Fig. 1.

Fear generalization is associated with decreased neuronal activation in the ZI. (A) Outline of the discriminative auditory fear conditioning protocol. On day 1, one group of mice received CS⁺ tone presentations paired with 0.3-mA foot-shocks (low-threat intensity) and unpaired CS⁻ tone presentations. Another group of mice received CS⁺ tone presentations paired with 0.8-mA foot-shocks (high-threat intensity) and unpaired CS⁻ tone presentations. On day 2, freezing responses were recorded for the CS⁺ and CS⁻ tone presentations. ITI, intertrial interval. (B) Animals trained under low-threat conditions show a low freezing response to CS⁻ and a high freezing response to CS⁺ (no fear generalization). In contrast, animals trained under high-threat conditions show an increased freezing response to both CS⁻ and CS⁺ (fear generalization). (C) Discrimination indices (DIs) reveal significant fear generalization in the animals trained under high-threat conditions. (D) Decreased C-FOS expression was observed in the ZI of animals that showed increased fear to CS⁻ presentations on the testing day. (E) Significant correlation was found between C-FOS expression in the ZI and behavioral fear responses. (F) Representative images of C-FOS expression in the ZI in response to tone presentations during the testing day after training under low-threat or high-threat conditions. (Scale bar: 100 μm.) **P < 0.01; ****P < 0.0001. Blue bars, low-intensity training; red bars, high-intensity training. Data are represented as mean ± SEM.