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. Author manuscript; available in PMC: 2021 Sep 4.
Published in final edited form as: Neuroscience. 2020 Jan 10;429:92–105. doi: 10.1016/j.neuroscience.2019.12.009

Fig. 2.

Fig. 2.

CO2 inhalation evokes differential freezing behavior in mice. Mice were exposed to the CO2 inhalation paradigm (see layout) comprising of habituation (A, D), CO2 inhalation (B, C and E) and context exposure (F). (A) No differences in freezing during habituation to context on Day 1 in mice exposed to air or CO2. (B) Significantly higher freezing was observed in mice exposed to CO2 in comparison with the air cohort on Day 2. (C) A frequency distribution histogram of Day 2 freezing revealed subpopulations within the CO2-exposed mice, with a group showing freezing behavior that largely overlapped with air exposed mice (CO2-low, CO2-L) and the other separated by higher freezing (CO2-high, CO2-H). (D) Dichotomization using a median split showed no significant group differences during habituation (Day 1). (E) During air or CO2 inhalation (Day 2), CO2-H mice showed significantly higher freezing in comparison to both air and CO2-L groups and no differences between air and CO2-L cohorts. (F) Exposure to context on Day 3 shows significantly higher freezing in the CO2-H group as compared to air and CO2-L mice. Data are represented as mean ± SEM; *p < 0.05 versus air group or CO2-L group (N = 9–19 mice per group).