The role of age-dependent behaviors in the retention of an approach-avoidance response in preweanling rats

Approach memory turns to avoidance memory with age

Ontogenetic modification of an early memory is relatively poorly understood. And an important question is whether the memory output is more determined by the age at acquisition or at retention? Here we explore the expression of odor-shock conditioning in the rat pup. Acquisition at post-natal day 6 (P6) leads to an approach response and at post-natal day 12 (P12) to an avoidance response when the retention test is 24h later. In both cases, anisomycin injected immediately post-acquisition induced a retrograde amnesia. Controls show that, in either case, short-term memory measured 4h after acquisition is not impaired and that anisomycin given after a 4h delay has no effect. Thus, at the two ages, memory involves a consolidation process. The main result is the spontaneous reversal of the conditioned response from approach acquired at P6 to avoidance when tested at P13. This phenomenon is robust as it is observed in three conditions. Moreover, amnesia induced at P6 is maintained at P13. Results are discussed in terms of maturation and/or competition of the memory traces.

Does infant memory expression reflect age at encoding or age at retrieval?

Do human infants express a memory acquired earlier in ontogeny in a manner appropriate to their age at encoding or their age at the time of retrieval? To answer this, we exploited the fact that retention is highly context dependent at 6 months but not at 8-9 months of age. Six-month-olds learned an operant response in one context, and their memory was maintained by monthly reinstatements in the original context. At 8 or 9 months of age, 1 month after the last (or only) reinstatement, infants were tested in either the same or a different context. During testing, infants' retention was no longer context dependent; rather, they responded robustly in both test contexts. These results revealed that infants expressed a memory acquired when they were younger in a manner appropriate to their test age. They were interpreted in terms of changes in the functional significance of context before and after infants self-locomote.