Emotion processing across and within species: A comparison between humans (Homo sapiens) and chimpanzees (Pan troglodytes).

For social species, recognizing and adequately yet quickly responding to the emotions of others is crucial for their survival. The current study investigates attentional biases toward emotions in two closely related species, humans and chimpanzees. Prior research has demonstrated that humans typically show an attentional bias toward emotions. We here build on that literature by studying the underlying unconscious mechanisms within and across humans and chimpanzees and aim to gain insight into the evolutionary continuity of expressions. Experiment 1 tested whether chimpanzees show an attentional bias toward the expressions of conspecifics and whether this putative bias is modulated by the stimulus presentation duration, being 33 ms or 300 ms. The stimuli were followed by a visual mask in the form of a neutral body image. This backward-masking procedure eliminated the visibility of the stimuli that were presented for 33 ms, rendering their presentation subliminal. In contrast to our prediction, no attentional bias toward emotions was observed in chimpanzees. The goal of Experiment 2 was to verify this finding and to investigate chimpanzees' reaction to human stimuli. Replicating Experiment 1, no evidence of an attentional bias toward emotions was observed in chimpanzees. In Experiment 3 we used the same chimpanzee and human expressions in 711 museum visitors and confirmed that humans do have an attentional bias toward emotions. Interestingly, this bias was independent of the stimulus presentation duration and most strikingly, independent of the species that was observed. Implications for theorizing about species differences in attentional mechanisms in processing emotions are discussed, as well as directions for future research, to investigate our preliminary findings and this potential species difference further. (PsycINFO Database Record (c) 2018 APA, all rights reserved).