Fear potentiation of the acoustic startle reflex using noises of various spectral frequencies as conditioned stimuli

Abstract

The effeetiveness of noise of various intensities and frequeneies in modulating the amplitude of the aeoustie startle reflex was evaluated, as a preliminary test of the eapaeity of these noises to produee eonsistent and reliable fear-potentiated startle in rats. It was determined that bands ofnoise eontaining high frequeneies (greater than 10 kHz) tend to uneonditionally reduee aeous­ tic startle responses, probably by masking the high-frequency startle-elieiting stimulus. Noise eontaining high frequeneies produeed reliable fear-potentiated startle only when its estimated uneonditioned startle suppression was subtraeted from the enhaneement obtained after pairing it with footshocks. Noises devoid ofhigh frequeneies produced modest uneonditioned startle en­ haneements and the most robust and reliable fear-potentiated startle when they were paired with footshocks. A nonmonotonie relationship between training shock intensity and the level of fear­ potentiated startle was also exhibited when a low-frequeney noise was used as a eonditioned stim­ ulus, a finding consistent with previous studies in whieh a visual stimulus was used. Finally, a differential Pavlovian eonditioning procedure indieated that rats eould readily discriminate between two different low-frequeney bands ofnoise. The outcome ofthese experiments indieates that it is easier to employ low-frequeney bands of noise to obtain auditory fear-potentiated star­ tle with high-frequeney startle-elieiting stimuli. The fear-potentiated startle paradigm, in whieh the am­ plitude of the aeoustie startle reflex is enhaneed in the presenee of a eonditioned stimulus (CS) previously paired with an aversive unconditioned stimulus (US), has repeat­ edly been used to study Pavlovian (classieal) eondition­ ing processes (Brown, Kalish, & Farber, 1951; ef. Davis, 1986). In most studies in whieh the fear-potentiated star­ tle paradigm has been used, CSs have been either visual or eompound visual-auditory stimuli. In only a few studies has fear-potentiated startle been found with the exclusive use of auditory CSs (Hitcheock & Davis, 1987; Siegel, 1967; Swerdlow, Britton, & Koob, 1989). Attempts at replieating auditory fear-potentiated startle in our labo­ ratory have usually generated ineonsistent results. Reli­ able startle potentiation in the auditory modality would allow tests of the generality of previous findings obtained with visual CSs, as weil as the study of higher order eonditioning phenomena, through the use of the fear­ potentiated startle paradigm.