By Victoria Gill
Science reporter, BBC News
A visual illusion has provided clues about how monkeys recognise faces.
In a study, rhesus monkeys responded to the "Thatcher effect", a strange phenomenon that makes it difficult to detect changes in an upside down face.
The study, in the journal Current Biology, is the first to show this effect in non-human animals.
The authors say this suggests that the ability to identify a familiar face may have evolved in an ancestor common to humans and rhesus monkeys.
The Thatcher effect is named after Margaret Thatcher, because images of the former UK prime minister's face were used its first demonstration, in experiments with humans.
A "thatcherised" image of a face has its mouth and eyes inverted relative to the rest of the face.
A familiar face
To test the monkeys' response to the effect, the team assessed their responses to photographs, observing how much attention the animals paid to the images.
"We showed monkeys pictures of other monkeys, either upright or upside down until they were bored (and stopped paying attention)," explained one of the authors, Robert Hampton from Emory University in Atlanta, US.
"Then we showed them 'thatcherised' faces, again either right side up or upside down."
When the faces were right side up, the monkeys regained interest, indicating that they noticed the change.
"When the faces were upside down, they did not," said Professor Hampton. "They treated it as just another boring presentation of the same picture."
"What is surprising is that, like humans, the monkeys detect these changes strongly in upright pictures of faces, but not in faces that are upside down," explained Professor Hampton.
"This shows that monkeys, like humans, are especially sensitive to the relationship [between] facial features in upright faces. This sensitivity is likely necessary for discriminating between different faces."
Because faces share many common features, with subtle differences in their layout, Professor Hampton explained that "detecting these differences in the layout is the key" to recognising a familiar face amongst many others.
Humans often describe upright "thatcherised" faces as gruesome, and the team are planning future studies taking physiological measures such as pupil size, to begin to address whether monkeys also find the images alarming.