They are light sensitive but not color sensitive. That's why inverted images look black and white. The human visual system is very efficient at processing information. When we see a face, for example, the eyes are usually the first thing people notice, so it makes sense that neurons would be activated when viewing an image of eyes. Neurons that respond to light coming from the back of the eye would be used by the brain to recognize faces because people have two eyes on their head and they want to make sure they are getting all the information they can from each eye. Inverted photographs are useful tools for scientists to study how the brain processes images. Some experiments have shown that if you show subjects inverted images of objects such as chairs or houses, they tend not to recognize them later. This may be because the brain is using information from the whole image to make a judgment, not just the shape of the object. In other words, the eyes and the rest of the image contribute to what the brain decides is a "chair." Inverted photographs have many other applications too.