The "Mona Lisa Effect" -- the impression that the eyes of a person in a portrait are following the viewer -- ironically is not seen in the Leonardo da Vinci's world-famous painting, scientists say.
"People are very good at gauging whether or not they are being looked at by others. Perceptual psychology demonstrated this in the 1960s," said Gernot Horstmann, of Bielefeld University.
"People can feel like they're being looked at from both photographs and paintings -- if the person portrayed looks straight ahead out of the image, that is, at a gaze angle of 0 degrees," said Horstmann, one of the authors of the study published in the scientific journal i-Perception.
"With a slightly sideward glance, you may still feel as if you were being looked at. This was perceived as if the portrayed person were looking at your ear, and corresponds to about 5 degrees from a normal viewing distance. But as the angle increases, you would not have the impression of being looked at," said Horstmann.
"Curiously enough, we don't have to stand right in front of the image in order to have the impression of being looked at -- even if the person portrayed in the image looks straight ahead," said Sebastian Loth, from Bielefeld University.
"This impression emerges if we stand to the left or right and at different distances from the image. The robust sensation of 'being looked at' is precisely the Mona Lisa effect," said Loth.
In his research on communication with robots and avatars, Loth repeatedly encountered the term "Mona Lisa Effect," coined after the famous 16th-century oil painting.
"The effect itself is undeniable and demonstrable. But with the Mona Lisa, of all paintings, we didn't get this impression," said Loth.
In order to test this observation, researchers had 24 study participants look at the Mona Lisa on a computer screen and assess the direction of her gaze. The participants sat in front of the monitor. A simple folding ruler was positioned between them and the screen at several distances. The participants indicated where Mona Lisa's gaze met the ruler.
In order to test whether individual features of Mona Lisa's face influenced the viewers' perception of her gaze, the researchers used 15 different sections from the portrait -- starting from her entire head to only her eyes and nose.
Each image was shown three times in random order. Halfway through the session, the researchers also changed the distance of the ruler from the monitor. Researchers gathered more than 2,000 assessments this way -- and almost every single measurement indicated that the Mona gaze is not straight on but to the viewer's right-hand side.
"The participants in our study had the impression that Mona Lisa's gaze was aimed to their right-hand side. More specifically, the gaze angle was 15.4 degrees on average," said Horstmann.
"Thus, it is clear that the term "Mona Lisa Effect" is nothing but a misnomer. It illustrates the strong desire to be looked at and to be someone else's centre of attention -- to be relevant to someone, even if you don't know the person at all," he said.
Gaze direction plays an important role in designing virtual characters or avatars for assistive systems or computer games.
"When communicating with an avatar, for example in a virtual environment, gaze improves our understanding of the avatar," said Loth.
"Using their eye gaze, the virtual agent can express its attention, and it can point at objects that are or will become relevant to the task -- just like a human," he said.