Fig. 1.

Fear conditioning in F₂ and F₈ mice. A three-day procedure was used to phenotype each subject. Each test lasted 5 min. On day 1, pre-training freezing was measured in F₂ (a) and F₈ (b) mice from 30 to 180 s, after which mice were exposed to two 30-s tones (indicated by hatched bars, labeled T1 and T2) that co-terminated with a 2-s, 0.5-mA foot shock (indicated by arrows, labeled S1 and S2). On day 2, freezing to context was measured in F₂ (c) and F₈ (d) mice from 30 to 180 s. On day 3, freezing to the altered context was measured in F₂ (e) and F₈ (f) mice from 30 to 180 s after which freezing to cue was measured (180–210 + 240–270 s); the time spent freezing to each tone was averaged to obtain the freezing to tone/cue variable. Each data point represents the average % freezing calculated across the 30 s time bin. Error bars represent the standard error of the mean