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

The concept of fear in animals, particularly how it manifests and is measured, remains a topic of ongoing research and debate. 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. The results demonstrate that 22-kHz USV align more closely with the subjective state of fear, while body freezing predominantly reflects an automated defensive mechanism. These findings support the extension of the two-system fear framework to rodent models and highlight the importance of incorporating measures like 22-kHz USV in studies exploring the subjective aspects of fear in animals. This study contributes to the understanding of fear in rodents and has implications for the broader field of animal behaviour and psychology, suggesting a more nuanced approach to studying fear responses in animal models.