There have long been anecdotal reports that the eyes of the Mona Lisa—Renaissance artist Leonardo da Vinci's most famous painting—sometimes seem to follow viewers as they move around the artwork. The phenomenon is even called the "Mona Lisa effect" because of it. But a new study published in the journal i-Perception found that she's really "looking" to the right-hand side of her audience.

"There is no doubt about the existence of the Mona Lisa effect," the authors wrote. "It just does not occur with the Mona Lisa herself."

The study grew out of ongoing research at Bielefeld University in Germany on human communication with robots and avatars. Directional gaze is key when designing gaming avatars or virtual agents, for instance. That's one way an avatar/agent can indicate attention, perhaps directing a player/user toward objects that are relevant to the task at hand.

Co-author Sebastian Loth has observed the Mona Lisa effect frequently in his research with robots and avatars and says the effect is "undeniable and demonstrable." It's also remarkably robust. "Curiously enough, we don't have to stand right in front of the image in order to have the impression of being looked at," he said. "This impression emerges if we stand to the left or right and at different distances from the image."

Since the 1960s, perceptual psychologists have known that we're very good at sensing when someone is looking at us, according to co-author Gernot Horstmann, whose expertise is in eye movement and attention. "It illustrates the strong desire to be looked at and to be someone else's center of attention—to be relevant to someone, even if you don't know the person at all," he said. That includes the eerie sensation of being watched by the subjects of paintings or photographs, which typically occurs when the subject is looking straight ahead out of the image, at an angle between 0 and 5 degrees.

While there have been several studies stating that this occurs with the Mona Lisa (aka La Gioconda), the authors claim those studies failed to cite convincing evidence of the phenomenon. So they decided to test it for themselves. Since directly measuring the objective direction of such a gaze isn't really feasible, Horstmann and Loch designed their experiment to measure the perceived line of the gaze.

This was a small study, with just 24 subjects. All were asked to look at a high-resolution recreation of the Mona Lisa on a computer monitor, with a folding ruler placed between them and the screen to track viewing distance. Subjects would signal where they perceived Mona Lisa's gaze met the ruler. The researchers sampled 15 sections of the famous portrait, ranging from the Mona Lisa's full head to just her eyes and nose, and they showed subjects each image three times in random order. They also changed the ruler's distance from the monitor halfway through the sessions.

Based on the more than 2,000 individual assessments, they found no evidence of the Mona Lisa effect with Leonardo's masterpiece. "We demonstrated that Mona Lisa gazes to her left-hand side [the viewer's right] from about 35.5 cm inside pictorial space, and 14.4 degrees to the viewer's right-hand side in real space," the authors wrote. "Thus, Mona Lisa does not fulfill the premise of the Mona Lisa effect. She does not gaze at the viewer."

This isn't the first time scientists have been intrigued by how the Mona Lisa impacts human perception. Back in 2015, scientists at Sheffield Hallam University in Great Britain published a paper in Vision Research announcing they had solved the mystery of Mona Lisa's strangely enigmatic "now you see it, now you don't" smile. It's an optical illusion they say Leonardo created deliberately using a sfumato ("soft" or "pale") technique for the coloring and shading around the mouth.

If viewers focus on the subject's eyes from a distance (or when digitally blurred), a slight smile can be seen. But that smile disappears at close distances or if the viewer focuses on the mouth instead. The Sheffield team found the same effect in an earlier painting by the Renaissance master, La Bella Principessa.

"Given Leonardo's mastery of the technique and its subsequent use in the Mona Lisa, it is quite conceivable that the ambiguity of the effect was intentional," psychologist and co-author Alessandro Soranzo told The Telegraph at the time—especially since one of the painter's maxims was that portraits should reflect "inner turmoil of the mind."

I guess we can add early perceptual scientist to Leonardo's extensive list of accomplishments.

DOI: i-Perception, 2019. 10.1177/2041669518821702 (About DOIs).