HABITUATION AND SENSITIZATION OF THE ACOUSTIC STARTLE RESPONSE IN RATS - AMPLITUDE, THRESHOLD, AND LATENCY MEASURES

The amplitude of the acoustic startle response habituates to repetitiv e stimulation. The input and output of the startle system were measure d to determine if the decrease in startle amplitude during repetitive stimulation is due to an increase in the startle threshold. Two experi mental approaches were used in 35 Sprague-Dawley rats to probe the rel ationship between the input (the sound pressure level of the stimulus) and the behavioral output (startle amplitude). The results show that the minimum threshold for a response does not change during habituatio n; rather, the slope of the dependence of startle amplitude on stimulu s level decreases. Because habituation does not influence startle thre shold we propose that the site for habituation is located in the neura l circuitry downstream from the site for startle threshold. Besides am plitude and threshold, as an additional parameter we measured startle latency. In general, the latency of the acoustic startle response is n egatively correlated with the response amplitude. This correlation has been repeatedly shown, therefore one would expect a latency increase during the amplitude decrease caused by habituation. However, the late ncy of the startle reaction also decreased during the course of repeti tive stimulation. According to the dual process theory of habituation, a stimulus has both a response-decreasing, i.e., habituating, as well as a response-increasing, i.e., sensitizing, influence on a behavior (Groves & Thompson, 1970). Our explanation of the present results is t hat startle amplitude is reduced following repetitive stimulation beca use it is mainly influenced by habituation; latency, however, is short ened because it is mainly influenced by sensitization. (C) 1996 Academ ic Press, Inc.