We can see because neurons in our eyes take in visible light and relay electric signals to the brain. But some of the neurons in our retinas detect light that we cannot actually see. In fact, people who lose all their other retinal cells except these neurons are blind. If you shine a light in their eyes and ask them to guess the color, however, they guess very well. It turns out these neurons feed this invisible light to many parts of the brain. In my latest column for Discover, I take a look at this hidden light. Check it out.