Factors influencing developmental differences in retention of Pavlovian fear conditioning.

Abstract

Modern nonhuman animal research on the rapid forgetting of memories formed early in life-often termed "infantile amnesia"-has focused on neurobiological changes occurring between learning and retention testing to explain age differences in memory. Developmental differences in initial learning have received less attention as a contributing factor to infantile amnesia effects. The present study identifies conditions under which associative learning and memory are comparable between pre and postweaning rats across multiple training-testing intervals. Postnatal day (P) 17-18 or P24-25 littermates were trained with white noise conditional stimuli (CSs) alone, forward-paired, or explicitly unpaired with floor shock unconditional stimuli (USs), and tested for retention at intervals ranging between 5 min and 15 days later. Findings from within- and across-institution replications revealed that age differences in CS freezing were influenced by (a) the associative nature of the CS and US at training, (b) the number of CS, US presentations at training, and (c) the interval between training and testing. Rats trained on P17 or 18 displayed robust retention comparable to rats trained on P24 or 25 only when training in younger rats involved additional forward-paired CS-US presentations. Poor long-term retention observed at multiple training-testing intervals in rats trained on P17 or 18 was overcome with many additional forward-paired CS-US presentations at training. Conditions necessary for appropriate developmental comparisons of learning and memory relevant to the future neurobiological studies are discussed.