Ontogenetic differences in expressed fear of context following aversive conditioning

A conditioned-emotional-response procedure was used to compare acquired fear of a light conditioned stimulus and the conditioning context in 17-, 21-, and 60-day-old rats. Separate groups at each age received either light and shock paired, light and shock unpaired, shock alone, or neither light nor shock before testing 24 h later. Differences in learned context fear, measured on the basis of group differences in baseline activity prior to presentation of the light, appeared in 17- and 21-day-old rats but not in adult rats. Activity also declined significantly over the baseline intervals at the two younger ages among all groups that received shock; activity levels in adults did not change over the baseline intervals. Although all three ages expressed conditioned fear of the light, indexed as a decrement in activity during the light relative to the no-light baseline period, adults also exhibited pseudoconditioning and sensitization. These results are discussed in terms of possible ontogenetic differences in context learning.