22-kHz Calling and Body Freezing May Represent Two Systems of Fear in Rodent Fear Conditioning Paradigms

The "two-system" framework of fear, as proposed by LeDoux and Pine (2016), challenges traditional approaches to understanding fear in animal models, particularly in rodents. This study investigates the role of 22-kHz ultrasonic vocalizations (USV) and body freezing, a traditional fear index, in rodent fear conditioning paradigms.

We conducted a series of experiments with varied parameters to explore whether these behaviours represent distinct aspects of fear: 22-kHz USV as an indicator of subjective emotional states and body freezing as an automatic defensive response. Additionally, we assessed these behaviours in fear generalization tests to explore their consistency and divergence under different conditions. Our findings reveal a clear distinction between body freezing and 22-kHz USV, suggesting that these behaviours may indeed arise from separate systems within the fear response framework.