Auditory fear conditioning with a lower-intensity footshock is associative in nature. (A) Schema of fear conditioning in the paired or unpaired paradigm. In the paired conditioning paradigm, mice were placed in the conditioning chamber for 2 min and then presented with a tone for 1 min. At the end of the tone presentation, the mice were given a scrambled electrical footshock (0.3 mA, 1 s). One minute after footshock, the mice were returned to their home cages. In the unpaired conditioning paradigm, mice received the footshock immediately after placement in the conditioning chamber. Two and half minutes after the placement, a tone was presented for 1 min. Half minute later, mice were returned to their home cages. Twenty-four hours after conditioning, mice were placed in a novel chamber for 3 min and then the tone was presented for 3 min. (B) Freezing responses of paired group (filled circles, n = 7) on the conditioning (left) and test (right) days. (C) Freezing responses of unpaired group (open circles, n = 6) on the conditioning (left) and test (right) days. Arrows and solid lines indicate footshock and tone, respectively.