The study, conducted by researchers at the Yerkes National Primate Research Center, Emory University, sheds light on the evolution of the critical human social skill of facial recognition, which enables us to form relationships and interact appropriately with others.
"Humans and other social primates need to recognize other individuals and to discriminate kin from non-kin, friend from foe and allies from antagonists," said lead researcher Robert R. Hampton of the Yerkes National Primate Research Center and Emory's Department of Psychology.
"Our research indicates the ability to perform this skill probably evolved some 30 million or more years ago in an ancestor humans share with rhesus monkeys," he added.
The remarkable capability humans have to distinguish among thousands of faces stems from our sensitivity to the unique configuration, or layout, of facial features.
"Because faces share so many features in common - eyes, nose, mouth, etc. - the simple detection of the collection of these features alone would not permit us to tell many faces apart," Dr. Hampton noted.
"It's our ability to perceive small changes in the relations among the features that enables us to distinguish thousands of faces and recognize those we know," he said.
Hampton and his colleagues used the Thatcher Effect, a perceptual illusion named for Margaret Thatcher because it was first demonstrated using an image of the former British prime minister, to determine if rhesus monkeys use configural perception to recognize other monkeys.
In the study, the researchers presented images of six different monkeys to four 4-year-old rhesus macaque monkeys raised for two to three years in large social groups at the Yerkes Research Center.
The researchers "thatcherized" the images of faces by positioning the eyes and mouths upside down relative to the rest of each face.
The researchers presented monkeys with normal images of each face upside down and right side up until the monkeys were bored and ceased looking at the pictures.
They then showed the monkeys the thatcherized faces. In the upright position, the monkeys were surprised by the distorted features and began looking at the pictures again.
On contrary, when the faces were upside down, they were not at all surprised and treated the faces as if nothing had been done to them.
This is similar to the human response to the Thatcher Effect, which shows that when the eyes and mouth are rotated and, thus, distorted, humans surprisingly process the upside-down version of the image more as a collection of features and with less emphasis on the relations among the features.
As a result, the face appears fairly normal despite being thatcherized. However, when viewed right side up, humans say the image looks awkward or grotesque, demonstrating they clearly see the eyes and mouth have been rotated.
"This study advances our understanding of social processes critical for a healthy and successful social life in primates. Early primates apparently solved the problem of recognizing each others' faces in this way well before humans arrived on the planet," Hampton concluded.
The study has been reported in the June 25 online issue of Current Biology.