Learning about potential threats is critical for survival. Fear learning can happen through direct experience or through others. Like most animals, rats can acquire fear through social transmission by sensing cues from conspecifics that experience fear. According to a new study, laboratory rats can also detect human fear.

To assess whether rats have the ability to detect negative emotional states in humans, the researchers analyzed the behavior of male Wistar rats during their interaction with male caregivers who had been subjected to a fear conditioning procedure a few minutes before. The fear conditioning task experienced by the caregivers involved the presentation of colored symbols and the administration of unpleasant electrical stimulation. Previous studies employing this fear conditioning procedure have shown that it induces robust responses to the unconditioned stimulus, indicating fear acquisition.

Analysis of human–rat interactions revealed that rats responded differently to human caregivers who underwent fear conditioning and to caregivers who were subjected to the sham procedure. Notably, rats tested with the caregivers who underwent fear conditioning explored the human’s hands less, spent more time exploring the cage and showed a decrease in ultrasonic communication, indicating increased anxiety.

Next, the researchers investigated the neural circuits responsible for the ability of rats to detect emotional states of humans. c-Fos immunochemistry conducted on brain sections confirmed that the centromedial and basolateral nuclei – two major parts of the amygdala, a structure critical for detecting threats and sensing conspecifics’ fear – were more active in rats interacting with the caregiver who underwent fear conditioning than in rats interacting with the caregiver subjected to the sham procedure.

In a final set of experiments, the investigators performed an observational fear learning procedure in humans, in which one participant in a pair was assigned the role of a demonstrator while another was given the role of an observer. Results of the experiments showed that the observers felt empathy towards their friends receiving an electric stimulation and fMRI analysis indicated that they responded with activation of the basolateral and centromedial amygdalar nuclei.

Altogether, these results suggest that cross-species and within-species social transmission of threat information involve activation of similar neural circuits in the amygdala, indicating an evolutionarily conserved brain system for processing fear.

Original reference: Kaźmierowska, A.M. et al. Proc. Natl. Acad. Sci. USA 120, e2302655120 (2023)