investigating-uncatchable-smile-leonardo-da-vinci-s-la-bella

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

This paper discusses how the &#39;Uncatchable Smile&#39; illusion in Leonardo da Vinci&#39;s La Bella Principessa portrait was discovered. Kemp and Cotte1 ...

Sheffield Hallam University

The intersection between an illumination and a reflectance edge is characterised by the 'ratio-invariant' property, that is the luminance ratio of the regions under different illumination remains the same. In a CRT experiment, we shaped two areas, one surrounding the other, and simulated an illumination edge dividing them in two frames of illumination. The portion of the illumination edge standing on the surrounding area (labelled contextual background) was the contextual edge, while the portion standing on the enclosed area (labelled mediating background) was the mediating edge. On the mediating background, there were two patches, one per illumination frame. Observers were asked to adjust the luminance of the patch in bright illumination to equate the lightness of the other. We compared conditions in which the luminance ratio at the contextual edge could be (i) equal (possible shadow), or (ii) larger (impossible shadow) than that at the mediating edge. In addition, we manipulated the reflectance of the backgrounds. It could be higher for the contextual than for the mediating background; or, vice versa, lower for the contextual than for the mediating background. Results reveal that lightness constancy significantly increases when: (i) the luminance ratio at the contextual edge is larger than that at the mediating edge creating an impossible shadow, and (ii) the reflectance of the contextual background is lower than that of the mediating one. We interpret our results according to the albedo hypothesis, and suggest that the scission process is facilitated when the luminance ratio at the contextual edge is larger than that at the mediating edge and/or the reflectance of the including area is lower than that of the included one. This occurs even if the ratio-invariant property is violated.

Impossible shadows and lightness constancy.

The relation between perceptual belongingness and lightness perception has historically been studied in the contrast domain (Benary, 1924 Psychologische Forschung 5 131 - 142). However, scientists have shown that two equal grey patches may differ in lightness when belonging to different reflecting surfaces. We extend this investigation to the constancy domain. In a CRT simulation of a bipartite field of illumination, we manipulated the arrangement of twelve patches: six squares and six diamonds. Patches of the same shape could be placed: (i) all within the same illumination field; or (ii) forming a row across the illumination fields. Furthermore, we manipulated proximity between the innermost patches and the illumination edge. The patches could be (i) touching (forming an X-junction); or (ii) not touching (not forming an X-junction). Observers were asked to perform a lightness match between two additional patches, one illuminated and the other in shadow. We found better lightness constancy when the patches of the same shape formed a row across the fields, with no effect of X-junctions. Since lightness constancy is improved by strengthening the belongingness across the illumination fields, we conclude that belongingness might help the visual system to aggregate the differently illuminated surfaces, and facilitate the scission process.

Does perceptual belongingness affect lightness constancy?

It has been shown that lightness constancy depends on the articulation of the visual field (Agostini & Galmonte, 1999). However, among researchers there is little agreement about the meaning of "articulation." Beyond the terminological heterogeneity, an important issue remains: What factors are relevant for the stability of surface color perception? Using stimuli with two fields of illumination, we explore this issue in three experiments. In Experiment 1, we manipulated the number of luminances, the number of reflectances, and the number of surfaces and their spatial relationships; in Experiment 2, we manipulated the luminance range; finally, in Experiment 3 we varied the number of surfaces crossed by the illumination edge. We found that there are two relevant factors in optimizing lightness constancy: (1) the lowest luminance in shadow and (2) the co-presence of patches of equal reflectance in both fields of illumination. The latter effect is larger if these patches strongly belong to each other. We interpret these findings within the albedo hypothesis.

Photometric, geometric, and perceptual factors in illumination-independent lightness constancy.

In this article, we present MLP, a MATLAB toolbox enabling auditory thresholds estimation via the adaptive maximum likelihood procedure proposed by David Green (1990, 1993). This adaptive procedure is particularly appealing for those psychologists who need to estimate thresholds with a good degree of accuracy and in a short time. Together with a description of the toolbox, the present text provides an introduction to the threshold estimation theory and a theoretical explanation of the maximum likelihood adaptive procedure. MLP comes with a graphical interface, and it is provided with several built-in, classic psychoacoustics experiments ready to use at a mouse click.

MLP: a MATLAB toolbox for rapid and reliable auditory threshold estimation.

The term simultaneous lightness constancy describes the capacity of the visual system to perceive equal reflecting surfaces as having the same lightness despite lying in different illumination fields. In some cases, however, a lightness constancy failure occurs; that is, equal reflecting surfaces appear different in lightness when differently illuminated. An open question is whether the luminance profile of the illumination edges affects simultaneous lightness constancy even when the ratio invariance property of the illumination edges is preserved. To explore this issue, we ran two experiments by using bipartite illumination displays. Both the luminance profile of an illumination edge and the luminance ratio amplitude between the illumination fields were manipulated. Results revealed that the simultaneous lightness constancy increases when the luminance profile of the illumination edge is gradual (rather than sharp) and homogeneous (rather than inhomogeneous), whereas it decreases when the luminance ratio between the illumination fields is enlarged. The results are interpreted according to the layer decomposition schema, stating that the visual system splits the luminance into perceived lightness and apparent illumination components. We suggest that illumination edges having gradual and homogeneous luminance profiles facilitate the luminance decomposition process, whereas wide luminance ratios impede it.

Lightness constancy: ratio invariance and luminance profile.

Luminance ratios along shadow edges remain the same even when they cross reflectance borders. According to Gilchrist (1988, Perception & Psychophysics 43 415-424) this so-called ratio-invariance property is a crucial factor in the perception of shadows. However, Soranzo and Agostini (2004, Perception 33 1359-1368) suggested that in some conditions (named 'impossible shadows'), a luminance pattern might still be perceived as a shadow even if the ratio-invariance property along its edge is violated. This can occur when an edge is collinear with another edge (contextual edge) which incorporates it, shares the same polarity, and generates a larger ratio. The hypothesis that impossible shadows are actually perceived as shadows is here tested by comparing the perceptual contrast of a luminance edge in the absence of a contextual edge (control condition) to that of both possible shadow edges (where the contextual and mediating edge share the same ratio) and impossible shadow edges (where the ratio of the contextual edge is larger rather than that at the mediating edge). We found that the perceived contrast of luminance edges shrinks in both possible and impossible shadow conditions rather than in the control condition. This evidence supports the hypothesis that a luminance pattern might be perceived as a shadow even when the ratio-invariance property is violated.

The perceptual contrast of impossible shadow edges.

Lightness contrast and lightness assimilation are opposite phenomena: in contrast, grey targets appear darker when bordering bright surfaces (inducers) rather than dark ones; in assimilation, the opposite occurs. The question is: which visual process favours the occurrence of one phenomenon over the other? Researchers provided three answers to this question. The first asserts that both phenomena are caused by peripheral processes; the second attributes their occurrence to central processes; and the third claims that contrast involves central processes, whilst assimilation involves peripheral ones. To test these hypotheses, an experiment on an IT system equipped with goggles for stereo vision was run. Observers were asked to evaluate the lightness of a grey target, and two variables were systematically manipulated: (i) the apparent distance of the inducers; and (ii) brightness of the inducers. The retinal stimulation was kept constant throughout, so that the peripheral processes remained the same. The results show that the lightness of the target depends on both variables. As the retinal stimulation was kept constant, we conclude that central mechanisms are involved in both lightness contrast and lightness assimilation.

Von Bezold assimilation effect reverses in stereoscopic conditions.

Simultaneous Lightness Contrast (SLC) is the phenomenon whereby a grey patch on a dark background appears lighter than an equal patch on a light background. Interestingly, the lightness difference between these patches undergoes substantial augmentation when the two backgrounds are patterned, thereby forming the articulated-SLC display. There are two main interpretations of these phenomena: The mid-level interpretation maintains that the visual system groups the luminance within a set of contiguous frameworks, whilst the high-level one claims that the visual system splits the luminance into separate overlapping layers corresponding to separate physical contributions. This research aimed to test these two interpretations by systematically manipulating the viewing distance and the horizontal distance between the backgrounds of both the articulated and plain SLC displays. An immersive 3D Virtual Reality system was employed to reproduce identical alignment and distances, as well as isolating participants from interfering luminance. Results showed that reducing the viewing distance resulted in increased contrast in both the plain- and articulated-SLC displays and that, increasing the horizontal distance between the backgrounds resulted in decreased contrast in the articulated condition but increased contrast in the plain condition. These results suggest that a comprehensive lightness theory should combine the two interpretations.

The effects of belongingness on the Simultaneous Lightness Contrast: a virtual reality study.

PSYCHOACOUSTICS is a new MATLAB toolbox which implements three classic adaptive procedures for auditory threshold estimation. The first includes those of the Staircase family (method of limits, simple up-down and transformed up-down); the second is the Parameter Estimation by Sequential Testing (PEST); and the third is the Maximum Likelihood Procedure (MLP). The toolbox comes with more than twenty built-in experiments each provided with the recommended (default) parameters. However, if desired, these parameters can be modified through an intuitive and user friendly graphical interface and stored for future use (no programming skills are required). Finally, PSYCHOACOUSTICS is very flexible as it comes with several signal generators and can be easily extended for any experiment.

PSYCHOACOUSTICS: a comprehensive MATLAB toolbox for auditory testing.

A portrait of uncertain origin recently came to light which, after extensive research and examination, was shown to be that rarest of things: a newly discovered Leonardo da Vinci painting entitled La Bella Principessa. This research presents a new illusion which is similar to that identified in the Mona Lisa; La Bella Principessa's mouth appears to change slant depending on both the Viewing Distance and the Level of Blur applied to a digital version of the portrait. Through a series of psychophysics experiments, it was found that a perceived change in the slant of the La Bella Principessa's mouth influences her expression of contentment thus generating an illusion that we have coined the "uncatchable smile". The elusive quality of the Mona Lisa's smile has been previously reported (Science, 290 (2000) 1299) and so the existence of a similar illusion in a portrait painted prior to the Mona Lisa becomes more interesting. The question remains whether Leonardo da Vinci intended this illusion. In any case, it can be argued that the ambiguity created adds to the portrait's allure.

The uncatchable smile in Leonardo da Vinci's La Bella Principessa portrait.

Recent proliferation of available virtual reality (VR) tools has seen increased use in psychological research. This is due to a number of advantages afforded over traditional experimental apparatus such as tighter control of the environment and the possibility of creating more ecologically valid stimulus presentation and response protocols. At the same time, higher levels of immersion and visual fidelity afforded by VR do not necessarily evoke presence or elicit a "realistic" psychological response. The current paper reviews some current uses for VR environments in psychological research and discusses some ongoing questions for researchers. Finally, we focus on the area of visual perception, where both the advantages and challenges of VR are particularly salient.

The Use of Virtual Reality in Psychology: A Case Study in Visual Perception.

It is well known that visible luminance gradients may generate contrast effects. In this work we present a new paradoxical illusion in which the luminance range of gradual transitions has been reduced to make them invisible. By adopting the phenomenological method proposed by Kanizsa, we have found that unnoticeable luminance gradients still generate contrast effects. But, most interestingly, we have found that when their width is narrowed, rather than generating contrast effects on the surrounded surfaces, they generate an assimilation effect. Both high- and low-level interpretations of this "phantom" illusion are critically evaluated.

Article	Year
Impossible shadows and lightness constancy. Perception\| PubMed ID: 15693676	2004
Does perceptual belongingness affect lightness constancy? Perception\| PubMed ID: 16583764	2006
Photometric, geometric, and perceptual factors in illumination-independent lightness constancy. Perception & psychophysics\| PubMed ID: 16617834	2006
MLP: a MATLAB toolbox for rapid and reliable auditory threshold estimation. Behavior research methods\| PubMed ID: 19182120	2009
Lightness constancy: ratio invariance and luminance profile. Attention, perception & psychophysics\| PubMed ID: 19304637	2009
The perceptual contrast of impossible shadow edges. Perception\| PubMed ID: 19400427	2009
Von Bezold assimilation effect reverses in stereoscopic conditions. Perception\| PubMed ID: 20677697	2010
The effects of belongingness on the Simultaneous Lightness Contrast: a virtual reality study. Vision research\| PubMed ID: 23664881	2013
PSYCHOACOUSTICS: a comprehensive MATLAB toolbox for auditory testing. Frontiers in psychology\| PubMed ID: 25101013	2014
The uncatchable smile in Leonardo da Vinci's La Bella Principessa portrait. Vision research\| PubMed ID: 26049039	2015
The Use of Virtual Reality in Psychology: A Case Study in Visual Perception. Computational and mathematical methods in medicine\| PubMed ID: 26339281	2015
The phantom illusion. Vision research\| PubMed ID: 26505683	2015

Alessandro Soranzo

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