Lightness constancy: ratio invariance and luminance profile

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

Individual differences in aesthetic preferences for multi-sensorial stimulation

The aim of the current project was to investigate aesthetics in multi-sensorial stimulation and to explore individual differences in the process. We measured the aesthetics of Interactive Objects (IOs) which are three-dimensional objects with electronic components that exhibit an autonomous behaviour when handled: e.g., vibrating, playing a sound, or lighting-up. The Q-sorting procedure of Q-methodology was applied. Data were analysed by following the Qmulti protocol. The results suggested that overall participants preferred IOs that (i) vibrate, (ii) have rough surface texture, and (iii) are round. No particular preference emerged about the size of the IOs. When making aesthetic judgment, participants paid more attention to the behaviour variable of the IOs than the size, contour or surface texture. In addition, three clusters of participants were identified, suggesting that individual differences existed in the aesthetics of IOs. Without proper consideration of potential individual differences, aesthetic scholars may face the risk of having significant effects masked by individual differences. Only by paying attention to this issue can more meaningful findings be generated to contribute to the field of aesthetics

Photometric, geometric and perceptual factors in illumination-independent 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

Reasoning about visibility in mirrors: A comparison between a human observer and a camera

Human observers make errors when predicting what is visible in a mirror. This is true for perception with real mirrors as well as for reasoning about mirrors shown in diagrams. We created an illustration of a room, a top-down map, with a mirror on a wall, and objects (nails) on the opposite wall. The task was to select which nails were visible in the mirror from a given position (viewpoint). To study the importance of the social nature of the viewpoint we divided the sample (N=108) in two groups. One group (N=54) were tested with a scene in which there was the image of a person. The other group (N=54) were tested with the same scene but with a camera replacing the person. Participants were instructed to think about what would be captured by a camera on a tripod. This manipulation tests the effect of social perspective taking in reasoning about mirrors. As predicted, performance on the task shows an overestimation of what can be seen in a mirror, and a bias to underestimate the role of the different viewpoints, i.e. a tendency to treat the mirror as if it captures information independently of viewpoint. In terms of the comparison between person and camera there were more errors for the camera, suggesting an advantage for evaluating a human viewpoint as opposed to an artificial viewpoint. We suggest that social mechanisms may be involved in perspective taking in reasoning rather than in automatic attention allocation

Changes in relative area produce equal and opposite effects on lightness and perceived illumination (Abstract only)

In prior work (Gilchrist & Soranzo, ECVP 2012) in which subjects looked into two windows in the far wall of a vision tunnel and adjusted the level of illumination in one window to match that of the othe, they matched the windows for highest luminance (not average). The fact that both lightness and perceived illumination are anchored by highest luminance directly implies Koffka’s invariance theorem. For a given luminance within a framework, changing highest luminance changes lightness and perceived illumination in equal but opposite ways. In this new study, we consider area effects. We found that when a darker region is less than half of the total area within a window, Koffka’s theorem applies directly. However, when the darker region covers more than half of total area, Koffka’s principle applies to the darker region, but not the lighter one, which is anchored at white. Perceived illumination level can now be incorporated systematically into anchoring theory

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 '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

Da Vinci's La Bella Principessa and the uncatchable smile

n 1998, a little known picture was sold for a modest sum in a New York saleroom and in so doing attracted the attention of the art world. Painstaking analysis by the Oxford art historian Martin Kemp and others, revealed it to be the work of Leonardo Da Vinci (Kemp & Cotte, 2010, Hodder & Stoughton). Given the considerable interest in La Bella Principessa, it is perhaps surprising now to suggest that it may contain an illusion. The Principessa mouth appears to change shape dependant on whether it is viewed in foveal or peripheral vision and this in turn changes the facial expression and ambience, generating an 'uncatchable smile' experience. Experimental data showed that the uncatchable smile experience can be obtained also by approaching the picture from distance and, when a digital version of the picture was used, by either blurring or pixelating the image. The effect is similar, and perhaps stronger, to that described by the Harvard neurobiologist Margaret Livingstone, in her account of the Mona Lisa's enigmatic smile (Livingstone, 2002, Abrahams).The question arises as to whether Leonardo deliberately used such an artefact as a subtle embellishment to the overall aesthetic - after all, who can resist an uncatchable smile

The 'uncatchable smile' illusion in Da Vinci's Bella Principessa depends on the viewing angle

The Mona Lisa is the most-visited, most written about and most parodied work of art in the world. However, the ‘uncatchable smile’ that makes Leonardo da Vinci’s Mona Lisa so special is not unique. In previous research (Pickard & Soranzo, 2012) we found that the technique which would later give his most famous subject her mysterious allure was first executed in the lesser-known painting by the Renaissance master: La Bella Principessa. Specifically, we found a gaze-dependent illusion: When viewed directly the slant of her mouth appears to turn downwards, but when viewed in peripheral vision the edges of her mouth take an upward turn. Unlike the Mona Lisa, the Bella Principessa's portrait was painted in profile and the present research discovered that the magnitude of the 'uncatchable smile' illusion depends on the viewing position: when viewed 40◦ from the left, the Principessa smiles to the viewer, but when it is viewed 40◦ from the right, she maintains a neutral expression. The question arises as to whether Leonardo created this subtle illusion to enhance the Principessa's portrait before reproducing it from a frontal position in the Mona Lisa

Virtual environments in visual perception : applications and challenges

A 3D virtual environment (a VR cave) is an extremely beneficial tool for the examination of visual perception. So far, very little work on Virtual Reality has been specifically dedicated to human perception. Nevertheless, this technology has a number of advantages over traditional computer- or paper-based experiments. Virtual reality provides precise control over the visual scene, to a degree that is extremely difficult to achieve by manipulating physical objects in a room. More specifically, it allows full control of the luminance and of the spatial arrangement of the surfaces in the environment. In addition, within the immersive system, depth perception can be elicited through a combination of binocular stereopsis, head motion parallax and maintaining constant luminance intensity. This chapter outlines how recent studies in visual perception have employed an immersive 3D environment and discusses directions for future research