How primate eye tracking reveals new insights into the evolution of language

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The human environment is highly social. Family, friends, co-workers, strangers: all provide us with a continuous stream of information that we must track and understand. Who is dating whom? Who is fighting with whom? While our ability to manage such a vast social network is impressive, it is not something that is particularly unique to humans. Other primates do it, too.

We humans and other primates like monkeys and apes have what is called the social knowledge which allows us to follow the social dynamics of our friends, neighbors and even enemies.

What may be different about humans is how we communicate these dynamics. If I see my neighbors saying hello, I can easily express it in one sentence: “David is greeting Iris.” Research suggests that other primates can’t do this.

They can communicate about individual entities, such as sounding an alarm when there is danger, or producing food calls when they find a food they like. But they do not seem to express how an action is related to the individuals involved.

And that's exactly what happens when I make a sentence like, "David greets Iris." First, I say who is doing the action (David—the agent), then I express what he is doing (the action), and finally, to whom he is doing the action (the patient).

This structuring of the event is not only the case in English. Most languages give priority to agents through grammar, suggesting that it is a universal phenomenon among humans.

Cross-linguistic studies have found similar biases when people look at pictures of events. In tasks where people have to describe a picture depicting an action, they are quickly able to identify the agentand spend more time looking at the agent than the patient.

This suggests that our ability to "deconstruct" events such as these, and our apparent bias toward agents, may have their roots in an era before the evolution of language.

Eye tracking

To test this, together with Swiss colleagues, I conducted a eye tracking study with human adults, six-month-old babies, chimpanzees, gorillas and orangutans in a zoo.

We showed participants videos of social interactions, such as an orangutan hugging another, and non-social interactions, such as a person pushing a shelf, using a technique called infrared eye tracking. This technique allows us to determine remotely where our eyes are when we look at a screen. This meant that we were able to work with monkeys who were watching the videos voluntarily, through a designated window.

Our results revealed that adults and monkeys were quick to identify agents, but only in scenes where patients were objects.

In social interactions, determining who was the agent and who was the patient seemed to take longer. Unexpectedly, only in scenes depicting food did participants primarily look at the agent (who was eating or carrying food).

This lack of agent prioritization in other scenes is likely due to the fact that we showed videos, rather than asking participants to make decisions from still images, where you have to follow the action as it unfolds.

It’s unclear exactly why food scenes trigger such strong attention in agents, but it may be because paying attention to who has food is important for survival. Interestingly, our results showed very similar gaze patterns between adult humans and monkeys. As each scene unfolded, their gaze alternated between the agent and the patient.

This suggests that monkeys make sense of these events in the same way that humans do. What about infants? The infants showed very different gaze patterns. They seemed to look primarily at the background of each scene, suggesting that they were unable to identify information in the same way that adults do.

This may be because at this age they cannot "compute" information at the same speed as adults, and they probably also need to acquire of visual experience to help them quickly identify agents and patients.

Our results therefore suggest that when presented with scenes in which people can easily identify causes and effects, monkeys appear to be able to identify agents and patients, just as humans can. This supports the idea that our propensity for "deconstructing" information about events is not something unique to language, but an ability we share with our closest living relatives.

Perhaps it provided a structure on which we later built language. The question then is why other primates do not communicate about events in the same way that we do. This is a question we do not yet have an answer to.

However, it seems very possible that the social world in which humans and other apes evolved did indeed contribute to feed this provision to identify agents and patients, keeping track of all those love-hate relationships.

So the next time you see your neighbors saying hello, remember that monkeys seem to see the world in much the same way we do.

This article is republished from The Conversation under a Creative Commons license. Read the original article.