So I used his Mondrian display to study the single cells in area V4. In this way, Zeki discovered that some of the cells in area V4 consistently respond to the actual surface color of a Mondrian patch, regardless of the lighting conditions. He believes these are the cells that perform color constancy. More recently, with the aid of PET scans, Zeki found an area similar in location to the monkeys' V4 that is specifically activated in humans when they look at Mondrian color displays. The color displays also stimulate the primary visual area and an area that is adjacent to it, V2.
Much controversy exists about all aspects of the color pathway beyond the retina, however.
Researchers disagree about the exact role of cells in human V1 and V2, about the importance of V4, about the similarities between monkey and human brains. To resolve such issues, scientists await the results of further experiments on humans. The new, noninvasive brain-imaging techniques that can show the brain at work may supply key answers. Within a few years, researchers hope, these techniques will reveal the precise paths of the neural messages that make it possible for us to see the wealth of colors around us. For more information on Jeremy Nathans and the study of vision, see 1997 Holiday Lectures .