Attention,brightness contrast,and assimilation: The influence of relative area

A model of information transmission in the visual system which describes the effect of attention on apparent brightness is examined. This model states in part that the luminance of the portion of the visual field which captures the attention is overweighted in arriving at an overall average luminance level across the visual field. As this average must be computed with respect to both luminance and relative area, it is hypothesized that increasing the relative area of the portion of the visual field that captures the attention will result in a greater effect on the apparent brightness of all parts of the visual field. Two predictions, which involve the effect of relative area on apparent brightness, are experimentally tested and confirmed.     

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.     

Involuntary attention and brightness contrast

Carrasco, Ling, and Read (2004) reported that involuntary attention increased perceived contrast. We replicated Carrasco et al. and then tested an alternative hypothesis: With stimuli near threshold, a peripheral cue biased observers to believe a stimulus had been presented in the cued location. Consistent with this hypothesis, the effect disappeared when we used higher-contrast stimuli. We further tested the guessing-bias hypothesis in three ways: (1) In a detection experiment, the cue affected bias, but did not increase d'; (2) when the cue followed the stimulus, we obtained the same results as when the cue preceded the stimulus; (3) in one experiment, some trials contained no stimulus, yet observers responded that the cued blank stimulus had higher contrast than the uncued blank stimulus. The results suggest that the effects of a noninformative peripheral cue are best described in terms of nonperceptual biases.     

Lightness,brightness, and brightness contrast: 2. Reflectance variation

Changes of annulus luminance in traditional disk-and-annulus patterns can be perceived to be either reflectance or illuminance changes. In the present experiments, we examined the effect of varying annulus reflectance. In Experiment 1, we placed test and standard patch-and-surround patterns in identical Mondrian patchworks. Only the luminance of the test surround changed from trial to triaL., appearing as reflectance variation under constant illumination. Lightness matches were identical to brightness matches, as expected. In Experiment 2, we used only the patch and surround (no Mondrian). Instructions said that the illumination would change from trial to trial. Lightness and brightness-contrast data were identical; illumination gradients were indistinguishable from reflectance gradients. In Experiment 3, the patterns were the same, but the instructions said that the shade of gray of the test surround would change from trial to trial. Lightness matches were identical to brightness matches, again confirming the ambiguity of disk-and-annulus patterns.     

Lightness,brightness, and brightness contrast: 1. Illuminance variation

Changes of annulus luminance in traditional disk-and-annulus patterns are perceptually ambiguous; they could be either reflectance or illuminance changes. In more complicated patterns, apparent reflectances are less ambiguous, letting us place test and standard patchesjnpxsurrounds perceived to be different grays. Our subjects matched the apparent amounts of light coming from the patches (brightnesses), their apparent reflectances (lightnesses), or the brightness differences between the patches and their surrounds (brightness contrasts). The three criteria produced quantitatively different results. Brightness contrasts matched when the patch/surround luminance ratio of the test was approximately equal to that of the standard. Lightness matches were illumination invariant but were not exact reflectance matches; the different surrounda of test and standard produced a small illumination-invariant error. This constant error was negligible for increments, but, for decrements, it was approximately 1.5 Munsell value steps. Brightness matches covaried substantially with illuminance.     

Reduced brightness contrast as a reading aid

A standard reading comprehension test, modified by printing half of the material on medium grey paper to lower the contrast of print-to-page, was administered to 54 college students, 21 of whom had previously been diagnosed as learning disabled. Comprehension of the control group was little affected by the contrast, but the mean score for the learning disabled students was 10% higher on the pages with reduced contrast. This is congruent with Meares's 1980 clinical observations that reducing contrast significantly aids some readers.     

The relationship between brightness contrast and illusory contours.

One group of subjects rated differences in brightness and another the clarity of illusory contours for eight figure-ground combinations of the Kanizsa and Ehrenstein patterns made from Munsell papers. For four combinations there was a difference in Munsell value (brightness) between figure and ground and for another four no difference. For the latter the pattern was derived from differences in hue or colour quality. For the combinations with a Munsell value difference the ratings of both brightness difference and contour clarity were high and for those of uniform value both were low. The results are interpreted as supporting the argument that illusory contours derive primarily from contrast-induced differences in brightness and possibly in colour between contiguous, physically uniform regions.     

Cross-modal enhancement of perceived brightness: sensory interaction versus response bias

Stein, London, Wilkinson, and Price (1996) reported the presence of cross-modal enhancement of perceived visual intensity: Participants tended to rate weak lights as brighter when accompanied by a concurrent pulse of white noise than when presented alone. In the present study, two methods were used to determine whether the enhancement reflects an early-stage sensory process or a later-stage decisional process, such as a response bias. First, enhancement was eliminated when the noise accompanied the light on only 25% versus 50% of the trials. Second, enhancement was absent when tested with a paired-comparison method. These findings are consistent with the hypothesis that the sound-induced enhancement in judgments of brightness reflects a response bias, rather than an early sensory process--that is, enhancement is the result of a relatively late decisional process.     

Increasing perceived variability reduces prejudice and discrimination

We examined whether increasing individuals' perceived variability of an out-group reduces prejudice and discrimination toward its members. In a series of 4 laboratory and field experiments, we attracted participants' attention to either the homogeneity or the heterogeneity of members of an out-group, and then measured their attitudes or behaviors. Perceived variability was manipulated by making subgroups salient, by portraying the out-group members as having diverse opinions, by making salient that out-group members have different characteristics, or by asking participants to think about differences among out-group members. Prejudice and discrimination were measured in terms of self-reported attitudes, distribution of rewards, helping an out-group confederate, and evaluation of an out-group candidate in a simulated hiring decision. In all experiments, perceived variability decreased prejudice and discrimination. This effect may be due to the fact that perceived variability decreases the role of group membership in the production of attitudes and behaviors toward other individuals.     

Response priming driven by local contrast, not subjective brightness

We demonstrate qualitative dissociations of brightness processing in visuomotor priming and conscious vision. Speeded keypress responses to the brighter of two luminance targets were performed in the presence of preceding dark and bright primes (clearly visible and flanking the targets) whose apparent brightness values were enhanced or attenuated by a visual illusion. Response times to the targets were greatly affected by consistent versus inconsistent arrangements of the primes, relative to the targets (response priming). Priming effects could systematically contradict subjective brightness matches, such that one prime could appear brighter than the other but could prime as if it were darker. Systematic variation of the illusion showed that response-priming effects depended only on local flanker-background contrast, not on the subjective appearance of the flankers. Our findings suggest that speeded motor responses, as opposed to conscious perceptual judgments, access an early phase of lightness and brightness processing prior to full lightness constancy.     

Amount of surround and test-inducing separation in simultaneous brightness contrast

Simultaneous brightness contrast is investigated in the fovea as a function of (1) amount of surround of the inducing field (Experiment 1) and (2) separation between the test and inducing fields (Experiment 2). Circular test and match fields subtending 14 min (radius) are used throughout. The inducing field, held constant in area, is a circular annulus (615 sq min) varying from a quadrant on one side of the test circle to an annulus completely surrounding the test circle. Test-field apparent brightness is not significantly affected by amount of inducing-field surround when the separation between centers of the test and inducing fields is held constant (Experiment 1). Experiment 2, though, shows that apparent brightness increases significantly as the separation between the centers of the test and inducing fields is increased.     

Articulation: brightness, apparent illumination, and contrast ratios

Luminance edges in the environment can be due to regions that differ in reflectance or in illumination. In three experiments, we varied the spatial organization of 10 achromatic (simulated) surfaces so that some arrangements were consistent with an ecologically valid and parsimonious interpretation of 5 surfaces under two different illuminants. A constant contrast-ratio along a luminance edge in the scene allows this interpretation. The brightness of patches in this condition was compared to their brightness with minimally different spatial arrangements that fail to maintain the constant contrast-ratio criterion. When the spatial arrangement of the 10 surfaces included a luminance edge satisfying the constant contrast-ratio criterion, brightness changed systematically, compared to arrangements without such a luminance edge. We account for the results by positing that a luminance edge with a constant contrast-ratio segments the scene into regions of lower and higher illumination, with the same effect as a difference in real physical illumination: all else equal, a given surface appears brighter under higher than under lower illumination.