6.4. Retinex Theory and Color Constancy

To summarize, the trichromatic theory explains how the cones code different wavelengths, and cells in the LGN and the primary visual cortex respond according to the opponent process theory. Further along in the ventral pathway, V4 has been found to contain neurons which respond to a range of colors i.e., not just red, green, blue and yellow (Zeki, 1980). People who have damage to V4 may experience achromatopsia, where the world looks like a black and white photo. This area receives input from V2 and sends information onwards to V8. V8 appears to combine color information with memory information, which also influences our perception of color (Zeki & Marini, 1998).

The detection of different wavelengths is clearly important for color vision to occur, but we also need to interpret the information that the retina sends to the brain. When lighting conditions change, the wavelengths of light entering the eye can change too. However, our general perception of colors remains constant. For example, imagine looking out of your window at the leaves on the trees outside in the sunshine. Now consider looking out a few hours later when the weather has become dull and overcast. Do you perceive that the leaves have changed color? Hopefully you answered ‘No’ this question because you know that the leaves have not changed color. But how do you know this? The fact that we can perceive color as unchanging despite overall changes in luminance is because of a phenomenon called color constancy. We can see this easily in Figure 6.9. The rubric cube on the right is under a blue light. However, due to color constancy, we can still recognize the colors of the squares as red, orange, yellow, blue, white and green.The brain compensates for differences in wavelengths by taking into account the average color across the visual scene. This is explained by the retinex theory, which suggests that color perception is dependent on the retina and the top-down influences of the cortex. Our top-down processing of the information from the retina takes into account three main factors:

1. Chromatic adaptation: if a light source contains a disproportionate amount of wavelengths in one section of the spectrum (e.g., under a blue light), the responses of the relevant photoreceptors are suppressed, shifting our perception away from the dominant wavelength.
2. Local context: we compare all the colors in a scene to normalize our perception. This process takes place at a conscious and unconscious level.
3. Prior knowledge: certain objects characterized by a distinct color will always be perceived as this color despite differential illumination (e.g., fire trucks are red).
   

   

To learn more, take a look at this video about color and light constancy with great examples!