Impaired free-operant avoidance behavior following ventromedial hypothalamic lesions in rats

Under each of 3 shock intensities, ventromedial hypothalamic lesioned rats emitted fewer responses and received more shocks than control animals in a lever-pressing free-operant avoidance paradigm. While the performance of the control animals improved across 10 days of testing and within each session, the between- and within-session performance of the lesioned rats, after improving moderately during the first few days of testing, deteriorated as training progressed. Although the response rates of the lesioned animals were relatively unrelated to the different current levels, their shock rates varied inversely with intensity. This resulted from an increase in the frequency of long interresponse times and a reduction in the number of consecutive shocks. Preoperative training substantially improved the avoidance behavior of lesioned animals, but their postoperative performance was nevertheless inferior to their preoperative levels.