Habituation of the acoustic startle response in rats after lesions in the mesencephalic reticular formation or in the inferior colliculus.

Extensive damage to the mesencephalic reticular formation (MRF) in rats altered long-term habituation of the acoustic startle response without disrupting short-term habituation. The MRF lesions did not alter initial startle amplitudes, but the animals with lesions were unable to attain as low a long-term asymptote of habituation as could control animals. Subsequent manipulation of stimulus intensity and interstimulus interval revealed no differences in short-term habituation between the two groups. Large lesions to the inferior colliculus (IC) did not disrupt long-term habituation of the acoustic startle response, but these animals were unable to suppress responding as much as controls to intense stimuli presented rapidly. The deficits in long-term habituation following MRF lesions suggested a disruption of an extrinsic, inhibitory mechanism of habituation. The deficits following IC lesions could be due either to a disruption of a short-term habituation mechanism or to an increase in response sensitization produced by the lesions.