was assessed by returning mice to the training chambers and measuring freezing behavior for 5 min. On Day 3, the chambers were reconfigured (i.e., floor bars were covered with a plastic insert and white wall inserts were replaced with black tent inserts to alter both the color and shape of the chamber) and cleaned with 0.1% acetic acid. Mice were then assessed for freezing behavior in response to 2 × 3-min CS presentations. Freezing was assessed by Video Freeze software (Med Associates, Inc.) and data are presented as percent time spent freezing.
