fMRI relies on changes in the natural magnetic properties of blood cells that carry oxygen. When brain cells are active and firing away -- when they are responding to visual or other stimuli -- they use energy and they need oxygen. The blood flow increases near active areas of the brain to meet the increased energy demands of the brain cells. The magnetic signals from blood cells carrying hemoglobin molecules filled with oxygen are different from the signals of oxygen-depleted cells.

fMRI is considered safe and "noninvasive." Subjects are placed in a magnetic field, but the field is not strong and not thought to be dangerous. The hemoglobin molecules serve as internal magnetic markers, so subjects are not exposed to dangerous radioactive dyes or "contrast agents."

Doctors have used MRI (without the "f") for many years to look at normal brain structures and to detect tumors in the brain. What is new is the application of the technology to studies of brain function. The magnets that are used are already in place in most hospitals, so adapting them to fMRI studies is, according to Eden, "just a matter of adding a bit more equipment to the system."

fMRI will provide insights into both the normal activities of human brains and changes in their activities as they mature or are altered by injuries and diseases.