Figure 1.

Animals receiving pairings of odor and foot shock showed increased freezing response during a cue test (odor presented in a novel test chamber) 24 h after training. (A) Percent freezing per minute across successive trials on test day. During trial 0 no odor was given to assess baseline freezing levels and potential contextual fear. The trained odor, E5 was presented into the chamber for 20 sec at the beginning of trials 1–5. Mice were placed into three groups based on training: the paired group (•); the foot shock only group (█); and the odor only group (▴). From trial 1 on, only the paired group displayed strong freezing responses to E5. (B) Mean time spent freezing across all odor trials for each group. The animals receiving shock only or odor only (white bar) during the training showed little freezing responses during the odor test, whereas those in the paired groups displayed significantly higher freezing levels (black bar). (*) Significant difference from the control groups, P < 0.05.