharges for the sample population of units of the central nucleus (ICC) and the external nucleus (ICX) were also inversely proportional to stimulus rate. However, only ICX units were inversely proportional to trial (i.e. as was the ASR). When response properties of individual units were examined, a variety of profiles was observed with regard to rate and trial sensitivity: units were either not sensitive to stimulus rate, inversely sensitive, directly sensitive, or complexly sensitive. Superimposed on these rate classifications were trial sensitivity, with some units being insensitive to trial, directly sensitive, or inversely sensitive. The incidence of response types differed in some respects between ICC and ICX and these differences were, in general, consistent with the mean responses of the neuronal populations.