The effects of selective ibotenate lesions of the hippocampus on conditioned inhibition and extinction.

Previously, Solomon (1977) reported that aspiration lesions of the dorsal hippocampus in rabbits had no effect either on the acquisition of Pavlovian conditioned inhibition or on performance during a subsequent retardation test. The present experiment confirmed and extended these findings by showing that rats with ibotenate lesions of the complete hippocampus (the dorsal and ventral hippocampus and the dentate gyrus) were also unimpaired on the same types of tasks. Additional tests with the same rats showed that removing the hippocampus significantly impaired extinction of responding to a stimulus that had been previously trained with an appetitive unconditioned stimulus. The performance of the lesioned rats on a summation test was also marginally, but not significantly, different from that of controls. The data are discussed with reference to the idea that the hippocampus is involved with the formation of some, but not all, types of inhibitory associations.