Retrieval difficulty and retention of reactivated memories over the first year of life

A newly acquired memory is remembered longer when its retrieval is more difficult (i.e., when the time since the last retrieval is longer). In two experiments using an operant conditioning task with 198 infants between 3 and 12 months of age, we presently asked if a reactivated memory also is remembered longer when its retrieval is more difficult. During their first year, infants remember a memory reactivated 1 week after forgetting and the newly acquired memory equally as long. After determining the upper limit (UL) of reactivation at all ages (Experiment 1), we reactivated the memory near the UL and then measured its subsequent persistence (Experiment 2). We found that increasing the training-reactivation interval enhanced retention at 6 months only. These data reveal that increasing retrieval difficulty affects infants' retention of newly acquired and reactivated memories differently. More generally, memory reactivation seems to disproportionately benefit retention in younger infants.