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Alessandro Soranzo: How we captured Leonardo Da Vinci's uncatchable smile

Media centre home > Feature and comment > Alessandro Soranzo: How we captured Leonardo Da Vinci's uncatchable smile

Published: 04/09/15

In my research I try to understand how and why visual illusions occur - to help us understand how our visual system works.

We seem to enjoy viewing illusions.  It is amusing viewing something that appears different or strange and it is the perceptual change that makes Illusions captivating. 

It is curious for us all  to see that an object can appear longer or tilted or squeezed or of a different colour when moved to a different area of the visual scene or when it is viewed in peripheral vision. This perceptual change is fascinating to us and artists are well aware of this.

Of course, artists’ interests in visual illusions are different from scientists’ interests but the boundary between the two is feeble and Leonardo Da Vinci worked in both spheres with great success - we all know of the Mona Lisa and the mystery of that enigmatic smile.

I was therefore very interested in Leonardo’s work and puzzled when I read that a portrait of uncertain origin had recently came to light which, after extensive research and examination, was shown to be that very rarest of things, a newly discovered Leonardo da Vinci painting entitled La Bella Principessa.

Among the reports on this newly discovered Leonardo, it was noticed by art experts that the expression of the princess is ambiguous and has been described as ‘‘subtle to an inexpressible degree’’.

By exploring the portrait, this ambiguity adds to the portrait’s allure. When we inspect the portrait, our experience and feelings are difficult to define. Furthermore, what is in the portrait that triggers these ambiguous feelings?

I started wondering if it was possible to define and maybe to measure, using the scientific method, this “ambiguity”.

The questions were: Is it possible to quantify this ambiguity? Might it be due to a perceptual change, attributable to a visual illusion? Given that a similar ambiguity in our feeling is present in the Mona Lisa, what are the similarities in the two portraits that trigger the “ambiguity”? Did Leonardo know something about how to represent a face in order to generate this ambiguity?

My own experience was in line with the one suggested by art experts: the expression of the princess is ambiguous because it is unstable.

Sometimes the princess looks melancholic, unhappy and hostile, but at other times she looks more happy and cheerful. Hence, it seems that there is a perceptual change - a visual illusion!

The first hypothesis I came up with was therefore that the ambiguity, or the allure of the portrait, is triggered by a perceptual change relating to the way we look at the portrait.

If there is a perceptual change, which has to do with the way we look at the portrait, then there must be a controllable way to elicit in the viewer one impression (for example, the melancholic one) rather than the other (the cheerful one).

Watch a video of Alessandro's discovery by clicking the above image

What can we do to control the viewer's impression of La Bella Principessa in order to generate one impression rather than the other?

To investigate this, my colleague Michelle Newberry and I considered the work that has been done on the Mona Lisa's enigmatic smile. It is known that the ambiguity of the smile is generated by how the portrait is viewed. The Mona Lisa appears to smile more in peripheral vision rather than when she is viewed directly.

However, it is difficult to test this in controlled experimental situations. It is difficult for an observer to look at the portrait in peripheral vision only, without her/him being tempted to look at the portrait directly. We reason that in peripheral vision objects look less detailed and more blurry. The same happens when we look at objects from far away. So, manipulating the viewing distance may generate the same effect but it is much more practical to create an experiment where distance is manipulated rather than peripheral or direct vision.

From this starting point, and assuming that there are similarities between the Mona Lisa and La Bella Principessa, we started to develop a controllable experimental situation. We had to face the problem that we didn’t want the supposed two impressions to interfere with each other. 

If an observer sees the portrait in one way and perceives the Princess to be, for example, melancholic, this experience might interfere with the successive way of viewing the portrait.

Sheffield Hallam's excellent print shop did a fantastic job in providing us with a good quality exact sized copy of La Bella Principessa, printed from a digital version.

This print was placed at the end of a long corridor so that it could be accessed from two doors (one close to the portrait and the other far away). We asked some observers to enter from the closest door and others to enter from the distant one. Both groups of observers were asked to rate on a scale how much they saw the Princess 'smiling'. They couldn’t move from the location indicated with a line on a floor to the other, so their impression couldn’t be affected by previous ones. For the same reason, the observers chosen for this first experiment weren’t familiar with the portrait.

It was thrilling to see that practically all observers in the far condition rated the princess as 'smiling' more than those in the close condition: the presence of an illusion distant dependent was empirically shown.

Since this first pilot experiment many others have been conducted to find out what it is about the portrait that generates this fascinating illusion. Many Sheffield Hallam students wanted to take part in the project as both observers and experimenters.

However, the project is ongoing, and we have been invited to the Louvre in Paris to apply the experimental method we developed to other art pieces - there are many more research questions to answer in this fascinating field of research. Stay tuned!

The author:

Alessandro Soranzo

Alessandro's work on La Bella Principessa has been written about extensively in the national and international press.

"We have been invited to the Louvre in Paris to apply the experimental method we developed to other art pieces - there are many more research questions to answer."