Although the blue-green color comparison method used by the parietal eye is not one shared by humans, it does reveal one potential step in the evolution of color vision, the Hopkins researchers say.
Human light-reception cells responsible for color vision are called cone cells or photoreceptors, and they contain only one kind of pigment per cell - red, green, or blue. A color image results when light-triggered signals in the three different types of cone cells are compared by other nerve cells in the retina as well as the brain.
The lizard's parietal eye photoreceptors contain two pigments per cell, blue and green. Having two different pigments allows the cell to respond to two different colors of light and process that information within the same cell.
According to the researchers, when the lizard's third eye sees blue light, the blue pigment triggers a molecule called gustducin, which is very similar to a molecule found in human photoreceptors as well as the lateral eyes of the lizard - those on the sides of its head. But when the lizard's third eye sees green light, the green pigment triggers a different molecule called Go, known as "G-other," which also signals light responses in the light-sensing cells of the scallop and other creatures without a backbone. That Go is found in spineless creatures suggests it is the evolutionarily more ancient light-triggering signal.
Although gustducin and Go are different molecules, they are similar and considered
Source:Johns Hopkins Medical Institutions