Contextual illusions reveal the limit of unconscious visual processing

The perception of even the most elementary features of the visual environment depends strongly on their spatial context. In the study reported here, we asked at what level of abstraction such effects require conscious processing of the context. We compared two visual illusions that alter subjective judgments of brightness: the simultaneous brightness contrast illusion, in which two circles of identical physical brightness appear different because of different surround luminance, and the Kanizsa triangle illusion, which occurs when the visual system extrapolates a surface without actual physical stimulation. We used a novel interocular masking technique that allowed us to selectively render only the context invisible. Simultaneous brightness contrast persisted even when the surround was masked from awareness. In contrast, participants did not experience illusory contours when the inducing context was masked. Our findings show that invisible context is resolvable by low-level processes involved in surface-brightness perception, but not by high-level processes that assign surface borders through perceptual completion.     

Objects and attributes in divided attention: Surface and boundary systems

A series of experiments investigated concurrent discriminations of surface and nonsurface attributes, including color, brightness, texture, length, location, and motion. In all cases but one, results matched those previously reported: Interference occurred when two discriminations concerned different objects, but not when they concerned the same one. In the two-object case, interference was the same whether discriminations were similar (e.g., two surface discriminations) or different (e.g., one surface, one boundary). Such results support a model of visual attention in which separate visual subsystems are coordinated, converging to work on surface and boundary properties of the same selected object. A partial exception is color: For reasons that are unclear, color escapes twoobject interference except from other, concurrent surface discriminations     

Local brightness mechanisms sketch out surfaces but do not fill them in: psychophysical evidence in the Kanizsa square.

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous contrast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that "sketches out" surfaces at their surrounds but does not "fill" them "in."     

Local rather than global processing of visual arrays in numerosity discrimination by pigeons (Columba livia)

A series of experiments investigated which stimulus properties pigeons use when they discriminate pairs of visual arrays that differ in numerosity. Transfer tests with novel stimuli confirmed that the birds’ choices were based on relative differences in numerosity. However, pigeons differed from other species in the non-numerical cues that affected their choices. In human and non-human primates, numerical discrimination is often influenced by continuous variables such as surface area or overall stimulus brightness. Pigeons showed little evidence of using those cues, even when summed area and brightness had been correlated with numerosity differences and reward outcome. But when array-element sizes were asymmetrically distributed across numerosities, the birds readily utilized information about item sizes as an additional discriminative cue. These novel results are discussed in relation to pigeons’ tendency to focus on local, rather than global dimensions when they process other non-numerical complex visual stimuli. The findings suggest there may be inter-specific differences in the type of perceptual information that provides the input stage for mechanisms underlying numerical processing. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Subjective contours independent of subjective brightness

Two theories of subjective contours are distinguished according to the interrelationship of subjective contours and subjective brightness effects. In one view, subjective contours are illusory brightness gradients generated from grouped local brightness effects. In another view, subjective contours are the edges of subjective forms created on the basis of gestalt factors; subjective brightness is a secondary consequence of form perception. Two experiments which use rating scales to separate judgments of subjective contour and subjective brightness are presented. The first shows that subjects may judge contour to be strong when there is no subjective brightness gradient. In the second, gestalt grouping factors are shown to be more important than factors which should influence brightness according to local effects theories. Both experiments support the view that subjective brightness occurs through interactions at the level of form perception.     

On the determinants of surface brightness

The brightness of an achromatic surface with luminance S on an achromatic background with luminance B varies with S, with B, and with the luminance step deltaL at the border of the surface. In agreement with previous findings indicating that the visual system can perform as a photometer, the results of the two experiments reported here show that S and B determined surface brightness independently of deltaL when the surface was adjacent to and when it was separated from the background. This finding suggests that surface brightness depends on the integration of neural signals representing magnitudes of absolute luminance. A weighted-average model of this integration is proposed.     

Asymmetrical contrast effects induced by luminance and color configurations.

In the two experiments, the use of a psychophysical procedure of brightness/darkness cancellation shed light on interactions between spatial arrangement and figure-ground contrast in the perceptual filling in of achromatic and colored surfaces. Achromatic and chromatic Kanizsa squares with varying contrast, contrast polarity, and inducer spacing were used to test how these factors interact in the perceptual filling in of surface brightness or darkness. The results suggest that the neuronal processing of surfaces with apparent contrast, leading to figure-ground segregation (i.e., perceptual organization), is governed by mechanisms that integrate both luminance contrast and spatial information carried by the inducing stimuli, while discarding information on contrast polarity or color. The findings are discussed in relation to earlier observations on brightness assimilation and contrast. They support theories of nonantagonistic neural mechanisms suppressing local contrast or color signs in brightness-based figure-ground percepts. Such mechanisms might be necessary to cancel potentially conflicting polarities in geometrically complex visual stimuli so that perceptual filling in resulting in the most plausible representation of figure and ground can be achieved.     

Asymmetrical contrast effects induced by luminance and color configurations

In the two experiments, the use of a psychophysical procedure of brightness/darkness cancellation shed light on interactions between spatial arrangement and figure-ground contrast in the perceptual filling in of achromatic and colored surfaces. Achromatic and chromatic Kanizsa squares with varying contrast, contrast polarity, and inducer spacing were used to test how these factors interact in the perceptual filling in of surface brightness or darkness. The results suggest that the neuronal processing of surfaces with apparent contrast, leading to figure-ground segregation (i.e., perceptual organization), is governed by mechanisms that integrate both luminance contrast and spatial information carried by the inducing stimuli, while discarding information on contrast polarity or color. The findings are discussed in relation to earlier observations on brightness assimilation and contrast. They support theories of nonantagonistic neural mechanisms suppressing local contrast or color signs in brightness-based figure-ground percepts. Such mechanisms might be necessary to cancel potentially conflicting polarities in geometrically complex visual stimuli so that perceptual filling in resulting in the most plausible representation of figure and ground can be achieved.     

Context effects,reliability, and internal consistency of intermodal joint scaling

Context effects, intraindividual variability, and internal consistency of intermodal joint scaling with magnitude estimation (“magnitude matching”) were studied by instructing 12 subjects to judge the three pairs of odor intensity, loudness, and brightness on a common scale of perceived intensity as well as to judge odor intensity separately (unimodal magnitude estimation). Significant context effects were found by comparing odor intensity judgments obtained by separate versus intermodal joint scaling as well as across different modalities (loudness vs. brightness) in joint scaling. But no such effects were found for loudness or brightness when compared across modality of joint scaling. Intraindividual variability in the estimates imply about equal reliability in intermodal joint scaling and separate scaling. Good internal consistency was found, indicating that subjects are successful in expressing perceived intensities of different modalities on a common scale.     

Local brightness mechanisms sketch out surfaces but do not fill them in: Psychophysical evidence in the Kanizsa square

In two experiments, brightness enhancement of the illusory surface in the Kanizsa square was investigated by means of a brightness matching procedure. The results show that specific properties of the inducing elements such as size, spacing, and luminance have effects on the matching threshold that are similar to those previously obtained in experiments on simultaneous con trast. The data from a third experiment demonstrate that increment thresholds measured within the Kanizsa square are elevated when the target is flashed on a position close to the inducing elements. The thresholds decrease considerably in the center of both test and control figures (representing or not representing an illusory square). These observations suggest that low-level mechanisms are likely to explain local brightness differences within the configurations but not global figure brightness. In other words, local contrast seems to generate brightness information that “sketches out” surfaces at their surrounds but does not “fill” them “in.”     

不同环境照度下手机屏幕亮度最优参数研究－以三星某手机为例

已有研究表明环境照度、计算机显示器的屏幕亮度和对比度会影响用户的任务操作绩效。本研究采用视觉搜索任务,从行为绩效、主观评价以及眼睛疲劳度指标来考察环境照度、手机屏幕亮度对视觉搜索绩效的影响,并提出在不同的环境照度下手机屏幕亮度的最优值设置参数。结果表明：（1）当手机屏幕亮度保持不变时,不同环境亮度下的搜索绩效差异显著。（2）当环境照度为0lx时,手机屏幕亮度的最优值在11cd/m2左右;环境照度为100lx时,手机屏幕亮度的最优值是68cd/m2左右,环境照度为500lx时,手机屏幕亮度最优值是257cd/m2左右。     

Toward a perceptual theory of transparency

Theories of perceptual transparency have typically been developed within the context of a physical model that generates the percept of transparency (F. Metelli's episcotister model, 1974b). Here 2 fundamental questions are investigated: (a) When does the visual system initiate the percept of one surface seen through another? (b) How does it assign surface properties to a transparent layer? Results reveal systematic deviations from the predictions of Metelli's model, both for initiating image decomposition into multiple surfaces and for assigning surface attributes. Specifically, results demonstrate that the visual system uses Michelson contrast as a critical image variable to initiate percepts of transparency and to assign transmittance to transparent surfaces. Findings are discussed in relation to previous theories of transparency, lightness, brightness, and contrast-contrast.     

Assimilation of achromatic color cannot explain the brightness effects in the achromatic neon effect

If the mouths of the pacmen of a Kanizsa square are colored, for example red, then an illusory red transparent square is seen. In many visual theories such 'neon color spreading' is explained by assimilation of chromatic and achromatic color. In this paper the achromatic case was investigated. In a two-alternative forced-choice task thirty observers judged the brightness of achromatic neon figures. The results suggest that assimilation of achromatic color inside and/or outside of the illusory figures cannot explain the brightness effects seen in achromatic neon color spreading. Although these displays may produce assimilation, it appears that contrast (perhaps acting nonlocally) is a stronger influence on their perceived brightness.     

The visual field paradox: a theoretical account on the reafference principle providing a common frame for the homogeneity and inhomogeneity of visual representation

Observations on the structure of the visual field and its central representation lead to a paradox. A functional dissociation is indicated in oculomotor or attentional control when different response modes are observed as a function of stimulus eccentricity. Alternatively, constancy of brightness throughout the visual field suggests its homogeneity. This paradox can be resolved, if perceptual and motor processes are not conceived of being controlled by separate neuronal mechanisms, but are interconnected within one frame of reference. The reafference principle allows to formulate such a common frame as it integrates afferent and efferent processes. On the basis of this concept, the visual field paradox can be interpreted as not being a paradox at all, but a necessary condition for optimal information processing.     

From interpretation to segmentation

In visual perception, part segmentation of an object is considered to be guided by image-based properties, such as occurrences of deep concavities in the outer contour. However, object-based properties can also provide information regarding segmentation. In this study, outer contours and interpretations of object configurations were manipulated to examine differences between image-based and object-based segmentation in a visual search task. We found that locating a two-dimensional object configuration with deep concavities in the outer contour depends on the type of outer contour of the surrounding distractors. In addition, locating a three-dimensional object configuration was harder when it was surrounded by object-based-disconnected distractors, as compared with object-based-connected distractors, regardless of image-based connections in these distractors. We conclude that segmentation based on the outer contours of a target facilitates its localization. However, when three-dimensional information is available, segmentation strongly depends on object-based properties, rather than on image-based properties.     

Context-dependent brightness priming occurs without visual awareness

Our visual systems account for stimulus context in brightness perception, but whether such adjustments occur for stimuli that we are unaware of has not been established. We therefore assessed whether stimulus context influences brightness processing by measuring unconscious priming with metacontrast masking. When a middle-gray disk was presented on a darker (or brighter) background, such that it could be consciously perceived as brighter (or darker) via simultaneous brightness contrast (SBC), reaction times were significantly faster to a bright (or dark) annulus than to a dark (or bright) annulus. We further show that context-dependent brightness priming does not correlate with visibility using an objective measure of awareness (Experiment 1) and that context-dependent, but not context-independent brightness priming, occurs equally strongly for stimuli below or above the subjective threshold for awareness (Experiment 2). These results suggest that SBC occurs at early levels of visual input and is not influenced by conscious perception.     

A scaling analysis of the snake lightness illusion

Logvinenko and Maloney (2006) measured perceived dissimilarities between achromatic surfaces placed in two scenes illuminated by neutral lights that could differ in intensity. Using a novel scaling method, they found that dissimilarities between light surface pairs could be represented as a weighted linear combination of two dimensions, "surface lightness" (a perceptual correlate of the difference in the logarithm of surface albedo) and "surface brightness" (which corresponded to the differences of the logarithms of light intensity across the scenes). Here we attempt to measure the contributions of these dimensions to a compelling lightness illusion (the "snake illusion"). It is commonly assumed that this illusion is a result of erroneous segmentation of the snake pattern into regions of unequal illumination. We find that the illusory shift in the snake pattern occurs along the surface lightness dimension, with no contribution from surface brightness. Thus, even if an erroneous segmentation of the snake pattern into strips of unequal illumination does happen, it reveals itself, paradoxically, as illusory changes in surface lightness rather than as surface brightness. We conjecture that the illusion strength depends on the balance between two groups of illumination cues signaling the true (uniform) illumination and the pictorial (uneven) illumination.     

Fusion prevents the redundant signals effect: evidence from stereoscopically presented stimuli

In a simple reaction time (RT) experiment, visual stimuli were stereoscopically presented either to one eye (single stimulation) or to both eyes (redundant stimulation), with brightness matched for single and redundant stimulations. Redundant stimulation resulted in two separate percepts when noncorresponding retinal areas were stimulated, whereas it resulted in a single fused percept when corresponding areas were stimulated. With stimulation of noncorresponding areas, mean RT was shorter to redundant than to single stimulation, replicating the redundant signals effect (RSE) commonly found with visual stimuli. With stimulation of corresponding areas, however, no RSE was observed. This suggests that the RSE is driven by the number of percepts rather than by the number of stimulated receptors or sensory organs. These results are consistent with previous findings in the auditory modality and have implications for models of the RSE.     

Brightness contrast in the cat

Using both the method of paired comparisons and the method of constant stimuli, degree of brightness contrast was determined for two cats. Brightness contrast varied with relative luminance of various parts of the stimulus configuration in a manner comparable to that found in human Ss; brightness constancy was not obtained. The data indicate a fixed, nonadaptive basic visual process rather than a cognitive or adaptive perceptual constancy.     

Is visual image segmentation a bottom-up or an interactive process?

Visualimage segmentation is the process by which the visual system groups features that are part of a single shape. Is image segmentation a bottom-up or an interactive process? In Experiments 1 and 2, we presented subjects with two overlapping shapes and asked them to determine whether two probed locations were on the same shape or on different shapes. The availability of top-down support was manipulated by presenting either upright or rotated letters. Subjects were fastest to respond when the shapes corresponded to familiar shapes—the upright letters. In Experiment 3, we used a variant of this segmentation task to rule out the possibility that subjects performed same/different judgments after segmentation and recognition of both letters. Finally, in Experiment 4,we ruled out the possibility that the advantage for upright letters was merely due to faster recognition of upright letters relative to rotated letters. The results suggested that the previous effects were not due to faster recognition of upright letters; stimulus familiarity influenced segmentation per se. The results are discussed in terms of an interactive model of visual image segmentation.