Before the brain can begin to interpret visual input, light that is focused by the lens of the eye onto the retina must be converted to neural signals that can be relayed to the brain. The retina is actually a small outpouching of brain on the back of the eye that connects directly to a two-dimensional array of cells called photoreceptors. This array is in some ways analagous to the the array of photodetectors in a handheld video camera. Each receptor indicates the light level for its very small portion of the image as do the camera detectors for each pixel.
When light is absorbed by pigment molecules in the photoreceptors, the configuration of these molecules changes and they can become “bleached.” This configuration change leads to a cascade of other molecular events which cause a change in potential (voltage) in the cell. This change in potential causes a release of chemical transmitters that excite neurons within the retina which then relay the signals to the brain.
By staring at a single image for a long period of time, you are differentially bleaching sections of your retina. When you close your eyes, all of the photoreceptors are getting about the same input. But those areas that were recently bleached by exposure to brighter parts of the image are not as sensitive because they have less pigment available, and so they appear relatively darker than those areas that were not exposed to the bright areas. In short, the bright areas appear dark and the dark areas appear light.
The story is a bit more complicated because there is more than one type of photoreceptor in the retina; there are rods and three types of cones. The rods are sensitive to low levels of light but are not sensitive to any particular color. They produce the black and white image of the face. The three types of cones have pigments sensitive to three different colors, red, green and blue. In the image, the colored areas differentially bleach the different types of cones, so when you close your eyes what you see is the “negative color” which is red for green and blue for yellow.
You might also have noticed that the formation of the negative color image and the negative black and white image did not appear at the same time, nor did they fade away at the same time. The rods and cones differ in properties other than color sensitivity, such as temporal response and contrast sensitivity. For this reason, they are not evenly distributed over the retina and serve different functions within the visual system. We will learn more about these parallel systems as we continue to explore vision in the brain.