Some profoundly blind people can "see" after all - although not in the way we traditionally think of vision. Cells at the back of their eyes monitor light levels and use them to set the body's clock to either night or day.
The new insights come from experiments involving two blind people who lack the rods and cones needed for normal vision. Although neither person could see anything, they unconsciously sensed whether it was light or dark by detecting blue light.
While this had been expected from earlier work in animals, the experiments provide compelling evidence for the way in which the brain tells night from day. The cells responsible are called retinal ganglion cells and are located in the outermost layer of cells lining the retina at the back of the eye (see Diagram). Retinal ganglion cells usually help to transmit visual information to the brain.