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Object Illuminance

The phenomenon of color constancy occurs when the source of illumination is not directly known. It is for this reason that color constancy takes a greater effect on days with sun and clear sky as opposed to days that are overcast. Even when the sun is visible, color constancy may affect color perception. This is due to an ignorance of all possible sources of illumination. Although an object may reflect multiple sources of light into the eye, color constancy causes objective identities to remain constant.

Dr. D.H. Foster (2011) states, “in the natural environment, the source itself may not be well defined in that the illumination at a particular point in a scene is usually a complex mixture of direct and indirect [light] distributed over a range of incident angles, in turn modified by local occlusion and mutual reflection, all of which may vary with time and position.” The wide spectrum of possible illuminances in the natural environment and the limited ability of the human eye to perceive color means that color constancy plays a functional role in daily perception. Color constancy allows for humans to interact with the world in a consistent or veridical manner and it allows for one to more effectively make judgements on the time of day.

Neural Mechanism

There are two possible mechanisms for color constancy. The first mechanism is unconscious inference. The second view holds this phenomenon to be caused by sensory adaptation. Research suggests color constancy to be related changes in retinal cells as well as well as cortical areas related to vision. This phenomenon is most likely attributed to changes in various levels of the visual system.

Cone Adaptation

Cones, specialized cells within the retina, will adjust relative to light levels within the local environment. This occurs at the level of individual neurons. However, this adaptation is incomplete. Chromatic adaptation is also regulated by processes within the brain. Research in monkeys suggest that changes in chromatic sensitivity is correlated to activity in parvocellular lateral geniculate neurons. Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain.

Metamerism

Metamerism, the perceiving of colors within two separate scenes, can help to inform research regarding color constancy. Research suggests that when competing chromatic stimuli are presented, spatial comparisons must be completed early in the visual system. For example, when subjects are presented stimuli in a dichoptic fashion, an array of colors and a void color, such as grey, and are told to focus on a specific color of the array, the void color appears different than when perceived in a binocular fashion. This means that color judgements, as they relate to spatial comparisons, must be completed at or prior to the V1 monocular neurons. If spatial comparisons occur later in the visual system such as in cortical area V4, the brain would be able to perceive both the color and void color as though they were seen in a binocular fashion.