The eye plays a significant role in the expressiveness of a face, and how much sclera - the white part of the eye - is shown can indicate the emotions or behavioural attitudes of a person.
Wide-open eyes, exposing a lot of white, indicate fear or surprise. A thinner slit of exposed eye, such as when smiling, expresses happiness or joy.
The study found that the ability to respond to eye cues apparently develops during infancy - at seven or so months.
Grossmann and colleague Sarah Jessen from the Max Planck Institute used electroencephalography or EEG to measure the brain activity of 7-month-old infants while showing images of eyes wide open, narrowly opened, and with direct or averted gazes.
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They found that the infants' brains responded differently depending on the expression suggested by the eyes they viewed, which were shown absent of other facial features.
"Their brains clearly responded to social cues conveyed through the eyes, indicating that even without conscious awareness, human infants are able to detect subtle social cues," Grossmann said.
The infants' brain responses displayed a different pattern to sclera depicting fearful expressions (wide-eyed) to non-fearful sclera. They also showed brain responses that differed when viewing direct gaze eyes compared to averted gaze.
"This demonstrates that, like adults, infants are sensitive to eye expressions of fear and direction of focus, and that these responses operate without conscious awareness," Grossmann said.
The infants in the study wore an EEG cap, like a small hat, which included sensors that could detect brain signals.
The study was published in the journal PNAS.