Brn3b-dependent circuits are necessary for visually triggered freezing responses.

Fear-related defensive behavior was analyzed by examining freezing responses to visual threat in a looming stimulus assay. (A) Schematic diagram of the behavioral paradigms. The looming stimulus was an expanding dark disk on a gray background presented overhead (0.25 s expansion; 0.25 s at largest disk size; 0.5 s interstimulus interval; 10 repeats). Following the looming stimulus assay, the mice were tested on light/dark discrimination. For the experiments involving single housing, the mice were housed individually for 2 weeks prior to the behavioral tests. (B) Quantification of freezing responses for group-housed (32.4 ± 6.5 s for control, 22.6 ± 6.1 s for cKO; n = 9 for control and 8 for cKO) and single-housed (78.1 ± 12.3 s for control, 35.3 ± 7.4 s for cKO; n = 12 for control and cKO) mice. A significant difference in total freezing time was observed between control and cKO in single-housed but not in group-housed mice. Tukey post hoc analysis (mean ± SEM, p = 0.903 for group-housed and p = 0.006 [**] for single-housed animals). (C) Quantification of the time spent in the brightly lit area for group-housed (132.5 ± 14.0 s for control, 137.5 ± 21.9 s for cKO; n = 9 for control and 8 for cKO) and single-housed (108.7 ± 15.6 s for control, 136.1 ± 12.9 s for cKO; n = 9 for control and cKO) mice. No significant difference was detected in the light/dark exploration test. Tukey post hoc analysis (mean ± SEM, p = 0.996 for group-housed, p = 0.618 for single-housed animals). The data underlying this figure can be found in S1 Data.