Mice were fear conditioned as described previously.26,27 The conditioning context (‘context A’) was a 27 × 27 × 11 cm3 chamber with a metal-rod floor, cleaned with a 79.5% water/19.5% ethanol/1% vanilla-extract solution. After a 180 s acclimation period, there were 3 × pairings (60–120s inter-pairing interval) of the conditioned stimulus (CS; 30s, 80 dB, 3 kHz tone) and the unconditioned stimulus (US; 2 s, 0.6 mA scrambled foot shock), in which the US was presented during the last 2 s of the CS. The session ended 120 s after the final CS–US pairing. Stimulus presentation was controlled by the Med Associates VideoFreeze system (Med Associates, Burlington, VT, USA). Freezing (no visible movement except respiration) was scored every 5 s by an observer blind to condition/treatment and converted to a percentage ((freezing observations/total observations) × 100). Freezing during extinction was averaged to 5-trial blocks for analysis.
