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(a) The training protocol for experiment 1, with each arrow indicating 1 day. Mean (±SEM) percentage of CS-elicited freezing is shown for each group at training (b), extinction (c), and test (d) in experiment 1. Rats (35 days old) show impaired extinction retention (ie, higher levels of freezing) at test compared with 24- and 70-day-old rats.
