Humans typically feel uneasy when they see a very realistic human-looking robot or computer avatar, a phenomenon called the "uncanny valley" response. According to a new study performed with monkeys, that reaction might have an evolutionary basis. Researchers hypothesize that the response stems from almost realistic images that signal HUMAN! to us, but then fail to live up to the initial excitement. The uncanny valley response has been documented in humans since the 1970s, and has been blamed for the unpopularity of some CGI films with realistic characters

[like The Polar Express and Final Fantasy]

, and it is touted as the reason Pixar stuck to characters with cartoonish features [New Scientist].

The response takes its name from a graph (pictured at left) of human emotional response as a function of a depiction's human-likeness. As human-likeness increases, a positive emotional response also increases, until likeness reaches somewhere around 80 percent, then the emotional response shoots down to revulsion on par with viewing a human corpse. Uncanny valley response has never been observed in another species. So to investigate the response's evolutionary basis, researchers checked monkeys for the reaction.

To test their preference, researchers showed macaque monkeys real pictures, digital caricatures and realistic reconstructions of other monkey faces. To the latter, the macaques repeatedly averted their eyes [Wired.com], suggesting that monkeys also fall into the uncanny valley.

However the researchers couldn't determine for certain whether the monkeys were repulsed by the almost real faces, or were simply more attracted or interested in the others. The best way to do that would be to repeat these experiments while looking for possible signs of unease--

sweaty skin, dilated pupils or clenched facial muscles, as examples [Not Exactly Rocket Science].

The study was published recently in the Proceedings of the National Academy of Sciences. Related content: 80beats: Maternal Monkey Love: Macaque Moms Coo Over Their Babies 80beats: Female Monkeys Chat More Than Males to Maintain Social Ties 80beats: Study Finds Chimps Do Die From HIV-Like Virus, Bucking Long-Held AssumptionImages: PNAS / Asif Ghazanfar and Shawn Steckenfinger