Ontogenesis of trace conditioning in young rats: dissociation of associative and memory processes.

Hooded rats from 15 to 30 days of age were trained with a Pavlovian trace fear conditioning procedure in order to study the development of their capacity to learn associations between events separated in time. For both the auditory and visual systems, the associative processes necessary to learn about temporally contiguous events emerged earlier during ontogenesis than the processes necessary to integrate events separated in time. Furthermore, the ability of the rats to integrate events separated by increasingly long intervals continued to improve as they got older. We suggest that the emergence of the capacity to integrate temporally separate events reflects the maturation of memory processes that retain representations of stimulus events over time, and that these processes continue to mature for a considerable period after the basic associative processes have become functional.