Habituation and sensitization of the rat startle response: effects of raphe lesions

In the first experiment raphe lesioned, sham operated and nonoperated rats were presented with 150 tones (50 at each of 3 intensities) a day for 5 days. No differences were found among the groups in the rate of startle response habituation either within or between sessions. However, overall levels of startle were much greater following raphe lesions and a tone by tone analysis indicated that this was caused by heightened tone-elicited sensitization in the raphe group. Further tests of startle sensitization in Experiment 2 found the raphe group to be more sensitized than the other groups by loud tones and footshocks but not by different levels of background white noise. The results support the theory that repetitive stimulus exposure produces both habituation and sensitization and that different neural systems may underly these two processes.