Long-term retention of early Pavlovian fear conditioning in infant rats

That organisms cannot remember events occurring during infancy may be the result of common forgetting processes exacerbated by the organism's increase in size during development or a unique process such as insufficient neurological development at the time of the early experience. To establish the uniqueness of infantile forgetting, size change was made irrelevant by exposing infant rats to "off-baseline" Pavlovian fear conditioning and assessing the effect of an apparatus-free conditioned stimulus upon independently established bar pressing. In Experiment 1, bar pressing by rats exposed to Pavlovian contingencies when 20-22 days old was substantially suppressed by the conditioned stimulus both 1 and 42 days after conditioning. In Experiment 2, pups conditioned when 17-19 and 20-22 days old again showed excellent retention, whereas pups conditioned when 11-13 and 14-16 days old showed total forgetting 42 days later. In Experiment 3, pups conditioned when 14-16 days old remembered well after 5 days, less well 10 days later, and not at all after 20 days. These findings suggest that size change may contribute to the forgetting of events occurring late in development, but that neurological immaturity may underly the forgetting of earlier events.