Investigating the mechanistic role of painful self-experience in emotional contagion: an effect of auto-conditioning?

Emotional contagion refers to the transmission of emotions from one conspecific to another. Previous research in rodents has demonstrated that the self-experience of footshocks enhances how much an observer is affected by the emotional state of a conspecific in pain or distress. We hypothesized auditory auto-conditioning to contribute to this enhancement: during the observers own experience of shocks, the animal associates its own audible nocifensive responses, i.e. its pain squeaks, with the negative affective state induced by the shock. When the animal later witnesses a cage mate receive shocks and hears it squeak, the previously strengthened connection between fear and squeaks could be a mechanism eliciting the enhanced fearful response in the observer. As hypothesized, in a first study, we found pre-exposure to shocks to increase freezing and 22 kHz vocalizations associated with distress upon the playback of pain squeaks. Freezing was also increased during the playbacks of phase-scrambled squeaks, but 22 kHz calls were more frequent during the playback of regular squeaks. Core to the notion of auto-conditioning is that the effect of pre-exposure is due to the pairing of a pain-state with hearing ones own pain squeaks. In a second study, we therefore compared the response to squeak playbacks after animals had been pre-exposed to pairings of a CO2 laser with a squeak playback against three control groups that were pre-exposed to the CO2 laser alone, to squeak playbacks alone or to neither of these conditions. We however could not find any differences in freezing or 22 kHz calls among all experimental groups. In summary, we demonstrate the sufficiency of pain squeaks to trigger fear in a way that critically depends on the nature of an animals prior experience and discuss why the pairing of a CO2 laser with pain squeaks cannot substitute footshock pre-exposure.