The perception of lightness in 3-D curved objects.

Lightness constancy in complex scenes requires that the visual system take account of information concerning variations of illumination falling on visible surfaces. Three experiments on the perception of lightness for three-dimensional (3-D) curved objects show that human observers are better able to perform this accounting for certain scenes than for others. The experiments investigate the effect of object curvature, illumination direction, and object shape on lightness perception. Lightness constancy was quite good when a rich local gray-level context was provided. Deviations occurred when both illumination and reflectance changed along the surface of the objects. Does the perception of a 3-D surface and illuminant layout help calibrate lightness judgments? Our results showed a small but consistent improvement between lightness matches on ellipsoid shapes, relative to flat rectangle shapes, under illumination conditions that produce similar image gradients. Illumination change over 3-D forms is therefore taken into account in lightness perception.

Illumination change at a depth edge can reduce lightness constancy.

Lightness constancy requires that a surface retain its lightness not only when the illumination is changed but also when the surface is moved from one background to another. Occlusion of one surface by another frequently results in a retinal juxtaposition of patches under different illuminations. At such edges, retinal luminance ratios can be much higher than in scenes with a single illumination. We demonstrate that such retinal adjacencies can produce failures of lightness constancy. We argue that they are responsible for departures from perfect lightness constancy in two prior experiments that examined the effects of depth relations on lightness constancy.

How much does illuminant color affect unattributed colors?

Does the light coming from a surface (as opposed to the surface color) appear the same after adaptation to a new illumination as it did before the illumination changed? Many answers have been proposed over the past century, but all have been unsatisfactory. The main measurement problem is to provide a comparison stimulus that is unaffected by the adaptation being tested. My observers used a mental standard. Under 4000-, 6500-, and 10,000-K adaptations (the extremes and the average of daylight) they produced on a CRT unique hues with a constant saturation that was memorized during training. The main evaluation problem is how to determine the theoretical chromaticity shifts that represent illumination invariance for comparison with the data. Like most previous investigators, I used light sources rather than actual surfaces and illuminants. Using a new technique, I determined theoretical surfaces that would have unique hues under the test illuminants. Using Cohen's basis vectors, I derived theoretical reflectances that under 6500 K would produce the chromaticities that the observer chose as unique hues. The chromaticities of those same reflectances under 4000 and 10,000 K are theoretical points representing illuminants-invariant appearance of the light coming from the surfaces. Even for this small range of illuminants the adaptive shifts were too small for invariance, i.e., the appearance of the light was different even after full adaptation. This result sharpens the question of the basis for humans' concept of color as a stable property of surfaces.

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 patches on surrounds 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 surrounds 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.

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.

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.

Contrast sensitivity to patch stimuli: effects of spatial bandwidth and temporal presentation.

Models of the spatial response of human vision are important for applied work, but the available contrast sensitivity function (CSF) data vary widely due to the diverse spatiotemporal stimuli used over the years. To assist selection, this paper: (1) reports measurements of the effects on the CSF of varying the spatial and temporal windows of grating patches; (2) demonstrates that the widely discrepant CSFs from previous studies can be accounted for by using these results; and (3) discusses simple criteria for choosing CSFs for practical applications. CSFs were measured for several combinations of spatial and temporal waveforms, using the same subjects under otherwise identical conditions. The CSF was measured over the range of 0.5-10 c/deg using Gabor-type patches of 1.0-, 0.5-, 0.25-, and 0.125-octave spatial bandwidths using both abrupt and gradual temporal presentations. The results were compared with the CSF obtained with a fixed aperture (4 deg x 4 deg) grating pattern. Increasing the number of cycles resulted in increased sensitivity at intermediate frequencies, changing the CSF to a narrower bandpass shape. For each patch bandwidth, the gradual presentation CSF had a narrower spatial pass band than with the abrupt presentation. The relevance of the large differences in the CSFs obtained with different stimuli to our understanding of visual performance is discussed.

Simultaneous color constancy: paper with diverse Munsell values.

Arend and Reeves [J. Opt. Soc. Am. A 3, 1743 (1986)] described measurements of color constancy in computer simulations of arrays of colored papers of equal Munsell value under 4000-, 6500-, and 10,000-K daylight illuminants. We report an extension of those experiments to chromatic arrays spanning a wide range of Munsell values. The computer-simulated scene included a standard array of Munsell papers under 6500-K illumination and a test array, an identical array of the same papers under 4000 or 10,000 K. Observers adjusted a patch in the test array in order to match the corresponding patch in the standard array by one of two criteria. They either matched hue and saturation or they made surface-color matches, in which the test patch was made to "look as if it were cut from the same pice of paper as the standard patch." The test and the standard patches were surrounded by a single color (annulus display) or by many colors (Mondrian display). The data agreed with those of our previous equal-value experiment. The paper matches were often approximately color constant. The hue-saturation matches were in the correct direction for constancy but were always closer to a chromaticity match (no constancy) than to the chromaticity required for hue-saturation constancy.

Lightness and brightness over spatial illumination gradients.

We extended our studies of lightness and brightness in complex scenes to cathode-ray-tube simulations of an array of 35 gray reflective patches under spatially varying illuminants. There were three illuminance profiles, an abrupt step, a linear gradient, and a simulation of side illumination, with nine steepnesses of each. In half the sessions observers adjusted a test patch at one end of the illumination gradient in order to match the lightness of a standard patch at the other end of the gradient. In the remaining sessions they matched the brightness of the test patch to that of the standard. For all three illuminance profiles the lightnesses of the patches matched when they had approximately the same simulated reflectance; i.e., there was excellent lightness constancy even though the illuminance gradients were clearly visible.

Simultaneous constancy, lightness, and brightness.

An achromatic surface in a complex scene has both an apparent reflectance attribute (lightness) and an overall intensitive attribute (brightness). We studied changes of these two attributes as a function of changes in illumination level and pattern complexity. Subjects observed simultaneously two arrays of simulated achromatic surfaces with identical reflectance distributions. The left-hand array (standard) was always illuminated at a moderate level. The right-hand array (test) had different illuminances from trial to trial. The subjects adjusted patches in the test array to match the corresponding patches in the standard array in either lightness or brightness. In complex patterns (32 grays) lightness constancy was nearly perfect; test reflectance settings were invariant over illuminance. In disk/annulus patterns (two grays), the lightness-match data confirmed previously published reports. At high illuminances, the standard patches could be matched with a smaller range of test-array reflectances than at low illuminances, i.e., lightness constancy was imperfect. Brightness matches varied substantially as a function of illuminance in all conditions.

What is psychophysically perfect stabilization? Do perfectly stabilized images always disappear?: reply to comment.

We acknowledge the earlier work cited by Ditchburn in his comment [R. W. Ditchburn, J. Opt. Soc. Am. A 4, 405-406 (1987).] and point out a crucial difference between that work and our novel approach [L. E. Arend and G. T. Timberlake, J. Opt. Soc. Am. A 3, 235 (1986)]. Our procedure provides measurements uncontaminated by residual errors of stabilization. The visual system's extreme sensitivity to small temporal changes and the indirectness of the evidence cited by Ditchburn leave the meaning of stabilized-image reappearance unclear.

Reading with a macular scotoma. I. Retinal location of scotoma and fixation area.

To investigate how patients with macular scotomas use residual functional retinal areas to inspect visual detail, a scanning laser ophthalmoscope (SLO) was used to map the retinal locations of scotomas and areas used to fixate. Three patients with dense macular scotomas of at least 20 months duration and with no explicit low vision training were tested. SLO stimuli were produced by computer modulation of the scanned laser beam, and could be placed on known retinal loci by direct observation of the retina on a television monitor. Videotaped SLO images were analyzed to produce retinal maps that are corrected for shifts of stimulus position due to fixational eye movement, thus showing the true retinal locations of scotomas and fixation loci. Major findings were as follows: 1) each patient used a single, idiosyncratic retinal area, immediately adjacent to the scotoma to fixate, and did not attempt to use the nonfunctional foveola, 2) fixation stability with the eccentric fixation locus was as good as, or better than, that of ocularly normal subjects trying to fixate at comparable eccentricities, 3) fixation stability was not systematically related to clinical visual acuity, and 4) there is good agreement as to the shape and overall size of SLO and standard clinical tangent screen scotoma maps for these three patients.

What is psychophysically perfect image stabilization? Do perfectly stabilized images always disappear?

High-contrast luminance gratings stabilized on the retina with a Purkinje image eyetracker do not disappear completely. This could be due to small errors of stabilization, or the visual system could include mechanisms capable of responding to temporally constant images. We examined the visual system's sensitivity to small movements of gratings. We (1) replicated previous measurements of contrast sensitivity for gratings with controlled retinal-drift velocities, (2) developed a method for calculating sensitivity to small oscillations of gratings using thresholds for flickering stabilized gratings, and (3) examined the calculations empirically. We calculated that movements of only 8 sec of arc peak to peak produce detectable temporal changes. Since existent stabilization systems cannot eliminate movements this small, residual stabilized-grating detectability does not require detectors sensitive to temporally constant images.