Contrast-dependent red-green hue shift.

On bright surrounds, red-green-balanced yellow targets become greenish brown with decreased target luminance, and red-green-balanced brown targets become reddish yellow with increased target luminance. These effects imply luminance- and/or contrast-dependent weighting of M- and L-cone signals in post-receptoral pathways. We show psychophysically that luminance contrast between the surround and the target is the primary determinant of the magnitude of red-green hue shift, requiring surround luminance at least twice the target luminance and increasing with further increases of surround/target contrast. There is a much smaller effect of absolute stimulus luminance, with dimmer stimuli showing slightly larger hue shifts. To evaluate a possible retinal origin of the changes in cone-signal weightings underlying the hue shift, we recorded spike responses from both ON- and OFF-center midget ganglion cells in peripheral primate retina. We found no evidence that the relative strength of L- and M-cone post-receptoral responses changed systematically with change of surround irradiance. Nor was there any systematic difference between ON- and OFF-subtypes. This suggests that the change in cone signal weighting occurs later in the visual system.

Contrast-dependent red-green balance shifts depend on S-cone activity.

Previous research from our lab has established that red-green-balanced yellow targets become greenish-brown as surround luminance increases, while red-green-balanced brown targets become reddish-yellow as surround luminance decreases. To help assess the generality and underlying processes of this contrast-dependent red-green hue shift, we investigated red-green hue shifts for target stimuli that appeared achromatic or blue as well as yellow/brown. Results confirmed that the red-green hue shift was largest for yellow/brown targets and was progressively reduced for achromatic and blue targets as target excitation of S cones increased. The magnitude of the hue shift could be predicted by the S/(L+M) excitation of the target when bright white surrounds are used. The hue shift also requires that the target and surround are presented to the same eye, consistent with processing in monocular pathways. Increased S-cone excitation by the surround was associated with red-green hue shifts for all targets equally. Thus, S-cone signals from bright white surrounds might play a role in the contrast-dependent red-green hue shift, but the source of the variation of the magnitude of the hue shift with variations in target S-cone excitation when presented on those surrounds is unknown.

Color Vision 2018: Introduction by the feature editors.

This feature issue of the Journal of the Optical Society of America A (JOSA A) reflects the basic and applied research interests of members of the color vision community. Most of the articles stem from presentations at the 24th Biennial Symposium of the International Colour Vision Society (ICVS).

Dichoptic perception of brown.

Two experiments assessed mechanisms underlying brown induction by presenting a foveal target disk and concentric annular surround stimuli that varied in contrast relative to larger backgrounds. Stimuli were presented under monocular, binocular, and dichoptic viewing conditions. Observers adjusted the luminance of the target disk to a criterion brown level. We found evidence for at least two separate mechanisms for brown induction: one mechanism that is dependent on physically contiguous contrast and operates in monocular pathways and another mechanism that responds to high luminance contrast anywhere in the visual field and can operate after convergence of signals from the two eyes.

Luminance-dependent long-term chromatic adaptation.

There is theoretical and empirical support for long-term adaptation of human vision to chromatic regularities in the environment. The current study investigates whether relationships of luminance and chromaticity in the natural environment could drive chromatic adaptation independently and differently for bright and dark colors. This is motivated by psychophysical evidence of systematic difference shifts in red-green chromatic sensitivities between contextually bright- versus dark-colored stimuli. For some broad classes of scene content, consistent shifts in chromaticity are found between high and low light levels within images. Especially in those images in which sky and terrain are juxtaposed, this shift has direction and magnitude consistent with the observed psychophysical shifts in the red-green balance between bright and dark colors. Taken together, these findings suggest that relative weighting of M- and L-cone signals could be adapted, in a luminance-dependent fashion, to regularities in the natural environment.

No effects of surround complexity on brown induction.

A yellow stimulus turns brown when it is made sufficiently darker than its surroundings. Most previous studies have used simple contiguous surround stimuli to induce brown, so we know little about how brown induction may be controlled by more distant and more complex surround features. We begin to address this issue by varying the complexity of two configurations of achromatic surround stimuli. It was shown that the area most immediately contiguous to the test stimulus has strong effects on brown induction. More importantly, we found that neither the number of surround features nor the distribution of light in the surround region had an effect on brown induction, as long as the overall size of the surround region remained constant. Instead, we found that brown induction depended on the total amount of light in the constant-size surround region, regardless of how that light was distributed. This potentially distinguishes the mechanisms of brown induction from those of brightness induction.

Influence of surround proximity on induction of brown and darkness.

A bright white surround makes a yellow long-wavelength target look both browner and darker. We explored the parallel between these two types of induction by examining their dependence on the proximity of the bright surround to the target at two different time scales with 27 ms and 1 s stimulus durations. We assessed (a) brown induction by adjustment of target luminance to perceptual brown and yellow boundaries and (b) darkness induction by a successive matching procedure. We found that brown induction is a quick process that is robust even for 27 ms stimuli. For darkness induction, there was a strong, spatially localized surround proximity effect for the 27 ms stimuli and much weaker proximity effect for the 1 s stimuli. For brown induction, proximity effects were generally weaker but still showed relatively stronger localized proximity effects for 27 ms stimuli than for 1 s stimuli. For these stimuli, darkness induction predicts the relative pattern but not the magnitudes of brown induction. Both brown and darkness inductions show the operation of quick, spatially localized processes that are apparently superseded by other processes for extended stimulus presentations.

Color vision: introduction by the feature editors.

This feature issue of the Journal of the Optical Society of America A (JOSA A) reflects the basic and applied research interests of members of the color vision community. Most of the articles stem from presentations at the 23rd Biennial Symposium of the International Colour Vision Society (ICVS).

Rod hue biases for foveal stimuli on CRT displays.

Signals from rod photoreceptors bias (shift) the hues determined by cone photoreceptors for extrafoveal mesopic stimuli, creating green, blue, and red rod hue biases at long, middle, and short wavelengths, respectively. The fovea contains far fewer rods and S cones but may not be immune to rod hue biases. Here, we determine the biases found for mesopic foveal stimuli presented on a CRT display. The rod green bias was observed at unique yellow for all but one observer with 2° tests and persisted for most observers with 0.5° tests. The rod red bias typically seen at unique blue in extrafoveal studies was not apparent for either size of foveal test stimulus, and it was sometimes replaced by a rod green bias. The rod blue bias typically seen at unique green and unique red in extrafoveal studies was weak on average and inconsistent for both sizes of foveal test stimuli. Thus, small mesopic foveal stimuli permit rod influence on M- and L-cone color pathways but disadvantage rod influence on S-cone pathways, perhaps because of the sparseness of foveal S-cones. However, some observers did show idiosyncratic foveal rod hue biases that do not follow the general trends.

Dark versus bright equilibrium hues: rod and cone biases.

Equilibrium (unique) red, green, blue, and yellow stimuli look bright in a black surround, but they look dark in a bright white surround, and yellow changes to brown. We investigated differences in equilibrium-hue chromaticity between bright and dark hues to reveal changes in weighting of cone and rod signals. The largest, most consistent shifts were found between yellow and brown, with equilibrium-brown chromaticity shifted toward red compared to equilibrium yellow at both photopic and mesopic levels. Also, at mesopic levels, rod influence reversed for most observers from a green bias for yellow to a red bias for brown. Bright/dark differences for blue, green, and red were much smaller and/or less consistent. Thus, shifts of cone and rod hue biases between bright and dark hues are most prominent in L-M-cone pathways, especially those activated by yellow and brown stimuli.

Color vision: Introduction by the feature editors.

This feature issue of the Journal of the Optical Society of America A (JOSA A) stems from the 22nd Biennial Symposium of the International Colour Vision Society (ICVS) and reflects the basic and applied research interests of members of the color vision community. A profile is included of the 2013 Verriest Medal recipient.

Rod hue biases produced on CRT displays.

Studies of rod hue biases using monochromatic stimuli have shown that rod stimulation can shift the balance of hues at mesopic light levels. We found that the CRT display produced all three previously identified rod hue biases, which shifted the loci of all four unique hues at low mesopic light levels. Rod hue biases occurred at 2.6 cd/m(2) for some observers but not at 26 cd/m(2). At optimal light levels below 0.5 cd/m(2), rod hue biases varied among observers but generally (1) enhanced green versus red at unique yellow and sometimes at unique blue, (2) enhanced blue versus yellow at both unique green and unique red, and (3) enhanced red versus green at unique blue. Rod hue biases persisted for some observers even for smaller foveal stimuli.

Time course of rod influences on hue perception.

Stimulation of dark-adapted rods can shift the hues associated with specific wavelengths throughout the spectrum: Rods exert a green bias (strengthen green relative to red) at longer wavelengths and a blue bias (strengthen blue relative to yellow) at short-to-middle wavelengths. A third rod influence at shorter wavelengths is more complicated because it has been shown to reverse direction with change of stimulus duration. Thus, for 30-ms stimuli, rods exert a green bias like that observed at longer wavelengths. However, for 1-s stimuli, rods exert a red bias that is observed nowhere else in the spectrum. We examined the latency (time course) of rod hue biases by measuring the shifts of the three spectral unique hues under dark-adapted versus bleached (cone plateau) conditions. The rod green bias at unique yellow (mean 10 nm) and, in contrast to some prior studies, the rod blue bias at unique green (mean 21 nm) were not systematically affected by test stimulus duration. A quick rod green bias (mean 5 nm) was shown at unique blue for two of three observers but was dominated by a slower rod red bias (mean 11 nm) after 30-50 ms of rod stimulation. These opposing rod influences may reflect competing effects of rod signals on ML-cone and S-cone pathways.

Color Constancy in Two-Dimensional and Three-Dimensional Scenes: Effects of Viewing Methods and Surface Texture.

There has been debate about how and why color constancy may be better in three-dimensional (3-D) scenes than in two-dimensional (2-D) scenes. Although some studies have shown better color constancy for 3-D conditions, the role of specific cues remains unclear. In this study, we compared color constancy for a 3-D miniature room (a real scene consisting of actual objects) and 2-D still images of that room presented on a monitor using three viewing methods: binocular viewing, monocular viewing, and head movement. We found that color constancy was better for the 3-D room; however, color constancy for the 2-D image improved when the viewing method caused the scene to be perceived more like a 3-D scene. Separate measurements of the perceptual 3-D effect of each viewing method also supported these results. An additional experiment comparing a miniature room and its image with and without texture suggested that surface texture of scene objects contributes to color constancy.

Brown.

In this Quick Guide, Steven Buck explains how, uniquely among the bright primary perceptual hues, yellow changes its hue when it appears dark, becoming the colour brown.

Time-dependent changes of rod influence on hue perception.

Hue-naming was used in conjunction with a probe-flash procedure to determine the time-course of rod-mediated effects on hue appearance across the spectrum. Two types of rod influence on hue are distinguishable on the basis of differences in both spectral specificity and time course of effect: (1) a "faster" rod influence enhances green relative to red and (2) a "slower" rod influence enhances short-wavelength red relative to green and blue relative to yellow. The results show that there are separable rod hue biases that operate over different time courses and that the overall rod influence on hue appearance depends importantly on the temporal properties of the stimuli, presumably because rods interact in different ways with different portions of the neural pathways that mediate human color vision.

Foveal and extra-foveal influences on rod hue biases.

Green, blue and short-wavelength-red rod hue biases are strongest and most reliable with large, dimly-mesopic, extra-foveal stimuli but tend to diminish when stimuli are confined to a small area of the central fovea. This study explores how the stimulation of foveal and extra-foveal areas interact in determining rod hue biases, and whether large stimuli are as effective for revealing rod hue biases when foveally centered as when eccentrically centered. We assessed rod influence by measuring wavelengths of unique green and unique yellow (with 1-s duration, 1 log scot td stimuli and a staircase procedure) under bleached and dark-adapted conditions. We measured unique hues with foveally centered 2 degrees - and 7.4 degrees -diameter disks, a 7.4 degrees (outer) x 2 degrees (inner) diameter annulus, and a 7 degrees -eccentric, 7.4 degrees -diameter disk. The rod green bias (shift of unique yellow locus) was typically <10 nm and remained fairly constant across spatial configurations, indicating no special foveal influence. The rod blue bias (shift of unique green) varied more among observers and spatial configurations, reaching up to 47 nm. However, stimuli covering the fovea typically produced no rod blue bias. Thus, the present results add differences in spatial dependence (i.e., foveal/extra-foveal interaction) between green and blue rod biases to previously demonstrated differences (e.g., differences in amount of light level dependence, in time course and in the spectral range influenced by each bias).

Do rods influence the hue of foveal stimuli?

To understand the generality and mechanisms of previously reported rod hue biases, we examined whether they are present for small foveal stimuli by comparing the wavelengths of the three spectral unique hues under dark-adapted and flash-bleached conditions. Rod green bias (shift of unique yellow) and rod blue bias (shift of unique green) were found for some observers with 1 degrees -diameter foveal stimuli, the size most likely to stimulate rods. Smaller stimuli (0.2 degrees and 0.6 degrees diameter), which were least likely to stimulate rods, produced no large or consistent differences between dark-adapted and bleached conditions. This suggests that rod hue biases result from the local stimulation of rods by light, not from remote suppression by dark-adapted, unstimulated rods, and not from bleaching light artifacts.

Generality of rod hue biases with smaller, brighter, and photopically specified stimuli.

This study tests the generality of previously demonstrated rod hue biases (red and blue biases at shorter wavelengths and a green bias at longer wavelengths) that cause the loci of the three spectral unique hues to shift to longer wavelengths. We found rod hue biases for 2-deg targets to be generally similar in magnitude and light-level dependence to those observed for 7.4-deg targets (the size most often studied) when measured at 7-deg eccentricity. The largest effects for both test sizes occurred at the lowest light levels tested, 1 log scotopic troland. All three rod hue biases were found with 0.6-deg targets, but were not reliably measurable at the lowest light levels and were reduced in magnitude and consistency across observers. The largest rod hue biases all occurred at the same scotopic light level, which corresponds to different photopic light levels for the three hue biases, because of differences in photopic and scotopic spectral sensitivity. This suggests that no single photopic light level will produce such large effects for all three rod hue biases. Finally, when the rod influence on a specific unique-hue locus was measured using photopically (rather than scotopically) constant stimuli, rod hue biases were still found but were more variable in magnitude and incidence across observers. We conclude that the rod hue biases we have previously described can be found with smaller stimuli, at somewhat higher light levels, and under photopically constant conditions, although our prior conditions tend to produce larger, more reliable rod hue biases.