Large depth effects on lightness in the absence of a large luminance range.

To investigate the role of luminance range for lightness computations in complex 3-dimensional scenes we measured the change in lightness of a surface embedded within a relatively low-luminance-range context, as its perceived spatial position shifted from one plane to another. Our experiment tested conflicting claims between the coplanar ratio principle, according to which large depth effects require a high overall luminance range, and the anchoring theory, which predicts that depth effects can occur with a low overall range, given a sufficiently large difference between the highest luminance values in the 2 planes. Our results show decisive support for the anchoring theory but also hint at a large expansion of the perceived range of reflectances (gray shades) relative to the actual range within each plane. This expansion is qualitatively consistent with anchoring theory's scale normalization principle, but it is surprising in magnitude. Together with our earlier findings showing a massive compression of the perceived reflectance range in unsegmented high-dynamic-range Mondrians, our results underline the urgency of the scaling problem in lightness theory (how luminance range is mapped onto lightness range), a companion of the anchoring problem (which point on the lightness scale is anchored to which point on the luminance scale). (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Computational-observer analysis of illumination discrimination.

The spectral properties of the ambient illumination provide useful information about time of day and weather. We study the perceptual representation of illumination by analyzing measurements of how well people discriminate between illuminations across scene configurations. More specifically, we compare human performance to a computational-observer analysis that evaluates the information available in the isomerizations of cone photopigment in a model human photoreceptor mosaic. The performance of such an observer is limited by the Poisson variability of the number of isomerizations in each cone. The overall level of Poisson-limited computational-observer sensitivity exceeded that of human observers. This was modeled by increasing the amount of noise in the number of isomerizations of each cone. The additional noise brought the overall level of performance of the computational observer into the same range as that of human observers, allowing us to compare the pattern of sensitivity across stimulus manipulations. Key patterns of human performance were not accounted for by the computational observer. In particular, neither the elevation of illumination-discrimination thresholds for illuminant changes in a blue color direction (when thresholds are expressed in CIELUV ΔE units), nor the effects of varying the ensemble of surfaces in the scenes being viewed, could be accounted for by variation in the information available in the cone isomerizations.

The relative contribution of color and material in object selection.

Object perception is inherently multidimensional: information about color, material, texture and shape all guide how we interact with objects. We developed a paradigm that quantifies how two object properties (color and material) combine in object selection. On each experimental trial, observers viewed three blob-shaped objects-the target and two tests-and selected the test that was more similar to the target. Across trials, the target object was fixed, while the tests varied in color (across 7 levels) and material (also 7 levels, yielding 49 possible stimuli). We used an adaptive trial selection procedure (Quest+) to present, on each trial, the stimulus test pair that is most informative of underlying processes that drive selection. We present a novel computational model that allows us to describe observers' selection data in terms of (1) the underlying perceptual stimulus representation and (2) a color-material weight, which quantifies the relative importance of color vs. material in selection. We document large individual differences in the color-material weight across the 12 observers we tested. Furthermore, our analyses reveal limits on how precisely selection data simultaneously constrain perceptual representations and the color-material weight. These limits should guide future efforts towards understanding the multidimensional nature of object perception.

Illumination discrimination for chromatically biased illuminations: Implications for color constancy.

We measured discrimination thresholds for illumination changes along different chromatic directions starting from chromatically biased reference illuminations. Participants viewed a Mondrian-papered scene illuminated by LED lamps. The scene was first illuminated by a reference illumination, followed by two comparisons. One comparison matched the reference (the target); the other (the test) varied from the reference, nominally either bluer, yellower, redder, or greener. The participant's task was to correctly select the target. A staircase procedure found thresholds for discrimination of an illumination change along each axis of chromatic change. Nine participants completed the task for five different reference illumination conditions (neutral, blue, yellow, red, and green). We find that relative discrimination thresholds for different chromatic directions of illumination change vary with the reference illumination. For the neutral reference, there is a trend for thresholds to be highest in the bluer illumination-change direction, replicating our previous reports of a "blue bias" for neutral reference illuminations. For the four chromatic references (blue, yellow, red, and green), the change in illumination toward the neutral reference is less well discriminated than changes in the other directions: a "neutral bias." The results have implications for color constancy: In considering the stability of surface appearance under changes in illumination, both the starting chromaticity of the illumination and direction of change must be considered, as well as the chromatic characteristics of the surface reflectance ensemble. They also suggest it will be worthwhile to explore whether and how the human visual system has internalized the statistics of natural illumination changes.

The perception of colour and material in naturalistic tasks.

Perceived object colour and material help us to select and interact with objects. Because there is no simple mapping between the pattern of an object's image on the retina and its physical reflectance, our perceptions of colour and material are the result of sophisticated visual computations. A long-standing goal in vision science is to describe how these computations work, particularly as they act to stabilize perceived colour and material against variation in scene factors extrinsic to object surface properties, such as the illumination. If we take seriously the notion that perceived colour and material are useful because they help guide behaviour in natural tasks, then we need experiments that measure and models that describe how they are used in such tasks. To this end, we have developed selection-based methods and accompanying perceptual models for studying perceived object colour and material. This focused review highlights key aspects of our work. It includes a discussion of future directions and challenges, as well as an outline of a computational observer model that incorporates early, known, stages of visual processing and that clarifies how early vision shapes selection performance.

Illumination discrimination in the absence of a fixed surface-reflectance layout.

Previous studies have shown that humans can discriminate spectral changes in illumination and that this sensitivity depends both on the chromatic direction of the illumination change and on the ensemble of surfaces in the scene. These studies, however, always used stimulus scenes with a fixed surface-reflectance layout. Here we compared illumination discrimination for scenes in which the surface reflectance layout remains fixed (fixed-surfaces condition) to those in which surface reflectances were shuffled randomly across scenes, but with the mean scene reflectance held approximately constant (shuffled-surfaces condition). Illumination discrimination thresholds in the fixed-surfaces condition were commensurate with previous reports. Thresholds in the shuffled-surfaces condition, however, were considerably elevated. Nonetheless, performance in the shuffled-surfaces condition exceeded that attainable through random guessing. Analysis of eye fixations revealed that in the fixed-surfaces condition, low illumination discrimination thresholds (across observers) were predicted by low overall fixation spread and high consistency of fixation location and fixated surface reflectances across trial intervals. Performance in the shuffled-surfaces condition was not systematically related to any of the eye-fixation characteristics we examined for that condition, but was correlated with performance in the fixed-surfaces condition.

Gender representation in the vision sciences: A longitudinal study.

Understanding the current status and historical trends of gender representation within a research field is an important component of fostering a diverse and inclusive scientific community. Here, we report on the gender representation of a large sample of the vision science research community--the attendees of the Annual Meeting of the Vision Sciences Society (VSS). Our analysis shows that the majority of scientists at all career levels in our sample are male. This imbalance is most pronounced for the senior scientists, whereas predoctoral students are nearly balanced between the genders. Historically, the gender imbalance was larger than it is at present, and it has followed a slow-but-steady trend toward gender parity over the past decade. A longitudinal analysis based on tracking individual attendees shows a larger dropout rate for female than male predoctoral trainees. However, among the trainees who continue in the vision science field after graduate school, evidence suggests that career advancement is quite similar between the genders. In an additional analysis, we found that the VSS Young Investigator awardees and the abstract review committee members reflect substantial gender imbalances, suggesting that these recognitions have yet to catch up with the greater gender balance of the rising generation of junior vision scientists. We hope that this report will encourage awareness of issues of diversity in the scientific community and further promote the development of a research field in which all talented scientists are supported to succeed.

The nature of instructional effects in color constancy.

The instructions subjects receive can have a large effect on experimentally measured color constancy, but the nature of these effects and how their existence should inform our understanding of color perception remains unclear. We used a factorial design to measure how instructional effects on constancy vary with experimental task and stimulus set. In each of 2 experiments, we employed both a classic adjustment-based asymmetric matching task and a novel color selection task. Four groups of naive subjects were instructed to make adjustments/selections based on (a) color (neutral instructions); (b) the light reaching the eye (physical spectrum instructions); (c) the actual surface reflectance of an object (objective reflectance instructions); or (d) the apparent surface reflectance of an object (apparent reflectance instructions). Across the 2 experiments we varied the naturalness of the stimuli. We find clear interactions between instructions, task, and stimuli. With simplified stimuli (Experiment 1), instructional effects were large and the data revealed 2 instruction-dependent patterns. In 1 (neutral and physical spectrum instructions) constancy was low, intersubject variability was also low, and adjustment-based and selection-based constancy were in agreement. In the other (reflectance instructions) constancy was high, intersubject variability was large, adjustment-based constancy deviated from selection-based constancy and for some subjects selection-based constancy increased across sessions. Similar patterns held for naturalistic stimuli (Experiment 2), although instructional effects were smaller. We interpret these 2 patterns as signatures of distinct task strategies-1 is perceptual, with judgments based primarily on the perceptual representation of color; the other involves explicit instruction-driven reasoning. (PsycINFO Database Record

Illumination discrimination in real and simulated scenes.

Characterizing humans' ability to discriminate changes in illumination provides information about the visual system's representation of the distal stimulus. We have previously shown that humans are able to discriminate illumination changes and that sensitivity to such changes depends on their chromatic direction. Probing illumination discrimination further would be facilitated by the use of computer-graphics simulations, which would, in practice, enable a wider range of stimulus manipulations. There is no a priori guarantee, however, that results obtained with simulated scenes generalize to real illuminated scenes. To investigate this question, we measured illumination discrimination in real and simulated scenes that were well-matched in mean chromaticity and scene geometry. Illumination discrimination thresholds were essentially identical for the two stimulus types. As in our previous work, these thresholds varied with illumination change direction. We exploited the flexibility offered by the use of graphics simulations to investigate whether the differences across direction are preserved when the surfaces in the scene are varied. We show that varying the scene's surface ensemble in a manner that also changes mean scene chromaticity modulates the relative sensitivity to illumination changes along different chromatic directions. Thus, any characterization of sensitivity to changes in illumination must be defined relative to the set of surfaces in the scene.

Color constancy in a naturalistic, goal-directed task.

In daily life, we use color information to select objects that will best serve a particular goal (e.g., pick the best-tasting fruit or avoid spoiled food). This is challenging when judgments must be made across changes in illumination as the spectrum reflected from an object to the eye varies with the illumination. Color constancy mechanisms serve to partially stabilize object color appearance across illumination changes, but whether and to what degree constancy supports accurate cross-illumination object selection is not well understood. To get closer to understanding how constancy operates in real-life tasks, we developed a paradigm in which subjects engage in a goal-directed task for which color is instrumental. Specifically, in each trial, subjects re-created an arrangement of colored blocks (the model) across a change in illumination. By analyzing the re-creations, we were able to infer and quantify the degree of color constancy that mediated subjects' performance. In Experiments 1 and 2, we used our paradigm to characterize constancy for two different sets of block reflectances, two different illuminant changes, and two different groups of subjects. On average, constancy was good in our naturalistic task, but it varied considerably across subjects. In Experiment 3, we tested whether varying scene complexity and the validity of local contrast as a cue to the illumination change modulated constancy. Increasing complexity did not lead to improved constancy; silencing local contrast significantly reduced constancy. Our results establish a novel goal-directed task that enables us to approach color constancy as it emerges in real life.

Color constancy supports cross-illumination color selection.

We rely on color to select objects as the targets of our actions (e.g., the freshest fish, the ripest fruit). To be useful for selection, color must provide accurate guidance about object identity across changes in illumination. Although the visual system partially stabilizes object color appearance across illumination changes, how such color constancy supports object selection is not understood. To study how constancy operates in real-life tasks, we developed a novel paradigm in which subjects selected which of two test objects presented under a test illumination appeared closer in color to a target object presented under a standard illumination. From subjects' choices, we inferred a selection-based match for the target via a variant of maximum likelihood difference scaling, and used it to quantify constancy. Selection-based constancy was good when measured using naturalistic stimuli, but was dramatically reduced when the stimuli were simplified, indicating that a naturalistic stimulus context is critical for good constancy. Overall, our results suggest that color supports accurate object selection across illumination changes when both stimuli and task match how color is used in real life. We compared our selection-based constancy results with data obtained using a classic asymmetric matching task and found that the adjustment-based matches predicted selection well for our stimuli and instructions, indicating that the appearance literature provides useful guidance for the emerging study of constancy in natural tasks.

Lightness perception in simple images: testing the anchoring rules.

One approach toward understanding how vision computes surface lightness is to first determine what principles govern lightness in simple stimuli and then test whether these hold for more complex stimuli. Gilchrist (2006) proposed that in the simplest images that produce the experience of a surface (two surfaces differing in luminance that fill the entire visual field) lightness can be predicted based on two anchoring rules: the highest luminance rule and the area rule, plus a scale normalization. To test whether these anchoring rules hold when critical features of the stimuli are varied, we probed lightness in simple stimuli, painted onto the inside of hemispheric domes viewed under diffuse lighting. We find that although the highest luminance surface appears nearly white across a large variation in illumination (as predicted by the highest luminance rule), its lightness tends to increase as its luminance increases. This effect is small relative to the size of the overall luminance change. Further, we find that when the darker region fills more than half of the visual field, it appears to lighten with further increases in area but only if it is a single surface. Splitting the dark region into smaller sectors that cover an equal cumulative area diminishes or eliminates the area effect.

Depth effect on lightness revisited: The role of articulation, proximity and fields of illumination.

The coplanar ratio principle proposes that when the luminance range in an image is larger than the canonical reflectance range of 30:1, the lightness of a target surface depends on the luminance ratio between that target and its adjacent coplanar neighbor (Gilchrist, 1980). This conclusion is based on experiments in which changes in the perceived target depth produced large changes in its perceived lightness without significantly altering the observers' retinal image. Using the same paradigm, we explored how this depth effect on lightness depends on display complexity (articulation), proximity of the target to its highest coplanar luminance and spatial distribution of fields of illumination. Importantly, our experiments allowed us to test differing predictions made by the anchoring theory (Gilchrist et al., 1999), the coplanar ratio principle, as well as other models. We report three main findings, generally consistent with anchoring theory predictions: (1) Articulation can substantially increase the depth effect. (2) Target lightness depends not on the adjacent luminance but on the highest coplanar luminance, irrespective of its position relative to the target. (3) When a plane contains multiple fields of illumination, target lightness depends on the highest luminance in its field of illumination, not on the highest coplanar luminance.

Lightness perception in high dynamic range images: local and remote luminance effects.

We measured the perceived lightness of target patches embedded in high dynamic range checkerboards. We independently varied the luminance of checks immediately surrounding the test and those remote from it. The data establish context transfer functions (CTFs) that characterize perceptual matches across checkerboard contexts. Several features of the CTFs are broadly consistent with previous research: Matched luminance decreases when overall context luminance decreases; matched luminance increases when overall context luminance increases; manipulating context locations near the target has a greater effect than manipulating locations far from the target patch. The measured CTFs are not well described, however, by changes with context in multiplicative gain alone or by changes in both multiplicative and subtractive adaptation parameters. We were able to fit the data with a three-parameter model of adaptation. This allowed us to characterize the CTFs by specifying the luminances that appeared white, black, and gray (white point, black point, and gray point, respectively). The white and black points depended additively on the local and remote contrasts, but accounting for the gray point required an interaction term. Analysis of this effect suggests that the target patch itself must be included in a description of the visual context.

The dynamic range of human lightness perception.

Natural viewing challenges the visual system with images that have a dynamic range of light intensity (luminance) that can approach 1,000,000:1 and that often exceeds 10,000:1 [1, 2]. The range of perceived surface reflectance (lightness), however, can be well approximated by the Munsell matte neutral scale (N 2.0/ to N 9.5/), consisting of surfaces whose reflectance varies by about 30:1. Thus, the visual system must map a large range of surface luminance onto a much smaller range of surface lightness. We measured this mapping in images with a dynamic range close to that of natural images. We studied simple images that lacked segmentation cues that would indicate multiple regions of illumination. We found a remarkable degree of compression: at a single image location, a stimulus luminance range of 5,905:1 can be mapped onto an extended lightness scale that has a reflectance range of 100:1. We characterized how the luminance-to-lightness mapping changes with stimulus context. Our data rule out theories that predict perceived lightness from luminance ratios or Weber contrast. A mechanistic model connects our data to theories of adaptation and provides insight about how the underlying visual response varies with context.

Adjacency and surroundedness in the depth effect on lightness.

Using two perpendicular planes, one brightly and one dimly illuminated, A. L. Gilchrist (1977) showed that target lightness can change nearly from black to white by changing its perceived spatial position, with no change in the retinal image, if the target has an adjacent coplanar neighbor in each position. Earlier L. Kardos (1934) found a modest depth effect for a target that was not adjacent to its coplanar neighbor but surrounded by it. Using Kardos' experimental arrangement, but articulated planes and a between-subjects design, we obtained a large depth effect on lightness without adjacency. We then explored the role of adjacency and surroundedness using Gilchrist's perpendicular planes arrangement. We replicated the large depth effect when the target was adjacent to its coplanar neighbor. However, most of this depth effect was lost when adjacency was eliminated, by moving each target within its plane away from its coplanar neighbor. When we surrounded each target by extending its non-adjacent coplanar background, half the effect provided by adjacency was restored, but only in the brightly illuminated, not the dimly illuminated plane. Our findings support the view that, to compute surface lightness, the visual system groups surfaces in the image that seem to be equally illuminated.

Functional frameworks of illumination revealed by probe disk technique.

We used a novel probe disk technique to test for the existence of functional frames of reference for lightness perception in complex images. Thirteen identical gray disks were electronically pasted into the photograph Trastevere, which shows two large regions of sunlight and shadow. Observers matched the lightness of each disk with a Munsell scale. The data revealed a framework effect. That is, lightness differences within either the sunlight or shadow region were small relative to the pronounced step function at the framework boundary. Additional experiments testing the perceived embeddedness of the disks showed that the framework effect was increased when disk size and shape were altered to conform to the perspective shown in the photograph and when the disks were blurred slightly to conform to the graininess of the photograph. The effect was further increased when the photograph was viewed through a pinhole and when the disks were presented one by one. The effect was reduced when paper disks of equal luminance and visual angle were pasted onto the glass front of the CRT screen. When the sunlight framework was covered with black paper, the remained disks within the shadow region appeared white, as predicted by the anchoring theory (A. Gilchrist, 2006).

Anchoring of lightness values by relative luminance and relative area.

Surface lightness is widely thought to depend on the relative luminance coming from neighboring surfaces. But relative luminance can produce only relative lightness values. Specific lightness values can be derived only with an anchoring rule that specifies how relative luminance values in the retinal image are mapped onto the lightness scale. We explored the anchoring rules governing very simple images consisting of two adjacent surfaces that fill the entire visual field. These were painted onto the interior of a large hemisphere that surrounded the observer's head. Lighter and darker radial sectors of the same two shades of gray were painted onto nine such hemispheres, but with different relative areas. The region of highest luminance was always seen as white. The lightness of the darker sector depended on relative area, appearing lighter as the darker sector became larger, but this effect was stronger when the darker sector was larger than the lighter, a pattern of results shown to be consistent with over a dozen prior studies of relative area and lightness.

Implicit and explicit features of paintings.

Implicit features of the paintings are properties that are imposed by the observer (e.g. how pleasant, interesting, tense a painting appears), whereas explicit features refer to properties that can be directly perceived (form, color, depth, etc.). The aim of Experiments 1 and 2 was to investigate the underlying structure of implicit and explicit features of paintings using the factor analysis of elementary judgments. In the preliminary studies, representative sets of paintings and elementary implicit and explicit dimensions (in the form of bipolar scales) were selected. Four implicit factors were extracted: Regularity, Relaxation, Hedonic Tone and Arousal. Four explicit factors were extracted: Form, Color, Space and Complexity. The following significant correlations between implicit and explicit factors were obtained: Regularity-Form, Regularity-Space, Hedonic Tone-Form and Arousal-Complexity. In Experiment 3 the role of implicit and explicit factors in similarity-dissimilarity ratings was specified. Significant correlations between the position of paintings in MDS space and mean judgments of explicit factors Color, Space and Complexity and implicit factor Relaxation were obtained, suggesting that similarity ratings of paintings are primarily based on explicit features. The causal relation of explicit and implicit features is discussed.