Humans are not the only animals that can produce emotions

While behavioral and biological characteristics that are exclusive to humans certainly exist, researchers in this field keep finding more and more similarities between humans and other animals. For a long time, it was believed that tool usage was unique to humans, but then Jane Goodall observed chimpanzees using sticks to fish termites from a termite mound. Since then it has been shown that great apes, who are our closest living relatives, express many more human-like behaviors, even producing similar physiological responses when aroused. Unsurprisingly chimpanzees, who have almost identical facial muscles to humans, are also capable of activating them in different combinations, constructing a facial expression which is essential for emotional communication. This prompts the question of whether such communication would work across species.

Attention to human and chimpanzee emotions captured with eye-tracking

To find this out, at Leiden University we conducted an eye-tracking experiment, showing our participants different emotions expressed by chimpanzees and humans. Analyzing gaze data, we aimed to find similarities in human processing of different species’ facial expressions. To shed light on these mental processes, we selected visual attention as an indicator of the influences of different emotional expressions. Our participants first saw a picture of either a chimpanzee or a human expressing a positive, negative, or neutral emotion. Right after seeing that picture, two other pictures depicting a friendly and a fearful human appeared on the screen. We then measured towards which of these two pictures the participants’ attention was drawn more, and whether this changed on the basis of the emotional expression they had previously seen.

Even though humans understand chimpanzee emotions, the influence on attention is confounded

We were indeed able to find a shift of attention towards the emotional picture that depicted the same expression as was shown on the previous screen. For example, if participants saw a negative picture, they would focus for longer on the negative facial expression on the following screen than on the positive facial expression. However, this only worked when they viewed human pictures, not when they viewed chimpanzee pictures. To rule out the possibility that our participants might not have understood the chimpanzee emotions correctly, we asked them to classify the pictures they had seen after the experiment. This way, we showed that understanding and interpreting the emotions was not a problem. Thus, we thought of two alternative explanations for this finding. First, time might play an important role in emotion classification. Since our participants were not used to detecting chimpanzee emotions, this might have taken them longer than detecting human emotions and might therefore, not have worked in the experiment, where the emotion was shown only briefly. However, when unlimited time was given to rate the emotions, the identification of other species’ emotions worked well. Another explanation for why the effect found did not work with chimpanzee pictures could be that a negative chimpanzee expression was confused with a positive one. The bared teeth display, for instance, which is usually expressed when chimpanzees experience distress, has remarkable similarities to a human smile. Although overlapping facial muscles are at work in both expressions, they communicate completely different emotions.

Emotional priming works better with an own-species primer

With our study, we showed that humans’ attention can indeed be influenced by emotional expressions. However, this only works reliably with human emotional expressions and not with chimpanzee emotional expressions. Nevertheless, the correct classification of emotions shows that humans did understand chimpanzees' emotional expressions. Therefore, other factors, such as viewing duration or leakage from one emotional category to another, might play an interesting role that should be explored further. Taking a closer look at other animals, especially those whom we share common ancestors with, will shed light on the origin of mental processes and bring us closer to answering the question about where emotions come from and what it is that makes us and them unique.