Jack Belliveau, a Harvard scientist whose quest to capture the quicksilver flare of thought inside a living brain led to the first magnetic resonance image of human brain function, died on Feb. 14 in San Mateo, Calif. He was 55.

The cause was complications of a gastrointestinal disorder, said his wife, Brigitte Poncelet-Belliveau, a researcher who worked with him at the Athinoula A. Martinos Center for Biomedical Imaging at Massachusetts General Hospital. He lived in Boston. His wife said he died suddenly while visiting an uncle at his childhood home, which he owned.

Dr. Belliveau was a 30-year-old graduate student at the Martinos Center when he hatched a scheme to “see” the neural trace of brain activity. Doctors had for decades been taking X-rays and other images of the brain to look for tumors and other lesions and to assess damage from brain injuries. Researchers had also mapped blood flow using positron emission tomography scans, but that required making and handling radioactive trace chemicals, whose signature vanished within minutes. Very few research centers had the technical knowledge or the machinery to pull it off.

Dr. Belliveau tried a different approach. He had developed a technique to track blood flow, called dynamic susceptibility contrast, using an M.R.I. scanner that took split-second images, faster than was usual at the time. This would become a standard technique for assessing blood perfusion in stroke patients and others, but Dr. Belliveau thought he would try it to spy on a normal brain in the act of thinking or perceiving.