Characterizing ACC involvement in emotional contagion circuits using functional ultrasound imaging in mice

We often feel distress when witnessing another individual in pain. The emotional state of others eliciting a similar state in the observer is defined as ‘emotional contagion’. This is observed in many species including humans and rodents. Human fMRI studies identified the anterior cingulate cortex (ACC) as a key region activated both by our own emotional states and by observing similar emotions in others. Rodent experiments show individual neurons in the ACC to be both activated by self-experience of pain and witnessing a conspecific in pain. To gain deeper insight into the mechanism underlying transfer of distress and to understand its evolutionary conservation, it is highly advantageous to compare rodent and human data using the same measure of brain activity. Functional Ultrasound Imaging (fUSI) is a breakthrough modality that is comparable to human fMRI, by recording local changes in cerebral blood volume induced by neural activity. Here, we show that socially witnessing pain (shock observation) synchronizes activity across the ACC-centred pain circuit including midbrain defensive hubs, a pattern absent during non-social fear recall. Tail-shock to the observer activates a similar but wider network and strengthens ACC-centred functional coupling, showing that the circuitry engaged to feel one’s own pain is repurposed to understand another’s. Thus, fUSI reveals a shared self/other pain code that spans cortical–subcortical nodes and can be mapped at mesoscale resolution in awake mice.