A developmental dissociation in compound summation following extinction

Recent studies suggest that extinction may involve different processes across development. The present study attempted to further clarify the nature of the processes involved in extinction of Pavlovian conditioned fear across development by employing a compound test procedure in rats. In this procedure, a comparison is made between the responding elicited by individual versus compound presentations of two extinguished CSs. The first two experiments showed that PND25 rats exhibit reliable compound summation (i.e., greater responding to the compound than to the individual CSs), at least when a between-subjects design is employed, but that PND18 rats do not exhibit such an effect. Experiment 3 demonstrated that this developmental difference in compound summation was not simply due to PND 25 rats acquiring a stronger CS–US association in the conditioning phase, and Experiment 4 showed that the results cannot be explained in terms of developmental differences in how the compound stimulus is processed at a perceptual level at test. Taken together, these results suggest that there are developmental differences in the amount of associative change that occurs to CSs during extinction training.