Humans' ability to easily distinguish among many faces and recognize people they know goes way, way back, say researchers reporting online on June 25th in Current Biology, a Cell Press publication. That assertion stems from new evidence that, like us, rhesus monkeys tell their friends from foes by picking up on the precise layout of facial features.
"We found that monkeys looking at faces perceive an illusion, the Thatcher effect, that humans experience," said Robert Hampton of Emory University's Department of Psychology and Yerkes National Primate Research Center. "This powerful perceptual effect shows that face perception depends on detection of the relations among features in a face, not just on detection of the collection of individual features."
The discovery means that primates have probably perceived faces in essentially the same way for 30 million years or more, he said.
The Thatcher effect refers to the impaired ability to recognize changes in the relations among features in upside-down versus right-side-up faces. (The phenomenon is so named because it was first shown with the image of Margaret Thatcher.) In demonstrating the effect, scientists manipulate the image of a face so that the eyes and mouth are upside down relative to the rest of the face.
"Surprisingly," Hampton said, "when the image of a face so modified is presented upside down, most people do not think it looks particularly odd. But when viewed right-side up, it looks awful." It shows that when we look at faces normally, we are especially sensitive to the relations among features. When faces are upside down, however, we process the image more as a collection of features, with less emphasis on their relations to one another.
In the current study, Hampton and his colleagues showed monkeys pictures of monkey faces until they became "bored," as indicated by a loss of interest in the images. They then showed them digitally manipulated faces, with the eyes and mouth upside down relative to the rest of the face. When the manipulated faces were shown upright, the monkeys took notice and began studying the pictures again. In contrast, upside-down faces held no new interest and the monkeys continued to ignore them as if nothing were amiss.
The findings offer the first demonstration that a non-human primate species shows the Thatcher effect. "This direct evidence of configural face perception in monkeys, collected under testing conditions that closely parallel those used with humans, indicates that perceptual mechanisms for individual recognition have been conserved through primate cognitive evolution," the researchers said.
The researchers include Ikuma Adachi, Dina P. Chou, and Robert R. Hampton, of Emory University, Atlanta, GA.