Dark versus bright equilibrium hues: rod and cone biases

Categorical consistency facilitates implicit learning of color-number associations

In making sense of the environment, we implicitly learn to associate stimulus attributes that frequently occur together. Is such learning favored for categories over individual items? Here, we introduce a novel paradigm for directly comparing category- to item-level learning. In a category-level experiment, even numbers (2,4,6,8) had a high-probability of appearing in blue, and odd numbers (3,5,7,9) in yellow. Associative learning was measured by the relative performance on trials with low-probability (p = .09) to high-probability (p = .91) number colors. There was strong evidence for associative learning: low-probability performance was impaired (40ms RT increase and 8.3% accuracy decrease relative to high-probability). This was not the case in an item-level experiment with a different group of participants, in which high-probability colors were non-categorically assigned (blue: 2,3,6,7; yellow: 4,5,8,9; 9ms RT increase and 1.5% accuracy increase). The categorical advantage was upheld in an explicit color association report (83% accuracy vs. 43% at the item-level). These results support a conceptual view of perception and suggest empirical bases of categorical, not item-level, color labeling of learning materials.

Perception of brown with variation in center chromaticity and surround luminance

Brown is a contrast color that depends on complex combinations of chromatic and achromatic signals. We measured brown perception with variations in chromaticity and luminance in center-surround configurations. In Experiment 1, the dominant wavelength and saturation in terms of S-cone stimulation were tested with five observers in a fixed surround luminance (60c d/m ²). A paired-comparison task required the observer to select the better exemplar of brown in one of two, simultaneously presented, stimuli (1.0° center diameter; annulus of 9.48° outer-diameter). In Experiment 2, the same task was tested with five observers in which surround luminance was varied (from 13.1 to 99.6c d/m ²) for two center chromaticities. The results were a set of win-loss ratios for each stimulus combination and converted to Z-scores. An ANOVA did not reveal a significant main effect of the observer factor but revealed a significant interaction with red/green (a ^∗) [but not with the dominant wavelength and the S-cone stimulation (or b ^∗)]. Experiment 2 revealed observer variation in interactions with surround luminance and S-cone stimulation. The averaged data plotted in 1976 L ^∗ a ^∗ b ^∗ color space indicate that high Z-score values widely distribute in the area of a ^∗ from 5 to 28 and b ^∗ over 6. The balance of the strength between yellowness and blackness differs among observers owing to the amount of induced blackness required for the best brown.

Melanopsin-driven surround induction on the red/green balance of yellow

To test the potential role of melanopsin-dependent ipRGCs in surround induction effects, we used a four-channel projector apparatus to hold the cone activity in a surround constant while varying the amount of melanopsin activity between two levels: low (baseline) and high (136% of the baseline). Rods were partially controlled by having the subjects complete conditions after either adapting to a bright field or darkness. The subjects adjusted the red/green balance of a 2.5° central target that varied in its ratio of L and M cones, but was equiluminant with the surround, to a perceptual null point (neither reddish nor greenish). When the surround melanopsin activity was higher, the subjects set their yellow balances at significantly higher L/(L+M) ratios, suggesting the high melanopsin surround was inducing greenishness into the central yellow stimulus. This is consistent with surround brightness effects that show the induction of greenishness into a central yellow test by high luminance surrounds. This potentially provides further evidence for a general role of melanopsin activity in brightness perception.

Testing the Cross-Cultural Generality of Hering's Theory of Color Appearance

This study examines the cross-cultural generality of Hering's (1878/1964) color-opponent theory of color appearance. English-speaking and Somali-speaking observers performed variants of two paradigms classically used to study color-opponency. First, both groups identified similar red, green, blue, and yellow unique hues. Second, 25 English-speaking and 34 Somali-speaking observers decomposed the colors present in 135 Munsell color samples into their component Hering elemental sensations-red,green,blue, yellow, white, and black-or else responded "no term." Both groups responded no term for many samples, notably purples. Somali terms for yellow were often used to name colors all around the color circle, including colors that are bluish according to Hering's theory. Four Somali Grue speakers named both green and blue elicitation samples by their term for green. However, that term did not name the union of all samples called blue or green by English speakers. A similar pattern was found among three Somali Achromatic speakers, who called the blue elicitation sample black or white. Thus, color decomposition by these Somali-speaking observers suggests a lexically influenced re-dimensionalization of color appearance space, rather than a simple reduction of the one proposed by Hering. Even some Somali Green-Blue speakers, whose data were otherwise similar to English, showed similar trends in yellow and blue usage. World Color Survey data mirror these results. These within- and cross-cultural violations of Hering's theory do not challenge the long-standing view that universal sensory processes mediate color appearance. However, they do demonstrate an important contribution of language in the human understanding of color.

Grouping ambiguous neural representations: neither identical chromaticity (the stimulus) nor color (the percept) is necessary

Multiple regions, each with the same ambiguous chromatic neural representation, are resolved to have the identical perceived color more often than chance [Proc. Natl. Acad. Sci. USA93, 15508 (1996)PNASA60027-842410.1073/pnas.93.26.15508; J. Opt. Soc. Am. A35, B85 (2018)JOAOD60740-323210.1364/JOSAA.35.000B85]. This reveals that the regions are grouped, but it is unclear whether they are grouped because each one has the identical competing representations of the same stimuli (that is, the same chromaticities) or, alternatively, identical competing representations of the same colors one sees. The current study uses chromatic induction, as in Nat. Neurosci.6, 801 (2003)NANEFN1097-625610.1038/nn1099, to disentangle whether grouping depends on identical (though ambiguous) stimulus chromaticities or on perceived colors, by (1) inducing one chromaticity to appear in two different colors or (2) inducing two different chromaticities to appear in the same color. All stimuli were equiluminant gratings with chromatic inducing and test fields. Three observers were tested, first completing color matches to measure induced color-appearance shifts and second completing grouping measurements using interocular-switch rivalry, a method with rivalrous dichoptic images swapped between the eyes at 3.75 Hz [J. Vis.17, 9 (2017)1534-736210.1167/17.5.9]. Each of two separate areas, one above and one below fixation, had dichoptic rivalry. The two sets of regions had either identical or different chromaticities that could appear either as the same color or not. Observers reported their percepts when both areas above and below fixation were grouped by color or by chromaticity (or neither in an additional experimental condition). All conditions showed significant groupings for every observer, including when neither color nor chromaticity was identical in a "group." Moreover, there was never a significant effect of chromaticity versus color for any observer. This is the result expected if neither color nor chromaticity must match between two regions in order for them to be grouped and suggests that, instead, some other feature drives grouping.

Paradoxical impact of memory on color appearance of faces

What is color vision for? Here we compared the extent to which memory modulates color appearance of objects and faces. Participants matched the colors of stimuli illuminated by low-pressure sodium light, which renders scenes monochromatic. Matches for fruit were not predicted by stimulus identity. In contrast, matches for faces were predictable, but surprising: faces appeared green and looked sick. The paradoxical face-color percept could be explained by a Bayesian observer model constrained by efficient coding. The color-matching data suggest that the face-color prior is established by visual signals arising from the recently evolved L-M cone system, not the older S-cone channel. Taken together, the results show that when retinal mechanisms of color vision are impaired, the impact of memory on color perception is greatest for face color, supporting the idea that trichromatic color plays an important role in social communication.

What is the function of color vision? Here, the authors show that when retinal mechanisms of color are impaired, memory has a paradoxical impact on color appearance that is selective for faces, providing evidence that color contributes to face encoding and social communication.

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.

Color weight photometry

We study the "color weight" for a number of rather different paradigms. In well researched heterochromatic photometry methods we find that the "weights" determined by settings of naive observers are closely determined by the CIE luminance functional. This is very different for tasks that involve mid- and high-level aspects of perception. In several cases we find equipollence for the display red, green and blue channels. Moreover, in such cases the very nonlinear maximum-rule fits the data rather better than a linear functional. These findings are of interest when photometry needs to be applied for stimuli that are different from the high temporal and low spatial frequency gratings typical for flicker photometry. These results are relevant for science, ergonomics and art.

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.

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.

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.

Vision under mesopic and scotopic illumination

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.