22kHz Ultrasonic Vocalizations and Body Freezing May Represent Two Systems of Fear in Rodent Fear Conditioning Paradigms

Summary The “two-system” framework of fear proposed by LeDoux and Pine (2016) sparked a discussion on the understanding of fear, prompting a reevaluation of rodent fear conditioning studies. We propose that 22kHz ultrasonic vocalizations (USV) may symbolize the subjective negative emotional states in rats. To evaluate this, we designed a series of fear conditioning experiments with varied parameters, comparing the expression of 22kHz USV and body freezing – a traditional fear index. These expressions were further assessed in fear generalization tests. Our findings suggest a distinct separation between body freezing and 22kHz vocalizations in fear conditioning under different conditions. The results indicate that 22kHz USV may more accurately signify the subjective state of fear, whereas body freezing may denote an automatic defensive response in rats. Consequently, we posit that the two-system fear framework may extend to rodent fear conditioning paradigms. Therefore, researchers should place greater emphasis on 22kHz vocalizations when investigating the subjective state of fear.