A developmental analysis of contextual fear conditioning.

Contextual fear conditioning by 18- and 23-day-old rats was compared in two training contexts, a transparent Plexiglas chamber or a black Plexiglas chamber. As measured by a conditioned defensive freezing response, older rats displayed more contextual fear than younger rats. At both ages conditioning was (a) stronger in the black chamber than in the clear chamber, (b) a nonmonotonic function of retention interval, with freezing being greater at the immediate and 24-hr retention interval than at the 10-min interval, and (c) preexposure to the context 24 hr before conditioning enhanced conditioned freezing observed at the 10-min retention interval. Additional experiments suggest that rats at both ages acquire independent representations of the visual and tactile features of the context. These results support Rudy and Morledge's (1994) hypothesis that contextual fear conditioning is mediated by both a short-term and a long-term memory system and that long-term memory for contextual fear requires the consolidation of a representation of the context. They challenge their view that there is a qualitative developmental difference in long-term memory processes between 18- and 23-day-old rats.