Chromatic Illumination Discrimination Ability Reveals that Human Colour Constancy Is Optimised for Blue Daylight Illuminations

Abstract

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.

Asymmetries in blue–yellow color perception and in the color of ‘the dress’

“…Our findings along with other recent work [10] reveal important asymmetries within this axis. The different phenomena we explored may all reflect an inference that indirect lighting and shadows are bluish, and a bias to attribute that blueness to the lighting rather than the surface, even when the surface is shown in isolation from all scene cues.…”

Robust colour constancy in red-green dichromats

“…Fig 7 shows high similarity in the mean discrimination accuracy along the yellow-blue line between the normal observers of this study and those of Pearce et al [14]. Although the task was the same in both studies, the stimuli were quite different.…”

The many colours of ‘the dress’

“…They had the task of adjusting the colour of a disc, displayed on the same screen, We are left with the open question how different people arrive at different conclusions when interpreting the same sensory data. The distribution of dress pixels along the daylight locus might be coincidental, but there is some evidence that this would make it much harder for the observers to disentangle illumination colour from object reflectance [7,8]. The bright blue tones present in the image could equally well be due to a dark bluish illumination on a white dress, or to a blue dress under a neutral bright light.…”

“…During the course of a day, more yellowish sunlight goes along with lower intensities [4,5]. Asymmetries between bluish and yellowish illuminations have been reported before [8,9]. Thus, it seems that observers do use this correlation to disentangle illumination and surface reflectance.…”