Monkeys recognize each other by comparing faces to a statistical average stored in their brains, not by memorizing what each individual looks like, scientists said on Wednesday. This probably applies to people, too, explaining how faces can be recognized in a fraction of a second, they said.
In their study, the scientists found that a monkey's brain does not keep track of different parts of a face, storing and then accessing the information to recognize others. Instead, it keeps a statistical average of the faces it has seen and uses it as a basis for comparison.
"When it sees a new face it compares it to this average and then it remarks upon the differences ... and that is how the face is seen," said David Leopold, of the National Institute of Mental Health in Bethesda, Maryland.
"It elucidates how it is possible that you can so quickly and effortlessly, in just a few hundred milliseconds, recognize faces," he added.
Leopold and his colleagues pinpointed the recognition system while studying neurons in an area of the brain called the inferotemporal cortex in two macaque monkeys that had been trained to recognize computer-generated human faces.
They monitored single neurons to understand how groups of the brain cells work together to recognize faces.
"What we found is that the neurons in this part of the monkey's brain respond in a way that is extremely sensitive to the small differences in information between faces of different identities," said Leopold, who reported the research in the science journal Nature.
The activity of the neurons was monitored as the monkeys were shown an average face of a person and as it was artificially morphed the full identity.
"The main finding was a striking tendency for neurons to show tuning that appeared centered about the average face," Leopold writes in the journal.
In psychological tests, humans identify faces in much the same way as monkeys so the researchers believe this aspect of the visual recognition system is similar in both species.