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Visual illusions have been studied extensively, but their time course has not. Here we show, in a sample of more than 550 people, that unrestricted presentation times—as opposed to presentations lasting only a single second—weaken the Ebbinghaus illusion, strengthen lightness contrast with double increments, and do not alter lightness contrast with double decrements. When presentation time is unrestricted, these illusions are affected in the same way (decrease, increase, no change) by how long observers look at them. Our results imply that differences in illusion magnitude between individuals or groups are confounded with differences in inspection time, no matter whether stimuli are evaluated in matching, adjustment, or untimed comparison tasks. We offer an explanation for why these three illusions progress differently, and we spell out how our findings challenge theories of lightness, theories of global-local processing, and the interpretation of all research that has investigated visual illusions, or used them as tools, without considering inspection time.

Lightness of a surface depends not only on its physical characteristics, but also on the properties of the surrounding context. As a result, varying the context can significantly alter surface lightness, an effect exploited in many lightness illusions. Computational models can produce outcomes similar to human illusory percepts, allowing for demonstrable assessment of the applied mechanisms and principles. We tested 8 computational models on 13 typical displays used in lightness research (11 Illusions and 2 Mondrians), and compared them with results from human participants (N = 85). Results show that HighPass and MiR models predict empirical results for simultaneous lightness contrast (SLC) and its close variations. ODOG and its newer variants (ODOG-2 and L-ODOG) in addition to SLC displays were able to predict effect of White’s illusion. RETINEX was able to predict effects of both SLC displays and Dungeon illusion. Dynamic decorrelation model was able to predict obtained effects for all tested stimuli except two SLC variations. Finally, FL-ODOG model was best at simulating human data, as it was able to predict empirical results for all displays, bar the Reversed contrast illusion. Finally, most models underperform on the Mondrian displays that represent most natural stimuli for the human visual system.

Lightness contrast and assimilation are opposite phenomena: in contrast grey targets appear\ud darker when bordering bright rather than dark surfaces; in assimilation grey targets appear\ud lighter when bordering bright rather than dark surfaces. The underlying neurophysiological\ud mechanisms of these phenomena are not known. The aim of this study was to investigate the\ud relationship between contrast and assimilation, and the timing and levels of perceptual and\ud cognitive processing using combined behavioural and electrophysiological methods. Thirty\ud undergraduate students (23 female, age range 18-48 years old) participated in a forced-choice\ud (grey target is lighter/darker than a comparison square) task, using stimuli designed such that the\ud inducers were in two configurations (small and large) and two shades (white and black). The\ud behavioural data (more consistent and faster responses) corroborated previous findings of\ud stronger contrast effects with white inducers and stronger assimilation effects with black\ud inducers. According to the Event Related Potentials (ERP) results the mean amplitude was larger\ud in conditions with less consistent and slower behavioural responses. Thus, with contrast\ud responses P1 amplitude was larger with black than white inducers, and N1 amplitude was larger\ud to assimilation than contrast when the configuration of the stimulus was held constant. These\ud results suggest contrast may occur as early as P1 (~110 ms) and assimilation may occur later in\ud N2 (~220 ms), whereas in some conditions, differences in ERPs associated with contrast versus\ud assimilation may happen as early as in N1 (~170m), in occipital and parietal cortical sites.

Humans are able to estimate the reflective properties of the surface (albedo) of an object despite the large variability in the reflected light due to shading, illumination and specular reflection. Here we first used a physically based rendering simulation to study how different statistics (i.e. percentiles) based on the luminance distributions of matte and glossy objects predict the overall surface albedo. We found that the brightest parts of matte surfaces are good predictors of the surface albedo. As expected, the brightest parts led to poor performance in glossy surfaces. We then asked human observers to sort four (2 matte and 2 glossy) objects in a virtual scene in terms of their albedo. The brightest parts of matte surfaces highly correlated with human judgments, whereas in glossy surfaces, the highest correlation was achieved by percentiles within the darker half of the objects’ luminance distributions. Furthermore, glossy surfaces tend to appear darker than matte ones, and observers are less precise in judging their lightness. We then manipulated different bands of the virtual objects’ luminance distributions separately for glossy and matte surfaces. Modulating the brightest parts of the luminance distributions of the glossy surfaces had a limited impact on lightness perception, whereas it clearly influenced the perceived lightness of the matte objects. Our results demonstrate that human observers effectively ignore specular reflections while evaluating the lightness of glossy objects, which results in a bias to perceive glossy objects as darker.

A key challenge for the visual system is to extract constant object properties from incoming sensory information. This information is ambiguous because the same sensory signal can arise from many combinations of object properties and viewing conditions and noisy because of the variability in sensory encoding. The competing accounts for perceptual constancy of surface lightness fall into two classes of model: One derives lightness estimates from border contrasts, and another explicitly infers surface reflectance. To test these accounts, we combined a novel psychophysical task with probabilistic implementations of both models. Observers compared the lightness of two stimuli under a memory demand (a delay between the stimuli), a context change (different surround luminance), or both. Memory biased perceived lightness toward the mean of the whole stimulus ensemble. Context change caused the classical simultaneous lightness contrast effect, in which a target appears lighter against a dark surround and darker against a light surround. These effects were not independent: Combined memory load and context change elicited a bias smaller than predicted assuming an independent combination of biases. Both models explain the memory bias as an effect of prior expectations on perception. Both models also produce a context effect, but only the reflectance model correctly describes the magnitude. The reflectance model, finally, captures the memory-context interaction better than the contrast model, both qualitatively and quantitatively. We conclude that (a) lightness perception is more consistent with reflectance inference than contrast coding and (b) adding a memory demand to a perceptual task both renders it more ecologically valid and helps adjudicate between competing models.

In a looking-time study, 24 infants 6 months of age were presented with continuously folding and unfolding patterns of stripes. The luminances in the dynamic lightness constancy pattern were changed in such way that adults attribute them to changes of the various regions’ orientation relative to the light source (lightness constancy display). The “reversed” lightness constancy stimulus consisted of a continuously folding and unfolding pattern, in which the luminance changes were not consistent with a striped surface illuminated from one side. The only difference between the animations was the relationship between the change in surface orientation and the change of luminances. The infants looked significantly longer at the reversed lightness constancy animation than at the lightness constancy display. This finding suggests that the infants detected the violation of the lightness constancy rules in the reversed lightness constancy stimulus. The infants were also presented with control animations to rule out the possibility that looking preferences were based on low-level properties of the display.

Recently we developed a model that reproduces the Kanizsa square illusion based on two principles: (1) a spatial 2-D integration of luminance ratio and differentiated depth signals creates a “primary” lightness map and a depth map, respectively, which is then followed by (2) a modification of the primary lightness values under influence of the perceived depth (Kogo, Strecha, Van Gool, & Wagemans, 2010). Within this model, the process of the spatial integration inevitably introduced an arbitrary offset. In order to obtain absolute values of depth and lightness, the offset values needed to be determined by other constraints. This is the anchoring problem of the depth and lightness measurements. Here we report the anchoring rules that were established by investigating the model’s responses to the Kanizsa square and its wide range of variations. For the primary lightness map, the highest value rule was applied, while the area rule appeared most plausible for the depth map. By applying the same principles to simple figures consisting of black and white areas of different size ratios, the model succeeded in reproducing published empirical results on lightness anchoring (Li & Gilchrist, 1999).

Natural scenes often contain variations in local luminance as a result of cast shadows and illumination from different directions. When making judgments about such scenes, it may be hypothesized that darker regions (with lower relative contrast due to a lack of illumination) are avoided as they may provide less detailed information than well-illuminated areas. We here test this hypothesis, first by presenting participants images of faces that were digitally modified to simulate the effect of a shadow over half of the image, and second by presenting photographs of faces taken with side illumination, also resulting in the appearance of a shadow across half of the face. While participants viewed these images, they were asked to perform different tasks on the images, to allow for the presentation of the different versions of each image (left shadow, right shadow, no shadow), and to distract the observers from the contrast and illumination manipulations. The results confirm our hypothesis and demonstrate that observers fixate the better illuminated regions of the images.

The variable resolution and limited processing capacity of the human visual system requires us to sample the world with eye movements and attentive processes. Here we show that where observers look can strongly modulate their reports of simple surface attributes, such as lightness. When observers matched the color of natural objects they based their judgments on the brightest parts of the objects; at the same time, they tended to fixate points with above-average luminance. When we forced participants to fixate a specific point on the object using a gaze-contingent display setup, the matched lightness was higher when observers fixated bright regions. This finding indicates a causal link between the luminance of the fixated region and the lightness match for the whole object. Simulations with rendered physical lighting show that higher values in an object’s luminance distribution are particularly informative about reflectance. This sampling strategy is an efficient and simple heuristic for the visual system to achieve accurate and invariant judgments of lightness.

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.

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.

The purpose of this work is to describe how the visual system groups surfaces of unequal lightness under complex patterns of illumination. We propose that the Gestalt principle of Grouping by Regularity explains this process better than the more often cited principle of Grouping by Similarity. In our first experiment we demonstrate that in a perceptual organization task, pitting proximity against illumination gradients, discounting the illuminant was contingent upon the periodicity of the illuminant. Traditional theories of lightness constancy and discounting the illuminant (Rock, Nijhawan, Palmer, & Tudor, 1992) cannot account for such effects. Three more experiments show that grouping is affected more by local luminance ratios than constant reflectance ratios. We conclude from these findings that Grouping by Regularity is a powerful grouping principle that operates pre-constancy.

A review of the field of lightness perception from Helmholtz to the present shows the most adequate theories of lightness perception to be the intrinsic image models. Nevertheless, these models fail on 2 important counts: They contain no anchoring rule, and they fail to account for the pattern of errors in surface lightness. Recent work on both the anchoring problem and the problem of errors has produced a new model of lightness perception, one that is qualitatively different from the intrinsic image models. The new model, which is based on a combination of local and global anchoring of lightness values, appears to provide an unprecedented account of a wide range of empirical results, both classical and recent, especially the pattern of errors. It provides a unified account of both illumination-dependent failures of constancy and background-dependent failures of constancy, resolving a number of long-standing puzzles.

A common-sense assumption concerning visual perception states that brightness and darkness cannot coexist at a given spatial location. One corollary of this assumption is that achromatic colors, or perceived grey shades, are contained in a one-dimensional (1-D) space varying from bright to dark. The results of many previous psychophysical studies suggest, by contrast, that achromatic colors are represented as points in a color space composed of two or more perceptual dimensions. The nature of these perceptual dimensions, however, presently remains unclear. Here we provide direct evidence that brightness and darkness form the dimensions of a two-dimensional (2-D) achromatic color space. This color space may play a role in the representation of object surfaces viewed against natural backgrounds, which simultaneously induce both brightness and darkness signals. Our 2-D model generalizes to the chromatic dimensions of color perception, indicating that redness and greenness (blueness and yellowness) also form perceptual dimensions. Collectively, these findings suggest that human color space is composed of six dimensions, rather than the conventional three., Author Summary Vision scientists have long adhered to the classic opponent-coding theory of vision, which states that bright–dark, red–green, and blue–yellow form mutually exclusive color pairs. According to this theory, it is not possible to see both brightness and darkness at a single spatial location, or an extended set of locations, such as a uniform surface. One corollary of this statement is that all perceivable grey shades vary along a continuum from bright to dark. At first glance, the notion that brightness and darkness cannot coexist on a single surface accords with our common-sense notion that a given grey shade cannot be simultaneously both brighter and darker than any other grey shade. The results presented here suggest that this common-sense notion is not supported by experimental data. Our results imply that a given grey shade can indeed be simultaneously brighter and darker than another grey shade. This seemingly paradoxical conclusion arises naturally if one assumes that brightness and darkness constitute the dimensions of a two-dimensional perceptual space in which points represent grey shades. Our results may encourage scientists working in related fields to question the assumption that perceptual variables, rather than sensory variables, are encoded in opponent pairs.

Studies on binocular contrast sensitivity have predominantly focused on flat, two-dimensional (2D) gratings. The underlying hypothesis of such studies is that contrast sensitivity is determined at the early stages of visual processing and is not influenced by the process of three-dimensional (3D) shape recovery. However, it can be argued that contrast detection involves identifying changes in albedo of a 3D surface rather than strictly determining the presence of 2D luminance changes. In support of this hypothesis, in three experiments we found that the relative salience of oriented luminance modulations was affected by the disparity content of the stimuli, when the same luminance distribution was assigned to a flat surface or to a surface modulated in depth. In the first experiment, in particular, we found that the relative salience of an oriented luminance grating decreased when it could be interpreted as the shading produced by a Lambertian illumination, rather than a change in reflectance. In the other two experiments, moreover, we found that this effect was reduced when the frequency of the luminance modulation did not match the frequency of the stereo corrugation. These results are consistent with the idea that the appearance of a luminance distribution is affected by the perceived 3D properties of the surface on which the luminance distribution is located.

A laminar cortical model of stereopsis and later stages of 3D surface perception is developed and simulated. The model describes how initial stages of monocular and binocular oriented filtering interact with later stages of 3D boundary formation and surface filling-in in the lateral geniculate nucleus and cortical areas V1, V2, and V4. In particular, it details how interactions between layers 4, 3B, and 2/3A in V1 and V2 contribute to stereopsis, and clarifies how binocular and monocular information combine to form 3D boundary and surface representations. Along the way, the model modifies and significantly extends the disparity energy model. Neural explanations are given for psychophysical data concerning: contrast variations of dichoptic masking and the correspondence problem, the effect of interocular contrast differences on stereoacuity, Panum’s limiting case, the Venetian blind illusion, stereopsis with polarity-reversed stereograms, da Vinci stereopsis, and various lightness illusions. By relating physiology to psychophysics, the model provides new functional insights and predictions about laminar cortical architecture.