Test for extinction retention at various time points after extinction acquisition. (A) Experimental design. (B) Extinction acquisition. Mean (±SEM) freezing to the tone averaged over two 20-sec stimulus presentations. All animals acquired comparable extinction. Test of extinction memory retention represents mean (±SEM) freezing to the tone averaged over three presentations. Animals were divided into four groups and tested 1 h, 4 h, 8 h, or 24 h after extinction acquisition, respectively. Rats in the short-delay groups (1 h and 4 h) exhibited high levels of freezing compared to the last block of extinction and to the longer delay testing groups ([*] P < 0.05).
