Ontogenetic differences in the expression of conditioned stimulus conditioning: effects of retention interval.

Preweanling 17-day-old rats were tested for retention of the conditioned emotional response after a 5-min or 24-hr retention interval. For a variety of conditioning parameters (i.e., variation in conditioned stimulus modality, unconditioned stimulus intensity, number of training trials), conditioned responding was consistently weaker after 5 min than after 24 hr. This apparent "incubation," or "hypermnesic," effect was not found in adult rats, even when comparable conditioning levels were indicated on the 24-hr test. The transient short-term retention deficit observed in 17-day-old preweanlings was alleviated by placing the pup in its home cage during the 5-min retention interval or by extending the conditioning session. Fifteen-day-old rat pups did not benefit from home cage exposure or extended training and displayed the transient short-term retention deficit regardless. The results are discussed in terms of age-related effects on time-dependent memory consolidation.