Medical imaging has come a long way since 1895, when german physicist Wilhelm Conrad Rntgen observed strange flickers cast by his cathode-ray instruments. Within months, Rntgen had used the mysterious "x-rays," as he called them, to produce an image of the bones of his wife's hand, revolutionizing medicine. For the first time, physicians could peek inside the body without cutting it open or probing an orifice. Today they can practically image our thoughts.

One of the latest technologies for seeing under our skin-functional magnetic resonance imaging (fMRI)-uses the combination of a powerful magnet and radio frequency pulses to see which parts of the brain are active. Neurons themselves are too small to image, but their activity causes changes in the flow of oxygenated and deoxygenated blood around them. For example, when you hear a loud noise, a patch of neurons fires on each side of your brain. Their activity requires an increase in blood supply. The oxygen-rich inrushing blood has different magnetic properties than the deoxygenated blood that it displaces. The magnet and the radio signals inside the functional MRI scanner work together to reveal where blood is rich in oxygen and where it is not. The resulting image shows the two patches of neural activity as bright regions on either side of the brain. From such maps, researchers can determine which parts of the brain are used for speech, vision, auditory and motor skills, and more.