Differential modulation of freezing and 22-kHz USVs by shock intensity, tone-duration matching, and anxiety levels in rodent fear-conditioning paradigms.

The concept of fear in animals, particularly its manifestation and measurement, remains a focal point in psychological research. This study builds on the systematic review and meta-analysis work of Bao et al. (Neuroscience & Biobehavioral Reviews, 157: 105537, 2024), which posits that freezing behavior and 22-kHz ultrasonic vocalizations (USVs) may represent objective and subjective fear states in rodents, respectively. We further investigated how these responses are modulated by shock intensity, tone-duration matching, and individual anxiety levels in rodent fear-conditioning paradigms. Experiment 1 manipulated shock intensity during fear learning and tone-duration matching between learning and cue tests, revealing that while freezing behavior was consistent across conditions, 22-kHz USVs varied significantly and appeared later than freezing. This divergence was more pronounced in fear generalization tests. Experiment 2 explored the response differences in rodents with high and low anxiety, demonstrating that highly anxious individuals exhibited more 22-kHz USVs but not increased freezing during cue tests. These findings suggest that while freezing may reflect automatic defensive reactions, 22-kHz USVs are more indicative of rodents' cognitive appraisal and their subjective experience of fear. This distinction provides valuable insights that could improve the translation of animal fear models to human psychiatric conditions related to fear and anxiety.