SUNDAY, Dec. 26 (HealthDay News) -- How do people recognize an image of a face as belonging to a living human being and not a replica such as a doll or mannequin? A new study suggests that the eyes have it: they are crucial to helping people distinguish real faces from those that are not.

"There's something fundamentally important about seeing a face and knowing that the lights are on and someone is home," study co-author Thalia Wheatley of Dartmouth College explained in a news release from the Association for Psychological Science.

In the study, published online and in the December issue of the journal Psychological Science, the researchers took photos of doll faces, then used software to morph the doll faces with faces of real people who looked similar. The end product was a continuum of faces from real to fake.

Volunteers looked at the photos and tried to figure out which ones were images of live humans. The tipping point -- the point where the volunteers realized that they were looking at an image of a person -- came about two-thirds along the continuum toward the human side.

Based on additional research, the study authors found that it appears that people search faces for signs of life and the most important facial feature in determining whether the subject is a living human being is the eyes.

"I think we all seek connections with others," Wheatley said in the news release. She added that when a person recognizes that a face is real, they think, "This is a mind I can connect with."

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Here's an example of the morphed faces used in the study.