Simultaneous lightness contrast on plain and articulated surrounds

Simultaneous lightness contrast is stronger when the dark and light backgrounds of the classic display (where one of the targets is an increment and the other is a decrement) are replaced by articulated fields of equivalent average luminances. Although routinely attributed to articulation per se, this effect may simply result from the increase in highest luminance in the light articulated, vs plain, background; by locally darkening the decremental target, such an increase would amplify the difference between the targets. We disentangled the effects of highest luminance and articulation by measuring, separately, the magnitude of lightness contrast on dark and light plain and articulated backgrounds. We found that highest luminance and articulation contribute separately to the final illusion.     

The possible loces of lightness contrast

Evidence that the magnitude of simultaneous contrast depends upon retinal orientation of a stimulus configuration is presented in two experiments. More specifically, the magnitude of contrast was found to be greater for stimuli presented horizontally and vertically than for stimuli presented obliquely. These results were interpreted in terms of a cortical analysis of simultaneous contrast.     

Mesopic lightness,brightness, and brightness contrast

At mesopic mean luminances, a fixed luminance contrast produces less brightness contrast than it does at photopic luminances. This suggests that lightnesses of surfaces might also be altered at low luminances. I measured lightness, brightness, and brightness contrast in CRT simulations of achromatic paper patchworks. The illuminance of the standard pattern was fixed, producing 0.12,1.2, or 12 cd/m². The illuminance on the test pattern was varied in a lightness constancy paradigm. Constant brightness contrast required more luminance contrast at lower mean luminances. Failures of lightness constancy occurred at the lowest mean luminances, but they were minor in comparison with the loss of brightness contrast in the same pattern. These results have implications for imaging applications. Often, image content falls in both the photopic and the mesopic ranges. Our results indicate that brightness contrast may decrease substantially in low-luminance regions without large changes of surface lightness.     

T-junctions,apparent depth,and perceived lightness contrast

Observers performed lightness matches for physically equivalent gray targets of a simultaneous lightness contrast display and displays in which both targets were on the same background. Targets either shared a common line-texture pattern with their respective backgrounds or did not. Results indicate that when targets share a line-texture pattern with their respective backgrounds, a contrast effect is obtained. However, when the target's pattern is different than the background's pattern, perceived contrast is significantly reduced and the target appears as a separate 3-D entity. This result applies to both vertically and horizontally oriented displays, to targets that are increments or decrements, and to line-texture patterns that are black or white. Line patterns that are shared by targets and backgrounds result in T-junctions that provide occlusion information. We conclude that targets and backgrounds perceived to be on separate planes because of T-junctions are less likely to be perceptually grouped together and that their luminance values are less likely to be compared with one another.     

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

Lightness induction can occur on the basis of the immediate surround of a region (local interactions) and also on the basis of global factors of perceptual organization. The experiments reported in this article used novel displays that made it possible to differentiate the contributions of these two kinds of factors. The experiments demonstrated, for the first time, that when higher-level factors act contemporaneously with lower-level factors, the contrast effect induced by the global-organization principle of perceptual belongingness overcomes the effect due to retinal lateral inhibition.     

A fair test of the effect of a shadow-incompatible luminance gradient on the simultaneous lightness contrast

Shadow-compatibility of simultaneous lightness contrast is discussed by Alexander D Logvinenko and Paola Bressan, with examples claiming to provide a test of the hypothesis.     

Chromatic rod vision VIII. Simultaneous contrast

Stabell, U. & Stabell, B. Chromatic rod vision. VIII. Simultaneous contrast. Scand. J. Psychol., 1973, 14, 1619.-Compared with the curve of additive opponent hue, the curve of scotopic hue of simultaneous contrast was found to be displaced toward blue in the orange and green-yellow regions of inducing field. The results are explained on the assumptions that (a) change of disposition for scotopic hue reflects variation in ratio of primary hue-related processes of inducing field, and (b) the chromatic-related opponent processes, in a dark-adapted and chromatically neutral eye, are of equal magnitude within the photochromatic interval.     

Spatial articulation affects lightness

In a recent paper, Agostini and Bruno (1996) showed that the size of simultaneous lightness contrast increases under Gelb lighting. To extend Agostini and Bruno's work, we applied their methodology to a set of more spatially articulated displays. In four experiments, we investigated the role of spatial articulation on the size of the simultaneous lightness contrast effect. In the first experiment, we found a decrease of the simultaneous lightness contrast effect as the spatial articulation increased. In the second experiment (the control experiment), performed under homogeneous illumination, we found that the effect of spatial articulation is not detectable, even though the data seem to show the same trend as that in the previous experiment. In the third experiment, we found that spatial articulation affects not only the middle reflectance region, but also the lowest one. As the spatial articulation increases, the effect on the lightnesses of both regions decreases. In the last experiment, performed with a reduced range of reflectances, we found a lightening effect for all the reflectances and, again, an effect of spatial articulation. The results of these experiments are interpreted according to the model proposed by Gilchrist et al. (in press).     

Explaining lightness illusions.

Grey looks darker when set against white than when set against black. In some complex figures this illusion becomes startling, and can be shown to depend on the perceptual organisation of regions within the image. The most widely accepted explanations of such effects are based on the analysis of the junctions formed where the boundaries of nearby regions meet. Even theories where junctions are not the subject of special concern underline their importance as grouping cues. In this paper I present several new families of figures that challenge both views, and conclude that junctions do not play any crucial role in lightness estimation.     

Capturing lightness between contours

Homogeneously coloured bars may exhibit lightness differences at the intersections. A well-known example is the Hermann grid illusion, where crossing white bars on a black background show dark patches at the crossings. Jung (1973, Handbook of Sensory Physiology volume VII/3, pp 1-152) found that the dark patches persist when thin outlines are drawn at the intersections, and are even visible in foveal vision. Recently, it has been shown that making distortions to the contours of a Hermann grid-like configuration results in the disappearance of the illusory dark spots (Geier et al, 2008 Perception 37 651 665). We show that thin outlines at the crossings of the distorted Hermann grid induce lightness differences in the same direction as in the original Hermann grid illusion, even in foveal vision and in displays consisting of two crossing bars. Our experiments reveal that the induced lightness differences are independent of the luminance polarity and shape of the contours at the intersection. We suggest that the effect results from lateral inhibition and an additional spreading and capturing of these differences between luminance contours. A similar capturing between collinear contours may play a role in peripheral vision in the original Hermann grid.     

Spatial and temporal lightness anchoring

Research in the area of lightness perception has not adequately addressed the influence of previously viewed visual fields on perceived surface reflectance. In the spatial realm, a spot-in-a-void will appear darker when a second surface of higher intensity is placed adjacent to it. The brighter surface takes the role of white, the anchor, and the dimmer is scaled accordingly. We find that when a spot-in-a-void is presented to observers in a light controlled chamber it is influenced by nonretinal temporal relationships mediated by the degree of complexity. We also find that the influence cannot be explained in terms of successive contrast at a high or low level in the visual system, but can be explained by an anchoring model. The present results follow the same rules governing spatial integration and anchoring and thereby support the currently proposed concept of temporal anchoring.     

Behavioural contrast and preference in a double chained schedule

Pigeons peck faster during a signal for reward (S+) when that signal alternates with one for absence of reward (S-). This “contrast effect” has been shown to involve diminished preference for S + compared with a stimulus not involved in a discrimination. The present experiment demonstrates that the signal produced by pecks to S+ in a chained schedule is responded to in proportion to the contrast effect during S +. The result suggests that a prior interpretation of contrast, in terms of Amsel's frustration theory, is not the correct one.     

The unbearable lightness of priming

Repetition priming from text to isolated words has been difficult to observe. One explanation for this difficulty is that previous attempts to observe this type of priming have utilised conditions that normally reduce priming. Two experiments were conducted to evaluate this hypothesis. Experiment 1 involved participants being presented with words in isolation and in text passages. The words were then presented again in a lexical decision test. Results indicated that priming occurred as a result of exposure to both isolated words and words in text, although priming was greater in the word-word condition. Experiment 2 investigated whether priming occurred in a lexical decision test on words that had been read prior to the test in Milan Kundera's novel "The Unbearable Lightness of Being." There was some evidence that participants who had read the book recently were faster at lexical decision to words from the book than participants who had not read the book. The two experiments therefore indicate that priming can occur from text to isolated words, although it is smaller in magnitude to that observed from word to word. Reasons for this difference, as suggested by Kirsner and Speelman (J. Exp. Psychol.: Learn. Mem. Cogn. 22 (1996) 563) model of repetition priming, are discussed.     

Impossible shadows and lightness constancy

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