Color constancy denotes the ability to assign a particular and stable color percept to an object, irrespective of its surroundings and illumination. The light reaching the eye confounds illumination and spectral reflectance of the object, making the recovery of constant object color an ill-posed problem. How good the visual system is at accomplishing this task is still a matter of heated debate, despite more than a 100 years of research. Depending on the laboratory task and the specific cues available to observers, color constancy was found to be at levels ranging between 15% and 80%, which seems incompatible with the relatively stable color appearance of objects around us and the consistent usage of color names in real life. Here, we show close-to-perfect color constancy using real objects in a natural task and natural environmental conditions, chosen to mimic the role of color constancy in everyday life. Participants had to identify the color of a (non-present) item familiar to them in an office room under five different experimental illuminations. They mostly selected the same colored Munsell chip as their match to the absent object, even though the light reaching the eye in each case differed substantially. Our results demonstrate that color constancy under ideal conditions in the real world can indeed be exceptionally good. We found it to be as good as visual memory permits and not generally compromised by sensory uncertainty.

Color constancy denotes the ability of human and animals to assign a particular color percept to an object. The light reaching the eye confounds illumination and spectral reflectance of the object, making the recovery of constant object color an ill-posed problem. How good the visual system is at solving this task is still a matter of debate. Depending on the laboratory task and the specific cues available to observers, color constancy was found to reach levels between 20% and 80%. If color constancy were indeed that poor, we would experience frequent color changes of objects in everyday life. This does not seem to be the case. Instead, we take it for granted that objects “have” a color, and we do use color terms to describe objects, e.g. a green scarf.

We investigated several sensory and cognitive determinants of colour constancy across 40 illumination hues. In the first experiment, we measured colour naming for the illumination and for the colour induced by the illumination on the colorimetric grey. Results confirmed that the induced colours are approximately complementary to the colour of the illumination. In the second experiment, we measured colour constancy using achromatic adjustments. Average colour constancy was perfect under the blue daylight illumination and decreased in colour directions away from the blue daylight illumination due to undershooting and a strong blue bias. Apart from this blue bias, colour constancy was not related to illumination discrimination and to chromatic detection measured previously with the same setup and stimuli. We also observed a strong negative relationship between the degree of colour constancy and the consensus of naming the illumination colour. Constancy coincided with a low naming consensus, in particular because bluish illumination colours were sometimes seen as achromatic. Blue bias and category consensus alone explained >68%, and all determinants together explained >94% of the variance of achromatic adjustments. These findings suggest that colour constancy is optimised for blue daylight.

Are objects remembered with a more saturated color? Some of the evidence supporting this statement comes from research using "memory colors"-the typical colors of particular objects, for example, the green of grass. The problematic aspect of these findings is that many different exemplars exist, some of which might exhibit a higher saturation than the one measured by the experimenter. Here we avoid this problem by using unique personal items and comparing long- and short-term color memory matches (in hue, value, and chroma) with those obtained with the object present. Our results, on average, confirm that objects are remembered as more saturated than they are.

Our visual system is capable of achieving constant color perception across large changes in illumination. Consequently, a physically constant, colorimetrically neutral patch of light, appearing gray under a neutral illumination, will appear vividly colored if the observer has the impression that the illumination of the scene changed. We wanted to test whether we can decode the brain's responses to color with respect to appearance, rather than mere retinal cone stimulation. We presented observers with a set of colored patches under three different simulated illumination conditions, while recording brain activity in a 3T fMRI scanner. Our stimuli consisted of a central square patch within a variegated surround. In neutral illumination trials, the surround remained neutrally illuminated while the central patches oscillated between gray and blue or gray and yellow. We used this neutral illumination condition to train a classifier to decode the central color patches into bluish and yellowish colors, based on the activity of the 300 most responsive voxels. These were obtained by means of a localizer that determined the retinotopic location of the target patches. Accuracies were between 50% and 79% correct (mean 67%). We then introduced simulated illumination changes. In these trials, the variegated surround oscillated between a neutral and either a bluish or a yellowish illuminant. This made the neutral gray patches appear to oscillate between gray and yellowish or bluish, respectively. When the classifier, trained on the real colors, was used to decode the appearance of these stimuli, accuracy was slightly reduced but still above chance (46% to 71%, mean 62%). Our results show color appearance can be decoded as early as V1/V2. This indicates that color constancy could be achieved very early in the visual pathways. Meeting abstract presented at VSS 2016

All established measures for color constancy come with methodological problems or serious constraints. They rarely capture our everyday experience of the phenomena. Here we present a new and intuitive approach that allows us to measure constancy for arbitrary colors without multiple illuminants in the scene. Participants (N=17) were asked to bring a personal object (e.g. scarf) that had for them a well-defined colour that they were confident they could identify in absence of the object. Without the object being present, participants were asked to select from the Munsell Book of Color (Glossy Edition) the chip that best represented the colour of their chosen object. They performed the task first in a room under neutral daylight illumination and in two other rooms that had non-daylight illuminations provided by windows covered with filters. The task was performed twice in each room but always on different days. The filters induced substantial changes in the objects' color coordinates by 24 (purple filter) and 28 (green filter) Lab units. Before selecting the chip, participants adapted to the illumination while performing a color sorting task. Under each of the three illuminations, we measured the Lab coordinates of the objects, the illumination and the selected chips. We also selected the chip that best matched the object in its presence. In this task, our participants were perfectly color constant. The change in the objects' color coordinates under the illumination changes was equalled by the changes in the color coordinates of the chips selected under the different illuminants. On average, 99.2% (+/- 3.6% s.e.) constancy was achieved. There was no significant difference for the two filter conditions. Our results show that perfect color constancy can be achieved in a task and conditions that are highly representative of the uses of color constancy in everyday life. Meeting abstract presented at VSS 2015.

We present an apparatus that allows independent stimulation of rods and short (S)-, middle (M)-, and long (L)-wavelength-sensitive cones. Previously presented devices allow rod and cone stimulation independently, but only for a spatially invariant stimulus design (Pokorny, Smithson, & Quinlan, 2004; Sun, Pokorny, & Smith, 2001b). We overcame this limitation by using two spectrally filtered projectors with overlapping projections. This approach allows independent rod and cone stimulation in a dynamic two-dimensional scene with appropriate resolution in the spatial, temporal, and receptor domains. Modulation depths were ±15% for M-cones and L-cones, ±20% for rods, and ±50% for S-cones, all with respect to an equal-energy mesopic background at 3.4 cd/m2. Validation was provided by radiometric measures and behavioral data from two trichromats, one protanope, one deuteranope, and one night-blind observer.

An established finding in colour memory research is that memory shifts to more saturated matches. These results are based on ‘memory colours’ – the typical colours of particular objects, for example the green of grass. The problematic aspect of these findings is that many different exemplars exist, some of which might exhibit a higher saturation than the one measured by the experimenter. Here we avoid this problem by using unique items. Participants (N=12) brought personal coloured objects (toys, etc.). In absence of the object, we secured from the owner a long-term memory match to it. The match was performed in a room under neutral daylight illumination by selecting the chip that best resembled the memory colour of the object from the Munsell Book of Color (Glossy Edition). Participants that were naïve to the objects (N=8) performed the same selection task immediately after looking at each object for 30 seconds (short-term memory match). Subsequently, the same participants provided an object-match under daylight with chips and objects present. We measured the colour coordinates of selected chips, objects and illumination. The Munsell collection did not provide exact matches for every object, so the object-match was used as a proxy for analysis. The comparison between the matches of the owner of the object and the naïve subjects matches showed that owners recalled the colour of their object as being more saturated (t(11)= -2.61, p=.02) while participants who had just seen the object did not show that effect (t(11)=-1.15, n.s.). In CIE-Lab, the saturation effect averaged over all participants and objects was 6.4 units for long-term and 2 units for short-term memory. Our results show that the shifts to higher levels of saturation only occur for objects stored in long-term memory and that this bias is not present for short-term memory. Meeting abstract presented at VSS 2015

Past research on color constancy has mainly focused on surfaces illuminated by a restricted set of illuminants. Since other visual functions related to color, such as color discrimination, exhibit systematic variations with hue, we wanted to investigate whether there are systematic differences in the degree of color constancy between illuminants of varying hues. We varied chromaticity and saturation of the illuminant in rendered two- and three-dimensional scenes displayed on a LCD screen extending a visual angle of 58.9° x 38.9°. The scenes depicted different versions of an illusion introduced by Lotto & Purves (2004). Ten naïve observers performed achromatic matches on scenes illuminated by illuminants of 20 different chromaticities and two saturation levels. The surface reflectances were chosen from the axes of DKL-Color-space, rotated in steps of 18° azimuth in accordance with the illuminants. Each Illuminant was chosen so that it exactly canceled the chromaticity of one of the surface colors used. Observers had to adjust a central patch in the scene until it appeared achromatic to them. Color constancy was defined as the magnitude of the correction observers used to adjust a gray, relative to the shift of the neutral patch under that illuminant. We observed levels of color constancy between 25% and 75% for different observers and conditions. There were only small differences between 2D- and 3D scenes and for the two different saturation levels. There was a trend towards higher constancy for illuminants varying in color directions close to the daylight locus. Overall, color constancy seems to be fairly stable across different illuminant directions. Lotto RB, Purves D (2004) Perceiving Colour. Review of Progress in Coloration 34: 12-25. Meeting abstract presented at VSS 2014

Signals of rods and cones are combined within the retina already. While two retinal pathways of rod-cone interaction have been identified (reviewed by Sun, Pokorny & Smith, JOV 2001b), consequences on perception are not fully understood yet. We have assessed rod-cone interaction with Gaussian blobs flickering at 1 and 10 Hz. Stimuli were presented by an image display device consisting of two filtered LED-projectors whose beam projections were combined by a semitransparent mirror. By careful calibration using a PR-650 radiospectrometer this device allows independent stimulation of rods and short (S)-, middle (M)-, and long (L)-wavelength-sensitive cones within a contrast range of at least ±10%. The flickering Gaussian blobs (σ=0.2°) were presented at 5° eccentricity on a uniformly gray mesopic background at 4.6 cd/sqm. Four dark-adapted trichromats were examined using a 4-AFC task and the method of constant stimuli. In the first experiment thresholds for rods and cones were determined separately. In agreement with previous findings cone thresholds were higher at 10 Hz than thresholds measured at 1 Hz, whereas rod thresholds did not differ between frequencies (Hess & Nordby, J. Physiol. 1986). In the second experiment each cone flicker was interfered with a constant sub-threshold rod flicker. Thresholds of rod influenced M- and S-cones decreased for both frequencies, suggesting a summation of rod and cone signals. L-cones showed an opposing pattern of interaction with rods: thresholds dropped at 10 Hz as well but increased significantly at 1 Hz. So far, in-phase combination of rod and cone flicker has been reported to augment cone sensitivities (Buck. In: Chalupa, Werner, editors. The Visual Neurosciences. MIT; 2004. pp. 863-878). Our results show that this is not mandatory for L-cones at low frequencies. Meeting abstract presented at VSS 2013