Recent extinction training of a second CS is sufficient to reinstate responding to a previously extinguished CS. (A) Experimental design. (B) Extinction acquisition for two conditioned stimuli, CS1 and CS2. Points represent mean (±SEM) freezing to the tone averaged over two 20-sec stimulus presentations. Extinction was comparable between groups for both stimuli. Extinction retention for the first CS was tested 1 h or 24 h after CS2 acquisition. Test represents mean (±SEM) freezing to the tone averaged over three presentations. Animals tested 1 h after cessation of CS2 extinction exhibited significantly more freezing to CS1 than animals tested 24 h later ([*] P < 0.05).
