Primate eye tracking reveals new insights into the evolution of language, writes Dr Vanessa Wilson, Lecturer in Psychology, University of Hull.
The human environment is a very social one. Family, friends, colleagues, strangers – they all provide a continuous stream of information that we need to track and make sense of. Who is dating whom? Who is in a fight with whom? While our capacity for dealing with such a large social network is impressive, it’s not something especially unique to humans. Other primates do it too.
We – humans and other primates such as monkeys and apes – have something called social knowledge that allows us to keep track of the social dynamics of our friends, neighbours and even enemies.
What is perhaps different about humans, though, is the way in which we communicate about these dynamics. If I see my neighbours saying hello, I can easily express this in a sentence: “David is greeting Iris.” As far as research has shown, other primates can’t do this.
They can communicate about individual entities, such as alarm calling when there’s danger, or producing food calls when they find a food they like. But they don’t seem to express how an action is linked to the individuals involved.
And this is 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. The majority of languages prioritise agents through grammar, suggesting that this is something that is universal among humans.
Cross-linguistic studies have revealed similar biases when people view images of events. In tasks where people have to describe an image depicting an action, they are rapidly able to identify the agent, and spend more looking at the agent than the patient.
This points to the possibility that our ability to “deconstruct” events such as these, and our apparent bias for agents, might have its roots in an era before language evolved.
Eye tracking
To test this, alongside colleagues from Switzerland, I conducted an eye-tracking study with human adults, six-month-old infants, chimpanzees, gorillas and orangutans in a zoo.
We showed participants videos of social interactions, such as one orangutan embracing another, and non-social interactions, such a person pushing a shelf, using a technique called infra-red eye tracking. This technique allows one to remotely determine the position of the eyes when looking at a screen. This meant that we could work with apes who watched the videos voluntarily, through a designated window.