Figure 8. CO₂ reactivity, fear conditioning, Extinction, and post-extinction LTM for the rats in the Lower (Blue) and Upper (Red) 40% range of LTM freezing.

CO₂ reactivity is shown here using 2 of the most important predictors of extinction phenotype: Rearing and Ambulation during the Flush-out periods (R3 and A3). The CO₂ reactivity measures (A) between the Lower and Upper 40% freezers approached significance, and there were significant differences between the Lower and Upper 40% freezers during extinction, and at the post-extinction LTM test, but not during Fear conditioning, preCS freezing, or freezing during the first 3 trials of extinction (B), suggesting that CO₂ reactivity may be specific to differences in extinction rather than fear learning. N=22, *= p<.05, #= p< .1, and p greater than .2 for all other comparisons.