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Great apes track events with their eyes in the same way that humans do, according to a study published November 26 in the open-access journal PLOS Biology by Vanessa Wilson from the University of Neuchatel, Switzerland, and colleagues.

When watching a cat chase a mouse, humans will alternate looking at cat and mouse, using the information to connect the two into what's called an agent-patient relationship—with the cat as the agent and the mouse as the patient. This cognitive mechanism is thought to be one of the bases for the evolution of human language, forming both how people think about events and structure speech.

To find out if great apes can identify agent-patient relationships, the authors of this study showed 84 short video clips to 14 humans and examined their visual responses. They compared human responses to the visual responses of five chimpanzees, two gorillas, and two orangutans at the Basel Zoo. They also performed the test with 29 six-month-old infants.

The authors found that both apes and adult humans paid the most attention to the agents and the patients compared with background information. They often alternated attention between the two, focusing more on the agent when video clips involved food.

Humans tended to focus entirely on the agents and patients, while apes showed more attention to the background. But while apes tended to track events like human adults, six-month-old human babies did not, instead paying attention mostly to the background.

The findings suggest that the way brains order events evolved before language, and that the way people break down events into agents and patients is not unique to humans, but instead is part of a cognitive spectrum between humans and other great apes. Future studies will be needed to understand why great apes do not communicate like humans, and to better understand how humans developed language.

The authors add, "Gaze patterns from eye tracking data suggest that apes, like human adults, can decompose causal actions into agent and patient roles, something that is crucial for language. Our findings are consistent with a shared cognitive mechanism between humans and apes, suggesting that event role tracking evolved long before language."