Positive emotional contagion by laughter-like 50-kHz ultrasonic vocalizations in rats

Rats display advanced empathic abilities, yet current research is focused on empathy toward negative affect, largely neglecting positive affective states. We therefore developed a paradigm suitable for assessing both negative and positive emotional contagion. It relies on ethotransmission (i.e., acoustic emotional communication) of affective states to conspecifics via ultrasonic vocalizations (USV), i.e., alarming 22-kHz USV (negative affect) and laughter-like 50-kHz USV (positive affect). Specifically, we subjected rats to playback of 22-kHz or 50-kHz USV, while assessing their acoustic startle response (ASR), which is known to be emotionally modulated (i.e., potentiated and attenuated by negative and positive affect, respectively). Here, 50-kHz but not 22-kHz USV strongly attenuated the ASR across sexes and development, suggesting that laughter-like 50-kHz USV induced positive affect in receivers, indicative of positive emotional contagion. Our paradigm provides the first quantitative assessment of positive emotional contagion in rats through capitalizing on ethological signals (USV) and simple reflexes (ASR).