Effects of mesencephalic reticular formation lesions on habituation of startle and lick suppression responses in the rat.

Extensive damage to the mesencephalic reticular formation (MRF) in rats altered long-term habituation of the acoustic startle response without disrupting short-term habituation. Animals with MRF lesions, although initially neither more nor less responsive than controls, could not attain as low a long-term asymptote of habituation as could control animals with repeated presentations of an auditory stimulus. Changing the quality of the auditory stimulus abolished the asymptotic difference in responsiveness. With repeated presentations of the second auditory stimulus, control animals again reached a significantly lower long-term habituation asymptote than did animals with MRF lesions. The course of long-term habituation for the two groups suggested a disruption of an extrinsic, inhibitory habituation process by the lesions. The effects of MRF lesions were specific to the acoustic startle response. Control animals and those with lesions showed comparable response levels, short-term habituation, and long-term habituation of the lick suppression response. responsiveness and habituation to tactile stimuli were comparable for the two groups in both lick suppression and startle response measures.