WASHINGTON - Rhesus monkeys and humans recognise faces the same way, according to the latest research. The study provides insight into the evolution of the critical human social skill of facial recognition, which enables us to form relationships and interact appropriately with others.

Both monkeys and humans share a specific perceptual mechanism, ‘configural perception’, for discriminating among the numerous faces they encounter daily.

“Humans and other social primates need to recognise other individuals and to discriminate kin from non-kin, friend from foe and allies from antagonists,” said Robert R. Hampton of Emory University, who led the study.

“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.

“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 explained.

Hampton and his research team used the Thatcher Effect, a perceptual illusion named after Margaret Thatcher to determine if rhesus monkeys use configural perception to recognise other monkeys.

In the study, the researchers presented images of six different monkeys to four 4-year-old rhesus monkeys raised for two to three years in large social groups at the Yerkes Research Centre.

The researchers “thatcherised” the images of faces by positioning the eyes and mouths upside down relative to the rest of each face.

They 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 ‘thatcherised’ faces. In the upright position, the monkeys were surprised by the distorted features and began looking at the pictures again.

In contrast, 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 ‘thatcherised’. 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, said an Emory release.

These findings were published in the June 25 online issue of Current Biology.