All animals were adult male Sprague-Dawley rats weighing 300–350 g. Rats were fear conditioned according to the paradigm schematized in Figure 1. Training consisted of a 5-minute acclimation to the novel fear conditioning chamber (Rat Test Cage, Coulbourn Instruments, Allen-town, PA), followed by three presentations of the CS (tone, 30 seconds, 75dB, 5 kHz), paired with an aversive US (foot shock, 1 second, 0.8 mA, co-terminating with the CS) that were separated by 5-minute intertrial intervals. In contrast to these fear-conditioned animals that received the CS and US in a paired fashion, the control animals received the same CSs and USs but in an explicitly unpaired fashion (unpaired): tone CSs were delivered on the same schedule as the paired group (5-minute inter-tone intervals), but the shock USs were delivered, on average, 120 seconds before the CSs. Unpaired controls using similar protocols have repeatedly been shown to produce little or no associative fear of the CS (Cain and LeDoux, 2007).
