Investigating the 'Uncatchable Smile' in Leonardo da Vinci's La Bella Principessa: A Comparison with the Mona Lisa and Pollaiuolo's Portrait of a Girl

Summary

This paper discusses the methodology used to reveal the 'Uncatchable Smile' illusion in Leonardo da Vinci's La Bella Principessa portrait. A combination of three methods was used (inter-observation, structured interviews, and psychophysical experiments), which led to an investigation that shaped itself without prior beliefs, thus reducing potential researcher bias.

Abstract

This paper discusses how the 'Uncatchable Smile' illusion in Leonardo da Vinci's La Bella Principessa portrait was discovered. Kemp and Cotte¹ described the expression of the Princess as ambiguous and "subtle to an inexpressible degree". A combination of three methods was used (inter-observation, structured interviews, and psychophysical experiments) to identify what may underlie this 'ambiguity'. The inter-observation and the structured interview methods were firstly applied to generate experimental hypotheses that were successively tested by a series of psychophysical experiments. The combination of these research methods minimizes the impact of the researcher's beliefs and biases in the development of the research design. It emerged that the ambiguity in La Bella Principessa is triggered by a change in the perceived level of contentment in her facial expression and that this perceptual change is attributable to a visual illusion relating to her mouth. Moreover, it was found that a similar effect can be observed in the Mona Lisa. As the smile in La Bella Principessa disappears as soon as the viewer tries to 'catch it', we named this visual illusion the 'Uncatchable Smile'. The elusive quality of the Mona Lisa's smile² is probably why the portrait is so famous, and so the existence of a similar ambiguity in a portrait painted by Leonardo prior to the Mona Lisa is even more interesting.

Introduction

This paper discusses the methodology used by Soranzo and Newberry³ to investigate the 'Uncatchable Smile' illusion in Leonardo da Vinci's La Bella Principessa portrait. The overall goal of the methodology employed was to identify what makes the expression of the Princess ambiguous. The portrait shows the profile of a young woman identified as Bianca, the illegitimate daughter of Ludovico, who at around 13 years of age was to be married to a commander of the Duke's Milanese forces⁴. According to Kemp⁵, the portrait was commissioned by the Duke in honor of the celebration of her marriage in 1496. Kemp and Cotte⁶

Protocol

The protocol was approved by the Faculty Research Ethics Committee at Sheffield Hallam University for studies on human subjects.

1. Inter-observation Stage

1. Identify how and why La Bella Principessa's expression appears ambiguous.
   1. Display a good quality, frameless and exact sized foam backed digital copy of La Bella Principessa (33.2 cm in height x 23.8 cm in width) on an easel in diffuse lighting.
   2. Have four participants discuss the ambiguous expression in La Bella Principessa's portrait. Record the results of the discussion on a piece of paper.
      Note: One participant must be the researcher a....

Results

From the negotiation process of the Inter-observation stage it was ascertained that the ambiguity in the Princess's expression might be due to a visual illusion of direction which depends on the viewing distance. Since spatial frequency changes with distance it was concluded that this change relates to spatial frequency. A Grounded Theory analysis of the descriptions obtained in the Structured Interview stage was performed and it was found that observers tended to describe her change of e.......

Discussion

The findings of the study conducted by Soranzo & Newberry³ indicate that the ambiguity of the expression in Leonardo da Vinci's La Bella Principessa portrait is attributable to a visual illusion. A new mixed-methodology which combined three methods: Inter-observation, structured interviews, and psychophysical experiments was adopted. Firstly, the inter-observation method was used to shed some light on the 'ambiguity' of the Princess's facial expression. This method which involves l.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
High quality same size reproductions of the following three portraits: The Mona Lisa and La Bella Principessa (Leonardo da Vinci) and Portrait of a Girl (Piero del Pollaiuolo)	The Print Shop - Sheffield Hallam University - Sheffield Hallam University, Howard Street, Sheffield, South Yorkshire S1 1WB, United Kingdom
Easel for resting portraits on	No particular brand needed
Laptop computer with wireless mouse for experiments	Lenovo
Programming software to write syntax to present the pictures in random order, record participant responses from the keyboard, and enable the measurement 'handle' to be adjusted by participants	True Basic software
Image editing software	Adobe Photoshop CC 2015