Functional magnetic resonance imaging

Functional MRI emerged from a Japanese physicist's observation at Bell Labs that blood changes its magnetic properties when it releases oxygen. In 1990, Seiji Ogawa noticed that high-resolution brain images showed dark lines that appeared and disappeared depending on the imaging sequence used. He had discovered the BOLD signal—Blood Oxygen Level Dependent contrast—and with it, a way to watch the living brain think.

The adjacent possible required structural MRI, powerful magnets, and fast imaging sequences. By 1990, MRI could produce detailed anatomical images, but they were static snapshots. The brain's blood supply, however, was dynamic. When neurons fire, they consume oxygen; fresh oxygenated blood rushes in to replace it. Ogawa realized that oxygenated and deoxygenated blood have different magnetic properties—deoxygenated hemoglobin is paramagnetic, distorting the magnetic field around it.

Working at AT&T Bell Labs with a 7 Tesla magnet—far stronger than clinical scanners—Ogawa demonstrated the BOLD effect in rat brains. When he changed physiological conditions, the dark lines in his images shifted. The blood vessels were becoming visible because deoxygenated blood altered the local magnetic field. More importantly, the effect was sensitive enough to detect changes in brain activity.

Convergent development occurred almost simultaneously. At Massachusetts General Hospital, Jack Belliveau showed in 1991 that cerebral activation could be imaged using injected contrast agents. But the breakthrough came in 1992, when three groups independently demonstrated BOLD imaging in humans without any injections. Ken Kwong at MGH showed visual cortex activation. Ogawa, now collaborating with Kamil Ugurbil at the University of Minnesota, demonstrated similar results at 4 Tesla. The brain's own blood became the contrast agent.

The cascade from fMRI transformed neuroscience. For the first time, researchers could watch which brain regions activated during specific tasks—reading, calculating, feeling emotions, making decisions. Psychology, which had largely studied behavior, could now observe the neural correlates of mental processes. Cognitive neuroscience emerged as a field.

Path dependence favored BOLD imaging over alternatives like arterial spin labeling. BOLD was technically simpler, produced stronger signals, and worked on existing MRI hardware. Once major neuroscience labs invested in BOLD expertise and analysis software, switching costs locked in the technique despite its limitations—BOLD measures blood flow, not neural activity directly, introducing temporal delays and interpretive complications.

Ogawa is regarded as the father of modern functional brain imaging. His 1990 papers describing BOLD contrast became among the most cited in neuroscience. The technology enabled thousands of studies correlating brain activity with cognition, emotion, and disease. fMRI became essential for presurgical mapping, research on psychiatric disorders, and understanding consciousness itself.

By 2026, fMRI scanners are standard equipment in research hospitals and neuroscience departments worldwide. The dark lines Ogawa noticed in his rat brain images led to a technology that lets humanity watch itself think.