Retardation of rabbit nictitating membrane conditioning following US preexposures depends on the distribution and number of US presentations

Retarded conditioned response (CR) acquisition produced by unconditioned stimulus (US) preexposures has been attributed either to interference resulting from contextual conditioning or to habituation of the US. Both perspectives assume that the amount of retardation is directly related to the number of US preexposures. This assumption was examined in two experiments. In Experiment 1, separate groups of rabbits received 200 paraorbital shock US preexposures either in one session or spread equally over 10 daily sessions. Subsequently, all subjects received 150 CS-US pairings. Acquisition of nictitating membrane CRs was retarded relative to a naive control group only in the group that received the preexposures over 10 sessions. Thus, the number of US preexposure sessions, rather than the number of US preexposures, determined whether or not retarded acquisition was observed. In Experiment 2, four groups of rabbits received 1, 5, 20, or 40 shock US preexposures in each of 10 sessions. Over the subsequent 150 CS-US pairings, similar levels of retarded CR acquisition were observed in groups that received 20 and 40 US preexposures per session, a weak retardation effect was observed with 5 preexposures per session, and no retardation was observed with 1 preexposure per session. Thus, Experiment 2 suggested that retarded CR development was not greatly influenced by increasing the number of US preexposures above some minimum threshold number of exposures per session. Implications for current theories were discussed.