Vocal communication of simulated pain

While evidence suggests that pain cries produced by human babies and other mammal infants communicate pain intensity, whether the pain vocalisations of human adults also encode pain intensity, and which acoustic characteristics influence listeners' perceptions, remains unexplored. Here, we investigated how trained actors communicated pain by comparing the acoustic characteristics of nonverbal vocalisations expressing different levels of pain intensity (mild, moderate and severe). We then performed playback experiments to examine whether vocalisers successfully communicated pain intensity to listeners, and which acoustic characteristics were responsible for variation in pain ratings. We found that the mean and range of voice fundamental frequency (F0, perceived as pitch), the amplitude of the vocalisation, the degree of periodicity of the vocalisation and the proportion of the signal displaying non-linear phenomena all increased with the level of simulated pain intensity. In turn, these parameters predicted increases in listeners' ratings of pain intensity. We also found that while different voice features contributed to increases in pain ratings within each level of expressed pain, a combination of these features explained an impressive amount of the variance in listeners' pain ratings, both across (76%) and within (31-54%) pain levels. Our results show that adult vocalisers can volitionally simulate and modulate pain vocalisations to influence listeners' perceptions of pain in a manner consistent with authentic human infant and nonhuman mammal pain vocalisations, and highlight potential for the development of a practical quantitative tool to improve pain assessment in populations unable to self-report their subjective pain experience.