Asymmetries in the time-course of chromatic adaptation and the significance of contrast

Differences in color fading and recovery under sustained fixation

More than two centuries ago, Swiss philosopher I. P. V. Troxler announced in 1804 that fixated images fade away during normal vision. Since this declaration, the phenomenon now known as Troxler fading has become the subject of intensive research. Many researchers were eager to find out why we experience image fading and under what conditions image restoration happens. Here, we investigated the dynamics of color stimulus fading and recovery under sustained eye fixation. The objective of the experiments was to find out which colors fade and recover faster under isoluminant conditions. The stimuli were eight blurred color rings extending to 13° in size. Four unique colors (red, yellow, green, and blue) and four intermediate colors (magenta, cyan, yellow-green, and orange) were used. Stimuli were displayed on a computer monitor with a gray background and were isoluminant to the background. The presentation of the stimulus lasted 2 min and subjects were required to look at the fixation point in the middle of the ring and suppress eye movements. The task for subjects was to report the moments of change in the stimulus visibility by four stages of stimulus completeness. We found that all investigated colors undergo fading and recovery cycles during 2 min of observation. The data suggest that magenta and cyan colors have faster stimulus fading and undergo more recovery cycles, while longer wavelength colors slow down stimulus fading.

Effects of screen-based retinal light stimulation measured with a novel contrast sensitivity test

Myopia is increasing worldwide hence it exists a pressing demand to find effective myopia control strategies. Previous studies have shown that light, spectral composition, spatial frequencies, and contrasts play a critical role in refractive development. The effects of light on multiple retinal processes include growth regulation, but also visual performance and perception. Changes in subjective visual performance can be examined by contrast sensitivity (CS). This study was conducted to investigate whether retinal light stimulation of different wavelength ranges is able to elicit changes in CS and, therefore, may be used for myopia control purposes. In total, 30 right eyes were stimulated with the light of different wavelength ranges, including dominant wavelengths of ∼480 nm, ∼530 nm, ∼630 nm and polychromatic light via a commercial liquid crystal display (LCD) screen. Stimulation was performed screen full-field and on the optic nerve head only. CS was measured before any stimulation and after each stimulation condition using a novel and time-efficient CS test. Post-stimulation CS changes were analyzed by ANOVA regarding the influencing factors spatial frequency, stimulation wavelength and stimulation location. A priorly conducted verification study on a subset of five participants compared the newly developed CS test to a validated CS test. The novel CS test exhibited good reliability of 0.94 logCS and repeatability of 0.13 logCS with a duration of 92 sec ± 17 sec. No clinically critical change between pre- and post-stimulation CS was detected (all p>0.05). However, the results showed that post-stimulation CS differed significantly at 18 cpd after stimulation with polychromatic light from short-wavelength light (p<0.0001). Location of illumination (screen full-field vs. optic nerve head) or any interactions with other factors did not reveal significant influences (all p>0.05). To summarize, a novel CS test measures the relationship between retinal light stimulation and CS. However, using retinal illumination via LCD screens to increase CS is inconclusive.

Chromatic fading following complete adaptation to unique hues

Profound vision loss occurs after prolonged exposure to an unchanging featureless visual environment. The effect is sometimes called visual fade. Here we investigate this phenomenon in the color domain using two different experiments. In the first experiment we determine the time needed for a colored background to appear achromatic. Four backgrounds were tested. Each represented the observers’ four unique hues. This adaptation time was compared with time to recover after adaptation Hue shifts at the end of the adaptation period were also measured. There were wide individual differences in adaptation times and recovery times. Overall recovery was faster than adaptation (p < 0.02). There were minimal shifts in hue. In the second experiment the changes in saturation (Munsell chroma) and lightness (Munsell value) of the background were monitored at six time intervals during the adapting process. Again asymmetric matching with Munsell samples was used. There were two distinct components to both the adaptation and recovery phases; one fast with time constant <1s, the other slow with time constant between 40 and 160s.

The experiments show that the special case of visual fade involving color represents the sensory basis for many color-related effects involving adaptation.

Contrast adaptation appears independent of the longitudinal chromatic aberration of the human eye

As ocular chromatic aberration was suspected to cue contrast adaptation in human vision, the purpose of this study was to investigate contrast adaptation under monochromatic light conditions. Single and complex frequency adaptation stimuli were used, and monochromatic conditions were achieved using band pass filters with short (470±2  nm), medium (530±2  nm), and long (630±2  nm) transmission wavelengths. Post-adaptational contrast sensitivity was shown to be significantly decreased for all wavelength conditions for the single frequency stimulus. A significant difference of contrast adaptation between short and long wavelengths was found. Consistently, adaptation led to a significant decrease in contrast sensitivity for the complex frequency stimulus. To conclude, contrast adaptation under mesopic illumination occurs independently of the longitudinal chromatic aberration of the eye; it can be inferred that this mechanism can be used to distinguish between the sign of optical defocus in poly- and monochromatic light conditions.

Development of Methodology and Study Protocol: Safety and Efficacy of a Single Subretinal Injection of rAAV.hCNGA3 in Patients with CNGA3-Linked Achromatopsia Investigated in an Exploratory Dose-Escalation Trial

Achromatopsia is an autosomal recessively inherited congenital defect characterized by a lack of cone photoreceptor function, leading to severely impaired vision. In this clinical study, achromatopsia patients were treated with a single subretinal injection of rAAV.hCNGA3 to restore cone function. The focus of this trial was on the safety of the treatment. After surgery, patients were monitored in eight extensive visits during the first year, followed by a 4-year follow-up period with annual visits. For essential complementation of the standard ophthalmological and systemic examinations, disease-specific methods were developed to assess the safety, efficacy, and patient-reported outcomes in this trial.

Surface color and predictability determine contextual modulation of V1 firing and gamma oscillations

The integration of direct bottom-up inputs with contextual information is a core feature of neocortical circuits. In area V1, neurons may reduce their firing rates when their receptive field input can be predicted by spatial context. Gamma-synchronized (30–80 Hz) firing may provide a complementary signal to rates, reflecting stronger synchronization between neuronal populations receiving mutually predictable inputs. We show that large uniform surfaces, which have high spatial predictability, strongly suppressed firing yet induced prominent gamma synchronization in macaque V1, particularly when they were colored. Yet, chromatic mismatches between center and surround, breaking predictability, strongly reduced gamma synchronization while increasing firing rates. Differences between responses to different colors, including strong gamma-responses to red, arose from stimulus adaptation to a full-screen background, suggesting prominent differences in adaptation between M- and L-cone signaling pathways. Thus, synchrony signaled whether RF inputs were predicted from spatial context, while firing rates increased when stimuli were unpredicted from context.

Relative colour cues improve colour constancy in birds

A ripe strawberry looks red to our eyes in sunlight and in the green light of a forest, although the spectrum of light reflected from its surface differs dramatically. This is caused by two effects: colour constancy and our ability to learn relative colour cues - the ripe strawberry remains relatively 'redder' than an unripe green strawberry. While colour constancy - the ability to recognize colours in shifted illumination - has been studied in many animals, the use of relative colour cues is investigated more rarely. In a previous study on chickens, we measured how large a shift in illumination their colour constancy mechanisms tolerate without reliable relative colour cues. Here, we show that chickens remain colour constant over larger illumination shifts, if they can use such relative colour cues. As relative colour cues are readily available in natural environments, we suggest that their use contributes strongly to colour constancy performance in nature.

The Verriest Lecture: Short-wave-sensitive cone pathways across the life span

Structurally and functionally, the short-wave-sensitive (S) cone pathways are thought to decline more rapidly with normal aging than the middle- and long-wave-sensitive cone pathways. This would explain the celebrated results by Verriest and others demonstrating that the largest age-related color discrimination losses occur for stimuli on a tritan axis. Here, we challenge convention, arguing from psychophysical data that selective S-cone pathway losses do not cause declines in color discrimination. We show substantial declines in chromatic detection and discrimination, as well as in temporal and spatial vision tasks, that are mediated by S-cone pathways. These functional losses are not, however, unique to S-cone pathways. Finally, despite reduced photon capture by S cones, their postreceptoral pathways provide robust signals for the visual system to renormalize itself to maintain nearly stable color perception across the life span.

What is white?

To shed light on the perceptual basis of the color white, we measured settings of unique white in a dark surround. We find that settings reliably show more variability in an oblique (blue-yellow) direction in color space than along the cardinal axes of the cone-opponent mechanisms. This is against the idea that white perception arises at the null point of the cone-opponent mechanisms, but one alternative possibility is that it occurs through calibration to the visual environment. We found that the locus of maximum variability in settings lies close to the locus of natural daylights, suggesting that variability may result from uncertainty about the color of the illuminant. We tested this by manipulating uncertainty. First, we altered the extent to which the task was absolute (requiring knowledge of the illumination) or relative. We found no clear effect of this factor on the reduction in sensitivity in the blue-yellow direction. Second, we provided a white surround as a cue to the illumination or left the surround dark. Sensitivity was selectively worse in the blue-yellow direction when the surround was black than when it was white. Our results can be functionally related to the statistics of natural images, where a greater blue-yellow dispersion is characteristic of both reflectances (where anisotropy is weak) and illuminants (where it is very pronounced). Mechanistically, the results could suggest a neural signal responsive to deviations from the blue-yellow locus or an adaptively matched range of contrast response functions for signals that encode different directions in color space.

Visual effects in great bowerbird sexual displays and their implications for signal design

It is often assumed that the primary purpose of a male's sexual display is to provide information about quality, or to strongly stimulate prospective mates, but other functions of courtship displays have been relatively neglected. Male great bowerbirds (Ptilonorhynchus nuchalis) construct bowers that exploit the female's predictable field of view (FOV) during courtship displays by creating forced perspective illusions, and the quality of illusion is a good predictor of mating success. Here, we present and discuss two additional components of male courtship displays that use the female's predetermined viewpoint: (i) the rapid and diverse flashing of coloured objects within her FOV and (ii) chromatic adaptation of the female's eyes that alters her perception of the colour of the displayed objects. Neither is directly related to mating success, but both are likely to increase signal efficacy, and may also be associated with attracting and holding the female's attention. Signal efficacy is constrained by trade-offs between the signal components; there are both positive and negative interactions within multicomponent signals. Important signal components may have a threshold effect on fitness rather than the often assumed linear relationship.

Chromatic illumination discrimination ability reveals that human colour constancy is optimised for blue daylight illuminations

The phenomenon of colour constancy in human visual perception keeps surface colours constant, despite changes in their reflected light due to changing illumination. Although colour constancy has evolved under a constrained subset of illuminations, it is unknown whether its underlying mechanisms, thought to involve multiple components from retina to cortex, are optimised for particular environmental variations. Here we demonstrate a new method for investigating colour constancy using illumination matching in real scenes which, unlike previous methods using surface matching and simulated scenes, allows testing of multiple, real illuminations. We use real scenes consisting of solid familiar or unfamiliar objects against uniform or variegated backgrounds and compare discrimination performance for typical illuminations from the daylight chromaticity locus (approximately blue-yellow) and atypical spectra from an orthogonal locus (approximately red-green, at correlated colour temperature 6700 K), all produced in real time by a 10-channel LED illuminator. We find that discrimination of illumination changes is poorer along the daylight locus than the atypical locus, and is poorest particularly for bluer illumination changes, demonstrating conversely that surface colour constancy is best for blue daylight illuminations. Illumination discrimination is also enhanced, and therefore colour constancy diminished, for uniform backgrounds, irrespective of the object type. These results are not explained by statistical properties of the scene signal changes at the retinal level. We conclude that high-level mechanisms of colour constancy are biased for the blue daylight illuminations and variegated backgrounds to which the human visual system has typically been exposed.

Slow updating of the achromatic point after a change in illumination

For a color constant observer, the color appearance of a surface is independent of the spectral composition of the light illuminating it. We ask how rapidly color appearance judgments are updated following a change in illumination. We obtained repeated binary color classifications for a set of stimuli defined by their reflectance functions and rendered under either sunlight or skylight. We used these classifications to derive boundaries in color space that identify the observer's achromatic point. In steady-state conditions of illumination, the achromatic point lay close to the illuminant chromaticity. In our experiment, the illuminant changed abruptly every 21 s (at the onset of every 10th trial), allowing us to track changes in the achromatic point that were caused by the cycle of illuminant changes. In one condition, the test reflectance was embedded in a spatial pattern of reflectance samples under consistent illumination. The achromatic point migrated across color space between the chromaticities of the steady-state achromatic points. This update took several trials rather than being immediate. To identify the factors that governed perceptual updating of appearance judgments, we used two further conditions, one in which the test reflectance was presented in isolation and one in which the surrounding reflectances were rendered under an inconsistent and unchanging illumination. Achromatic settings were not well predicted by the information available from scenes at a single time point. Instead, the achromatic points showed a strong dependence on the history of chromatic samples. The strength of this dependence differed between observers and was modulated by the spatial context.

The influence of depth segmentation on colour constancy

In real scenes, surfaces in different depth planes often differ in the luminance and chromatic content of their illumination. Scene segmentation is therefore an important issue when considering the compensation of illumination changes in our visual perception (lightness and colour constancy). Chromatic adaptation is an important sensory component of colour constancy and has been shown to be linked to the two-dimensional spatial structure of a scene (Werner, 2003 Vision Research 43 1611 - 1623). Here, the question is posed whether this cooperation also extends to the organisation of a scene in depth. The influence of depth on colour constancy was tested by introducing stereo disparity, whereby the test patch and background were perceived in either the same or one of five different depth planes (1.9-57 min of arc). There were no additional cues to depth such as shadows or specular highlights. For consistent illumination changes, colour constancy was reduced when the test patch and background were separated in depth, indicating a reduction of contextual influences. An interaction was found between the influences of stereo depth and spatial frequency on colour constancy. In the case of an inconsistent illumination change, colour constancy was reduced if the test patch and background were in the same depth plane (2-D condition), but not if they were separated in depth (3-D condition). Furthermore, colour constancy was slightly better in the 3-D inconsistent condition than in the 2-D inconsistent condition. It is concluded that depth segmentation supports colour constancy in scenes with inconsistent illumination changes. Processes of depth segmentation are implemented at an early sensory stage of colour constancy, and they define visual regions within which the effects of illuminant changes are discounted for separately. The results support recent models that posit such implementation of scene segmentation in colour constancy.

Almost complete colour constancy achieved with full-field adaptation

A successive asymmetric colour-matching task was used to study the changes in colour appearance of simulated Munsell samples. Colour shifts were induced with two Planckian illuminants, standard illuminant A (u'=0.256, v'=0.524) and illuminant S (u'=0.174, v'=0.392). Measurements were conducted with a 20 degrees field and a 120 degrees field. Adaptation period varied from 1 to 30s with the smaller field and from 1 to 60s with the larger field. Colour shifts were specified in terms of a modified Brunswik ratio (BR). Higher values of BR were associated with longer adaptation periods but only when the larger background was used. Supplementary experiments showed that the changes in colour appearance were related to a slight shift in the perceived colour of the background. The timing of the colour shifts are modelled in terms of cone opponent responses. High values of BR correspond to almost complete von Kries adaptation in all three cone types.

Psychophysical estimation of the best illumination for appreciation of Renaissance paintings

A variety of light sources are used in museum environments where the main concern is to prevent damaging effects of the light on paintings. Yet, the visual impression of an artistic painting is strongly influenced by the intensity and spectral profile of the illumination. The aim of this work was to determine psychophysically the spectral profile of the illumination preferred by observers when seeing paintings dated from the Renaissance époque and to investigate how their preferences correlate with the color temperature of the illumination and with the chromatic diversity of the paintings. Hyperspectral images of five oil paintings on wood were collected at the museum and the appearance of the paintings under five representative illuminants computed. Chromatic diversity was estimated by computing the representation of the paintings in the CIELAB color space and by counting the number of nonempty unit cubes occupied by the corresponding color volume. A paired-comparison experiment using precise cathode ray tube (CRT) reproductions of the paintings rendered with several illuminant pairs with different color temperatures was carried out to determine observers' preference. The illuminant with higher color temperature was always preferred except for one pair where no clear preference was expressed. The preferred illuminant produced the larger chromatic diversity, and for the condition where no specific illuminant was preferred the number of colors produced by the illuminant pair was very similar, a result suggesting that preference could have been influenced by chromatic diversity.

Minimalist surface-colour matching

Some theories of surface-colour perception assume that observers estimate the illuminant on a scene so that its effects can be discounted. A critical test of this interpretation of colour constancy is whether surface-colour matching is worse when the number of surfaces in a scene is so small that any illuminant estimate is unreliable. In the experiment reported here, observers made asymmetric colour matches between pairs of simultaneously presented Mondrian-like patterns under different daylights. The patterns had either 49 surfaces or a minimal 2 surfaces. No significant effect of number was found, suggesting that illuminant estimates are unnecessary for surface-colour matching.

Color contrast: a contributory mechanism to color constancy

Color constancy--by which objects tend to appear the same color under changes in illumination--is most likely achieved by several mechanisms, operating at different levels in the visual system. One powerful contributory mechanism is simultaneous spatial color contrast. Under changes in natural illumination the spatial ratios of within-type cone excitations between natural surfaces tend to be preserved (Foster and Nascimento, 1994); therefore, the neural encoding of colors as spatial contrasts tends to achieve constancy. Several factors are known to influence the strength of chromatic contrast induction between surfaces, including their relative luminance, spatial scale, spatial configuration and context (Ware and Cowan, 1982; Zaidi et al., 1991). Here we test the hypothesis that color contrast is weakened by differences between surfaces which indicate that they may be under distinct illuminants. We summarize psychophysical measurements of the effects of relative motion, relative depth and texture differences on chromatic contrast induction. Of these factors, only texture differences between surfaces weaken chromatic contrast induction. We also consider neurophysiological and neuropsychological evidence and conclude that the mechanisms which mediate local chromatic contrast effects are sited at low levels in the visual system, in primary visual cortex (V1) or below, prior to image segmentation mechanisms which require computation of relative depth or motion. V1 and lower areas may therefore play a larger role in color constancy than previously thought.

The spatial tuning of chromatic adaptation

A key question in colour research is how the colour and spatial analysis of an image interact. Traditionally, colour and form analysis have been regarded as parallel and separate processes, and documented effects of image complexity on chromatic adaptation have been attributed to a temporal integration process during eye movements. Evidence is presented here for a spatial mechanism, which tunes chromatic adaptation to the luminance structure (spatial frequency and orientation) of an image. This in turn suggests a close cooperation between colour and form analysis during chromatic adaptation. The results are discussed in relation to the "segregated pathway hypothesis" and the role of spatial aspects for the computation of colour constancy and adaptation to natural scenes.