Ontogeny of second-order odor-aversion conditioning in neonatal rats

In Experimental 1, rat pups 1, 3, 5, or 7 days old received a Phase 1 S1-US (Stimulus 1-unconditioned stimulus) pairing calculated to establish a first-order conditioned aversion to S1 (lemon scent). On the next day each pup received a Phase 2 S2-S1 pairing designed to establish a second-order aversion to S2 (orange scent). Pups at all ages displayed a first-order aversion to S1, but only pups given Phase 2 training when 6 or 8 days old displayed a second-order aversion to S2. Experiment 2 revealed that pups that received the S1-US pairing when 3 days old acquired an aversion to S2 if they were 6 days old when they received the S2-S1 pairing but not if they received the pairing when 4 days old. In Experiment 3, two Phase 2 treatments were compared. One was identical to that employed in Experiments 1 and 2: Pups 4 days old received a sequential exposure to S2 and S1. Another set of pups received a modified treatment: They were exposed first to S2 and then jointly to both S2 and S1. Only pups in the latter set displayed an aversion to S2. It is suggested that these results reflect developmental differences in the memory processes mediating the rat's experience with olfactory stimulation.