In the last two decades, new scanning technology has given us a better idea of what is happening in the brain as we process visual information.
One of the discoveries that has come out of these new data is the realization that that visual processing divides into two streams, which end up in very different parts of the brain.
According to Dr. Margaret Stratford Livingston, Professor of neurobiology at Harvard University, the visual brain processes tonal information separately from color information.
(Addendum: I changed the image thanks to a different way of processing them--Thanks, Nick! The image on the left has only luminance, but the one on the right has color information with luminance removed.)
The two streams originate in the retina, which begins some low-level processing. The information pathways route back to the optical cortex at the back of the brain. Although there is some crossover and interaction, the two streams are largely kept separate, from the level of the retina all the way to the higher-level vision centers of the brain.
The area of the brain that interprets tone is several inches away from the area that interprets color, making the experience of tone and color distinct physiological experiences, as distinct as sight and hearing.
The color stream (in blue), is also called the ventral stream or the “what” stream. It is more concerned with recognizing, identifying, and responding to objects. Color processing through the ventral stream is a capacity that is shared only by higher primates, not the bulk of other mammals.
I asked Dr. Livingstone if she would describe more differences between the "where" and "what" streams? In particular, is one stream more associated with emotional response or higher cognitive function?
“I don't know about emotions," she said, "but the dorsal (what) stream is certainly more associated with higher, conscious functions and awareness.” (Addendum: I believe Dr. Livingstone misspoke here, and meant the ventral stream.)
The difference between these two streams may explain why classically-trained artists use a strategy of planning the tonal organization of their compositions separately from the color scheme.
"The Artist at the Easel” by Sadie Dingfelder, February 2010 Monitor magazine of the American Psychological Association.
Two-Streams Hypothesis on Wikipedia