The protocol was approved by the Faculty Research Ethics Committee at Sheffield Hallam University for studies on human subjects.
1. Inter-observation Stage
2. Structured Interview Stage
3. Psychophysical Experiments Stage
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 expression in terms of contentment. Therefore, the term ''contentment'' was adopted as the dependent variable for the subsequent experimental stage. Five psychophysical experiments were conducted to test the hypotheses generated in the previous qualitative stages. Experiment 1 supported Hypothesis 1: When La Bella Principessa and the Mona Lisa were observed from a distance their Perceived Contentment was significantly higher than when the portraits were viewed from close-up, whereas there was no significant effect for Pollaiuolo's Portrait of a Girl (see Figure 1). Specifically, in the Far condition the average Perceived Contentment for La Bella Principessa and the Mona Lisa was about 2 and 1.5 units higher (in the range 1 – 7) rather than in the Close condition, respectively;, whilst there was no significant difference for Pollaiuolo's Portrait of a Girl. This finding supported Hypothesis 1. Second, the level of blur affected the Perceived Contentment of La Bella Principessa and the Mona Lisa but not the Perceived Contentment of Pollaiuolo's Portrait of a Girl. As can be seen in Figure 2, increasing the level of blur increased Perceived Contentment in a linear fashion, whilst this experimental manipulation did not affect the Perceived Contentment of Pollaiuolo's Portrait of a Girl. This finding supported Hypothesis 2. Third, the Perceived Contentment of La Bella Principessa increased with the Level of Blur in both the No Mask and Eye Mask conditions whereas there was no significant linear trend between Level of Blur and Perceived Contentment for either the Mouth and both Mouth and Eye mask conditions (see Figure 3). This supported Hypothesis 3 that the uncatchable smile illusion is mainly attributable to the mouth area of the portrait. Fourth, the Perceived Mouth Slant of La Bella Principessa changed with viewing distance; her mouth was perceived to take an upward slant when viewed from a distance, whereas it was perceived to take a downward direction when observed from close-up (see Figure 4). Specifically, in the Close condition the mouth was perceived to take a downwards direction (an average of about 7.5 degrees), whilst in the Far condition, the mouth was perceived to take an upwards direction (an average of about 1 degree). This finding supported Hypothesis 4. Finally, the Perceived Mouth Slant of La Bella Principessa changed with Level of Blur; when viewed with an increasing level of blur her mouth was perceived to take a more upward slant (see Figure 5). Specifically, the mouth slant was perceived to take a downwards slant (an average of about 8 degrees) when no blur was applied; and it took a linear upwards direction when the level of blur was gradually increased. This supported Hypothesis 5.
Figures 1 a 5 show examples of results from Soranzo & Newberry3 to illustrate how the findings from the experiments could appear.
Figure 1. The Effect of Distance on the Perceived Contentment of La Bella Principessa, Mona Lisa and Portrait of a Girl. This figure shows how distance could influence the perceived contentment of La Bella Principessa, Mona Lisa and Portrait of a Girl. Bars indicate standard errors. This figure has been modified from Soranzo and Newberry3.
Figure 2. The Effect of Blur on the Perceived Contentment of La Bella Principessa, Mona Lisa and Portrait of a Girl. This figure shows how blur could influence the perceived contentment of La Bella Principessa, Mona Lisa and Portrait of a Girl. Dotted lines favor the view of the trend. Bars indicate standard errors. This figure has been modified from Soranzo and Newberry3.
Figure 3. The Effect of Blur and Masking on the Perceived Contentment of La Bella Principessa. This figure shows how blur could affect the perceived contentment of La Bella Principessa when different parts are masked. Bars indicate standard errors. This figure has been modified from Soranzo and Newberry3.
Figure 4. The Effect of Distance on the Perceived Mouth Slant in La Bella Principessa's Portrait. This figure shows how distance could influence the perceived mouth slant in La Bella Principessa's portrait. Bars indicate standard errors. This figure has been modified from Soranzo and Newberry3.
Figure 5. The Effect of Blur on the Perceived Mouth Slant in La Bella Principessa's Portrait. This figure shows how blur could influence the perceived mouth slant in La Bella Principessa's portrait. Dotted lines favor the view of the trend. Bars indicate standard errors. This figure has been modified from Soranzo and Newberry3.
Supplemental file 1. Code for experiment 2. Please click here to download this file.
Supplemental file 2. Code for experiment 3. Please click here to download this file.
Supplemental file 3. Code for experiment 4. Please click here to download this file.
Supplemental file 4. Code for experiment 5. Please click here to download this file.
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 |
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.
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.
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.