Perceptual organization overcomes the effects of local surround in determining simultaneous lightness contrast

Perceptual Phenomena Cannot Be Approached from a Single Perspective

This article explores the relationship between neurophysiology and phenomenology in the context of ambiguous figures. Divided into three parts, the study investigates new forms of stimulus and experience errors that arise from ambiguous figures. Part 1 discusses the limitations of a single-disciplinary approach and cautions against relying only on neurophysiological explanations for perceptions. A sole reliance on neurophysiological explanations can lead to stimulus and experience errors, as well as to the development of an unfounded mind/body dualism. Part 2 focusses on the stimulus error associated with ambiguous figures. It also shows how the Mona Lisa's ambiguous expression can cause the experience error. Unlike other forms of ambiguous figures, different expressions of Mona Lisa are perceived when seen in different definitions. It is shown how assigning a higher ontological status to one of the expressions because it aligns with our knowledge of the nervous system, as conjectured by some authors, gives rise to the experience error. Part 3 emphasises the importance of complementing neurophysiological interpretations with phenomenological ones for a better understanding of perceptual phenomena. Phenomenology provides constraints and corrections to neurophysiology, whereas neurophysiology informs phenomenology through empirical findings. The theory of levels of reality is introduced as a framework to underlie the connections and dependencies between different perspectives. Using both neurophysiological and phenomenological approaches, a comprehensive understanding of perceptual phenomena emerges, surpassing the limitations of each discipline. This method encourages a holistic view of perception, where neurophysiology and phenomenology coexist, complementing and enriching each other's insights.

Simple Assumptions to Improve Markov Illuminance and Reflectance

Murray recently introduced a novel computational lightness model, Markov illuminance and reflectance (MIR). MIR is a promising new approach that simulates human lightness processing using a conditional random field (CRF) where natural-scene statistics of reflectance and illumination are implemented. Although MIR can account for various lightness illusions and phenomena, it has limitations, such as the inability to predict reverse-contrast phenomena. In this study, we improved MIR performance by modifying its inference process, the prior on X-junctions, and that on general illumination changes. Our modified model improved predictions for Checkerboard assimilation, the simplified Checkershadow and its control figure, the influence of luminance noise, and White's effect and its several variants. In particular, White's effect is a partial reverse contrast that is challenging for computational models, so this improvement is a significant advance for the MIR framework. This study showed the high extensibility and potential of MIR, which shows the promise for further sophistication.

Lightness contrast & assimilation: testing the hypotheses

Lightness contrast alters lightness of a target decreasing its similarity with neighbouring surfaces (inducers), while lightness assimilation has an opposite effect, similarity is increased. Previous studies emphasized some aspects of stimulation that favour occurrence of one or both of these two phenomena: spatial frequency of the inducers, magnitude and direction of the reflectance difference between the target and the inducers. More importantly, based on previous studies three precise hypotheses can be formulated that predict occurrence of the two phenomena: spatial frequency, differential stimulation and assimilation asymmetry. We manipulated target and inducers’ reflectance, and inducers’ spatial frequency. This enabled us not only to test the importance of these factors, but to predict lightness for each stimulus, according to all three hypotheses. Our results confirmed the importance of tested factors for both lightness contrast and assimilation. Unfortunately, the proposed hypotheses were poor in predicting the obtained data. Differential stimulation hypothesis correctly predicted obtained effect in less than half situations, since small reflectance differences produced contrast, and large differences produced assimilation. Spatial frequency hypothesis did not correctly predict the strength of obtained effects, and we obtained largest assimilation effects with low spatial frequency inducers. Finally, assimilation asymmetry hypothesis did not predict a single obtained effect. Contrary to this hypothesis predictions, we obtained contrast with decrement, and assimilation with increment inducers.

Contrasting a Misinterpretation of the Reverse Contrast

The reverse contrast is a perceptual phenomenon in which the effect of the classical simultaneous lightness contrast is reversed. In classic simultaneous lightness contrast configurations, a gray surrounded by black is perceived lighter than an identical gray surrounded by white, but in the reverse contrast configurations, the perceptual outcome is the opposite: a gray surrounded by black appears darker than the same gray surrounded by white. The explanation provided for the reverse contrast (by different authors) is the belongingness of the gray targets to a more complex configuration. Different configurations show the occurrence of these phenomena; however, the factors determining this effect are not always the same. In particular, some configurations are based on both belongingness and assimilation, while one configuration is based only on belongingness. The evidence that different factors determine the reverse contrast is crucial for future research dealing with achromatic color perception and, in particular, with lightness induction phenomena.

Depth Plane Separation Affects Both Lightness Contrast and Assimilation

Lightness contrast and assimilation are two opposite phenomena: contrast occurs when a gray target perceptually acquires a complementary color than the bordering, inducing, surfaces; assimilation is when a gray target perceptually acquires the same color component as the inducers. Previous research has shown that both phenomena are affected by the manipulation of depth between the inducers and target. However, different results have been reported; it is not clear whether contrast persists when inducers are non-coplanar with the target. Previous studies differ for the spatial configuration of the stimuli and the technique adopted to manipulate depth. The aim of this research was to measure the effects of manipulating the depth between inducers and target in comparable conditions. Results show that contrast persists, but largely reduces, after depth manipulation while assimilation reverses to contrast. Furthermore, interesting asymmetries between white and black inducers emerged with white inducers favoring contrast and black inducers favoring assimilation. These results provide further evidence that high-level processes of visual processing are involved in both phenomena, with important consequences for lightness theories.

Mechanisms underlying simultaneous brightness contrast: Early and innate

In the phenomenon of simultaneous brightness contrast, two patches, one on a dark background and the other on a light one, appear to have different brightness despite being physically equi-luminant. Elucidating the phenomenon's underlying mechanisms is relevant for the larger question of how the visual system makes photometric judgments in images. Accounts over the past century have spanned low-, mid- and high-level visual processes, but a definitive resolution has not emerged. We present three studies that collectively demonstrate that the computations underlying this phenomenon are low-level, instantiated prior to binocular fusion, and available innately, without need for inferential learning via an individual's visual experience. In our first two studies, we find that strong brightness induction is obtained even when observers are unaware of any luminance differences in the neighborhoods of the probe patches. Results with dichoptic displays reveal that eye of origin, although not evident consciously, has a marked influence on the eventual brightness percept of the probe patches, thereby localizing brightness estimation to a site preceding binocular fusion. The third study uses conventional simultaneous brightness contrast displays, but an unusual group of participants: Congenitally blind children whom we were able to treat surgically. The results demonstrate an immediate susceptibility to the simultaneous brightness illusion after sight onset. Together, these data strongly constrain the search for mechanisms underlying a fundamental brightness phenomenon.

Layer and framework theories of lightness

Lightness (the perceived dimension running from black to white) represents a problem for vision science because the light coming to the eye from an object totally fails to specify the shade of gray of the object, due to the confounding of surface gray and illumination intensity. The two leading approaches, decomposition theories and anchoring theories, split the retinal image into overlapping layers and adjacent frameworks, respectively. Because each approach has important strengths and some weaknesses, an integration of them would mark an important step forward for the lightness theory. But the problem remains how this integration can actually be realized.

Functional symmetry of the primary visual pathway evidenced by steady-state visual evoked potentials

The primary visual pathway exhibits a symmetrical anatomical structure, initially arising from the left and right retinas, passing through the lateral geniculate nucleus, and finally projecting to the left and right primary visual cortices. However, to our knowledge, studies based on scalp EEG have not provided adequate evidence of the functional symmetry of the primary visual pathway, as the usual visual ERP is often related to other higher-level brain areas. Steady-state visual evoked potentials (SSVEPs) can be considered as the direct response of the primary visual pathway to a repetitive stimulus, with a very limited correlation with responses of higher-level brain areas. Therefore, SSVEPs can be used to evaluate the functional symmetry of the primary visual pathway. In this study, we draw a comparison among the powers and distributions of SSVEPs of different frequencies when the left or right eye alone is stimulated, and when both the eyes are stimulated together. Our results indicate that the primary visual pathway is almost symmetrical in generating SSVEPs from either eye and that there is some functional interaction between the left and right primary visual pathways.

Lightness Constancy in Surface Visualization

Color is a common channel for displaying data in surface visualization, but is affected by the shadows and shading used to convey surface depth and shape. Understanding encoded data in the context of surface structure is critical for effective analysis in a variety of domains, such as in molecular biology. In the physical world, lightness constancy allows people to accurately perceive shadowed colors; however, its effectiveness in complex synthetic environments such as surface visualizations is not well understood. We report a series of crowdsourced and laboratory studies that confirm the existence of lightness constancy effects for molecular surface visualizations using ambient occlusion. We provide empirical evidence of how common visualization design decisions can impact viewers' abilities to accurately identify encoded surface colors. These findings suggest that lightness constancy aids in understanding color encodings in surface visualization and reveal a correlation between visualization techniques that improve color interpretation in shadow and those that enhance perceptions of surface depth. These results collectively suggest that understanding constancy in practice can inform effective visualization design.

Does Perceptual Belongingness Affect Lightness Constancy?

The relation between perceptual belongingness and lightness perception has historically been studied in the contrast domain (Benary, 1924 Psychologische Forschung5 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.

Perceptual belongingness determines the direction of lightness induction depending on grouping stability and intentionality

Contrast and assimilation are two opposite perceptual phenomena deriving from the relationships among perceptual elements in a visual field. In contrast, perceptual differences are enhanced; while, in assimilation, they are decreased. Indeed, if contrast or assimilation occurs depends on various factors. Interestingly, Gestalt scientists explained both phenomena as the result of perceptual belongingness, giving rise to an intriguing paradox. Benary suggested that belongingness determines contrast; conversely, Fuchs suggested that it determines assimilation. This paradox can be related both to the grouping stability (stable/multi-stable) and to the grouping intentionality (intentional/non-intentional). In the present work we ran four experiments to test whether the contrast/assimilation outcomes depend on the above-mentioned variables. We found that, intentionality and multi-stability elicit assimilation; while, non-intentionality and stability elicit contrast.

Grouping Factors and the Reverse Contrast Illusion

In simultaneous lightness contrast, two identical gray target squares lying on backgrounds of different intensities appear different in lightness. Traditionally, this illusion was explained by lateral inhibitory mechanisms operating retinotopically. More recently, spatial filtering models have been preferred. We report tests of an anchoring theory account in which the illusion is attributed to grouping rules used by the visual system to compute lightness. We parametrically varied the belongingness of two gray target bars to their respective backgrounds so that they either appeared to group with a set of bars flanking them, or they appeared to group with their respective backgrounds. In all variations, the retinal adjacency of the gray squares and their backgrounds was essentially unchanged. We report data from seven experiments showing that manipulation of the grouping rules governs the size and direction of the simultaneous lightness contrast illusion. These results support the idea that simultaneous lightness contrast is the product of anchoring within perceptual groups.

A Neurocomputational account of the role of contour facilitation in brightness perception

A new filling-in model is proposed in order to account for challenging brightness illusions, where inducing background elements are spatially separated from the gray target such as dungeon, cube and grating illusions, bullseye display and ring patterns. This model implements the simple idea that neural response to low-contrast contour is enhanced (facilitated) by the presence of collinear or parallel high-contrast contours in its wider neighborhood. Contour facilitation is achieved via dendritic inhibition, which enables the computation of maximum function among inputs to the node. Recurrent application of maximum function leads to the propagation of the neural signal along collinear or parallel contour segments. When a strong global-contour signal is accompanied with a weak local-contour signal at the same location, conditions are met to produce brightness assimilation within the Filling-in Layer. Computer simulations showed that the model correctly predicts brightness appearance in all of the aforementioned illusions as well as in White's effect, Benary's cross, Todorović's illusion, checkerboard contrast, contrast-contrast illusion and various variations of the White's effect. The proposed model offers new insights on how geometric factors (contour colinearity or parallelism), together with contrast magnitude contribute to the brightness perception.

A Gestalt Account of Lightness Illusions

Illusions of lightness offer valuable clues to how lightness values are computed by the visual system. The traditional domain of lightness illusions must be expanded to include failures of constancy, as there is no distinction between these categories. Just as lightness is (relatively) constant in the face of changes in illumination level, so it is equally constant in the face of changes in background reflectance. Simultaneous lightness contrast, the most familiar lightness illusion, is fairly weak, and represents a failure of background-independent lightness constancy. It is argued that a combination of the highest-luminance rule of anchoring plus the Kardos idea of codetermination can account for most lightness illusions. Kardos suggested that the lightness value of a target surface is partly determined relative to the field of illumination (or framework) in which it is embedded, and partly relative to the neighboring field of illumination. Although Kardos did not apply his principle of codetermination to failures of background-independent constancy such as the simultaneous contrast illusion, this can be done rather easily by defining a framework as a perceptual group instead of identifying it strictly with an objective field of illumination.

Reversing the Reversed Contrast

Galmonte and Agostini (1998 Investigative Ophthalmology & Visual Science, 39(4), S158), Agostini and Galmonte (2002 Psychological Science, 13, 89–93), Bressan (2001 Perception, 30, 1031–1046), and Gilchrist and Annan (2002 Perception, 31, 141–150) reported three different lightness contrast configurations in which grouping factors make a gray target totally surrounded by black appear darker than an equal gray target surrounded by white, reversing the classical contrast effect. In this paper we demonstrate that the three configurations known as ‘reversed contrast’ are based on different mechanisms. Sixteen participants judged the lightness of the gray targets of the original and modified versions of the three configurations. Our results highlight that the Agostini and Galmonte effect is reversed when the global grouping factors are removed, while in a number of variations of Bressan's and Gilchrist and Annan's displays the direction of the effect does not change, even in absence of global grouping factors. Our results indicate that the factors determining the Agostini and Galmonte effect are different from those acting on the other two configurations, in which the lightness change is also due to factors other than belongingness.

Is artists' perception more veridical?

Figurative artists spend years practicing their skills, analyzing objects, and scenes in order to reproduce them accurately. In their drawings, they must depict distant objects as smaller and shadowed surfaces as darker, just as they are at the level of the retinal image. However, this retinal representation is not what we consciously see. Instead, the visual system corrects for distance, changes in ambient illumination and view-point so that our conscious percept of the world remains stable. Does extensive experience modify an artist's visual system so that he or she can access this retinal, veridical image better than a non-artist? We have conducted three experiments testing artists' perceptual abilities and comparing them to those of non-artists. The subjects first attempted to match the size or the luminance of a test stimulus to a standard that could be presented either on a perspective grid (size) or within a cast shadow. They were explicitly instructed to ignore these surrounding contexts and to judge the stimulus as if it were seen in isolation. Finally, in a third task, the subjects searched for an L-shape that either contacted or did not contact an adjacent circle. When in contact, the L-shape appeared as an occluded square behind a circle. This high-level completion camouflaged the L-shape unless subjects could access the raw image. However, in all these tasks, artists were as much affected by visual context as novices. We concluded that artists have no special abilities to access early, non-corrected visual representations and that better accuracy in artists' drawings cannot be attributed to the effects of expertise on early visual processes.

Motion perception induced by dynamic grouping: a probe for the compositional structure of objects

A new method is described for determining how the visual system resolves ambiguities in the compositional structure of multi-surface objects; i.e., how the surfaces of objects are grouped together to form a hierarchical structure. The method entails dynamic grouping motion, a high level process in which changes in a surface (e.g., increases or decreases in its luminance, hue or texture) transiently perturb its affinity with adjacent surfaces. Affinity is determined by the combined effects of Gestalt and other grouping variables in indicating that a pair of surfaces forms a subunit within an object's compositional structure. Such pre-perturbation surface groupings are indicated by the perception of characteristic motions across the changing surface. When the affinity of adjacent surfaces is increased by a dynamic grouping variable, their grouping is transiently strengthened; the perceived motion is away from their boundary. When the affinity of adjacent surfaces is decreased, their grouping is transiently weakened; the perceived motion is toward the surfaces' boundary. It is shown that the affinity of adjacent surfaces depends on the nonlinear, super-additive combination of affinity values ascribable to individual grouping variables, and the effect of dynamic grouping variables on motion perception depends on the prior, pre-perturbation affinity state of the surfaces. It is proposed that affinity-based grouping of an object's surfaces must be consistent with the activation of primitive three-dimensional object components in order for the object to be recognized. Also discussed is the potential use of dynamic grouping for determining the compositional structure of multi-object scenes.

Do artists see their retinas?

Our perception starts with the image that falls on our retina and on this retinal image, distant objects are small and shadowed surfaces are dark. But this is not what we see. Visual constancies correct for distance so that, for example, a person approaching us does not appear to become a larger person. Interestingly, an artist, when rendering a scene realistically, must undo all these corrections, making distant objects again small. To determine whether years of art training and practice have conferred any specialized visual expertise, we compared the perceptual abilities of artists to those of non-artists in three tasks. We first asked them to adjust either the size or the brightness of a target to match it to a standard that was presented on a perspective grid or within a cast shadow. We instructed them to ignore the context, judging size, for example, by imagining the separation between their fingers if they were to pick up the test object from the display screen. In the third task, we tested the speed with which artists access visual representations. Subjects searched for an L-shape in contact with a circle; the target was an L-shape, but because of visual completion, it appeared to be a square occluded behind a circle, camouflaging the L-shape that is explicit on the retinal image. Surprisingly, artists were as affected by context as non-artists in all three tests. Moreover, artists took, on average, significantly more time to make their judgments, implying that they were doing their best to demonstrate the special skills that we, and they, believed they had acquired. Our data therefore support the proposal from Gombrich that artists do not have special perceptual expertise to undo the effects of constancies. Instead, once the context is present in their drawing, they need only compare the drawing to the scene to match the effect of constancies in both.

Measuring the Breathing Light Illusion by means of induced simultaneous contrast

By blurring the margins of a surface, both its brightness and the perceived contrast against a superimposed figure with sharp boundaries increase. Also, if one approaches a blurred white spot on a grey background, this spot will appear wider and brighter: this phenomenon is known as the Breathing Light Illusion (BLI) (Gori and Stubbs, 2006 Perception 35 1573-1577). We studied the increment of the achromatic contrast of a grey sharp-boundary disk when it was superimposed on the BLI. This augmentation of the perceived contrast in the dynamic presentation of the BLI was significantly stronger than the effect that Agostini and Galmonte (2002a Psychonomic Bulletin and Review 9 264-269) obtained in static presentation. Our study leads to an indirect quantification of the BLI. Two control experiments showed that the increment of the achromatic contrast depends on the blurred spot and is independent of the dynamic increment in angular size. These results argue for a causal relationship between the increase in brightness due to the BLI and the darkening of the superimposed disk.

The effect of figural manipulations on brightness differences in the Benary cross

The Benary cross is a classical demonstration showing that the perceived brightness f an area is not solely determined by its luminance, but also by the context in which it is embedded. Despite the fact that two identical grey triangles are flanked by an equal amount of black and white, one of the triangles is perceived as being lighter than the other. It has been argued that the junctions surrounding a test area are crucial in determining brightness. Here, we explored how different aspects influencing perceptual organisation influence perceived figure-background relations in the Benary cross and, with that, the perceived brightness of the triangular patches in our stimuli. The results of a cancellation task confirm that the alignment of contours at junctions indeed has a strong influence on an area's brightness. At the same time, however, the Benary effect is also influenced by the overall symmetry of the cross and its orientation.

Von Bezold Assimilation Effect Reverses in Stereoscopic Conditions

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.

Paradoxical lightness contrast

The visual system's computation of lightness (perceived reflectance) leads to contrast effects in which a gray target region appears lighter on a black background than on a white one. Here we show a paradoxical contrast effect in which targets look lighter after adding regions that increase the scene's average luminance, and darker after adding regions that decrease this luminance. The paradoxical effect emerges if the target sits either on a black local background surrounded by a white remote background, or on a white local background surrounded by a black remote background. It does not occur if both backgrounds have the same luminance. The effect is consistent with Bressan's double-anchoring theory, and likely also with those edge-integration theories that assume gain control, but differs from previously reported effects of assimilation, articulation, reverse contrast, and remote contrast.

Multiplicative and additive Adelson's snake illusions

Two different versions of Adelson's snake lightness illusion are quantitatively investigated. In one experiment an additive version of the illusion is investigated by varying the additive component of the atmosphere transfer function (ATF) introduced by Adelson [2000, in The New Cognitive Neuroscience Ed. M Gazzaniga (Cambridge, MA: MIT Press) pp 339-351]. In the other, a multiplicative version of the illusion is examined by varying the multiplicative component of the ATE In both experiments four observers matched the targets' lightness of the snake patterns with Munsell samples. Increasing the additive or the multiplicative component elicited an approximately equal increase in the magnitude of the lightness illusion. The results show that both components, in the absence of other kinds of information, can be used as heuristics by our visual system to anchor luminance of the object when converting it into lightness.

Multiresolution wavelet framework models brightness induction effects

A new multiresolution wavelet model is presented here, which accounts for brightness assimilation and contrast effects in a unified framework, and includes known psychophysical and physiological attributes of the primate visual system (such as spatial frequency channels, oriented receptive fields, contrast sensitivity function, contrast non-linearities, and a unified set of parameters). Like other low-level models, such as the ODOG model [Blakeslee, B., & McCourt, M. E. (1999). A multiscale spatial filtering account of the white effect, simultaneous brightness contrast and grating induction. Vision Research, 39, 4361-4377], this formulation reproduces visual effects such as simultaneous contrast, the White effect, grating induction, the Todorović effect, Mach bands, the Chevreul effect and the Adelson-Logvinenko tile effects, but it also reproduces other previously unexplained effects such as the dungeon illusion, all using a single set of parameters.

Brightness Induction: Rate Enhancement and Neuronal Synchronization as Complementary Codes

In cat visual cortex, we investigated with parallel recordings from multiple units the neuronal correlates of perceived brightness. The perceived brightness of a center grating was changed by varying the orientation or the relative spatial phase of a surrounding grating. Brightness enhancement by orientation contrast is associated with an increase of discharge rates of responses to the center grating but not with changes in spike synchronization. In contrast, if brightness enhancement is induced by phase offset, discharge rates are unchanged but synchronization increases between neurons responding to the center grating. The changes in synchronization correlate well with changes in perceived brightness that were assessed in parallel in human subjects using the same stimuli. These results indicate that in cerebral cortex the modulation of synchronicity of responses is used as a mechanism complementary to rate changes to enhance the saliency of neuronal responses.

Influence of target size and luminance on the White-Todorovic effect

Variants of a lightness effect described by [Todorovic's, D. (1997). Lightness and junctions. Perception, 26, 379] were studied to quantify the failure of lightness constancy as a function of target luminance and target size. Todorovic's effect is similar to White's effect. Simultaneous lightness contrast appears to operate selectively between stimuli belonging to the same perceptual group, and not between stimuli of equal proximity belonging to different perceptual groups. We found that mid-gray targets grouped with a white contextual stimulus were matched on average to a darker-than-veridical gray. Those grouped with a black contextual stimulus were matched on average veridically. This is consistent with 'anchoring' effects observed in simple two-stimulus displays. However, target luminance had an effect that was not captured by mid-level target luminance data or data averaged across target luminances. For both white and black contextual stimuli, light-gray targets were matched to a darker-than-veridical gray and the direction of this error shifted toward the lighter-than-veridical direction as the luminance of the target was lowered. The result was a constant difference between the perceived lightnesses of targets presented with white and black contextual stimuli. Target size had no effect on perceived lightness. These data imply that the Todorovic-White effect can be characterized as lightness assimilation rather than as lightness contrast. By accounting for compression as well as the Todorovic-White effect, assimilation is the more general explanation.

The effects of global grouping laws on surface lightness perception

Previous studies of lightness perception have shown that local surface grouping laws such as proximity and T junction were powerful determinants of target surface lightness. Recent lightness theories also emphasize the importance of local grouping of surfaces. In this study, we further examined the effects of three global grouping laws--symmetry, repetition, and alternation--on lightness perception. Local surface grouping laws such as proximity and good continuation were controlled across all of our stimulus displays. Participants' lightness perception consistently depended on a given surface's belongingness as determined by these laws--that is, global grouping laws affected a target surface's lightness perception. Our results indicate that global grouping laws determine a target surface's lightness when local surface grouping does not produce any distinct surface belongingness. Implications of our basic results are discussed in terms of a recent lightness theory.

Mechanisms of contrast induction in heterogeneous displays

This study examines how judgments of a region's contrast are influenced by components of a heterogeneous surround. Each stimulus comprised a 5x5 grid of squares in a homogeneous background of fixed mean luminance, with the central square the target. On a given trial, the task was to judge (with feedback) whether the (Weber) contrast of the target was 0.04 or -0.04 (relative to the background); the contrasts assigned (in random order) to the 24 surrounding squares were drawn from the values -0.98, -0.33, 0.33, 0.98 in conformity to one of nine pre-chosen histograms. Presentations were brief (80 ms) in one condition and long (800 ms) in another. A novel psychophysical method was used to estimate the impact exerted on judged target contrast (JTC) by a given contrast in a given grid position. Results were similar for four observers. For both display durations, the four squares sharing an edge with the target influenced JTC 2.4-9 times more than any other surrounding squares. In long presentations, abutting squares of extreme contrast repelled target contrast: squares of contrast -0.98 (0.98) increased (decreased) JTC. However, lower contrast abutting squares attracted target contrast: squares of contrast -0.33 (0.33) decreased (increased) JTC. This central finding can be explained by supposing that: (a) JTC is strongly correlated with the average boundary contrast from surround to target, as registered by linear, edge-selective neurons, and, crucially, (b) the responses of these neurons are themselves subject to lateral inhibition from the rectified responses of other similarly tuned neurons. Finally, in brief presentations, a polarity-specific asymmetry was observed: the two positive abutting-square contrasts continued to influence JTC as they did in long presentations, but contrasts -0.33 and -0.98 ceased to exert much impact, suggesting that lateral influences on target appearance propagate more quickly from positive than from negative contrast abutting regions.

Brightness assimilation in bullseye displays

In simultaneous brightness contrast displays, a gray target square G(B) bordered by black appears brighter than an identical gray target square G(W) bordered by white. Here we demonstrate that this effect can be reversed if G(B) is surrounded by bands that alternate outward from black to white, while G(W) is surrounded by bands that alternate outward from white to black. With these simple "bullseye" displays assimilation generally occurs--G(B) appears darker than G(W). Experiments 1 and 2 used a 2AFC design with a 2.2 s display duration. The results of these experiments indicate that (i) substantial assimilation occurs for target Weber contrasts (relative to the gray background) of -0.25, 0, and 0.25, but assimilation was maximal when target contrast was -0.25 and decreased as target contrast increased, (ii) assimilation effects were the same whether the width of the four surround bands was 20% of the target or 40% of the target, and (iii) assimilation occurs with as few as 2 surround-bands and the magnitude of the effect increases slightly as the number of bands increase. When experiment 1 was re-run using the method of matching (experiment 3), however, the results changed dramatically: (moderate) assimilation effects were found only when target contrast was -0.25; when target contrast was 0.25, there was a brightness contrast effect; when target contrast was 0, there was no illusion. Assimilation effects in bullseye displays are not predicted by the CSF model described in DeValois and DeValois [Spatial Vision, Oxford University Press, New York, 1988], the anchoring model of Gilchrist et al. [Psychological Review, 106(4) (1999) 795], or Blakeslee and McCourt's [Vision Research 39 (1999) 4361] ODOG model. We propose that this assimilation effect is the result of a contrast inhibition mechanism similar to that proposed by Chubb et al. [Proceedings for the National Academy of Science, vol. 86, 1989, p. 9631] to underlie contrast effects.