Samuel Williamson, who died April 25 at age 65, was a pioneer in mapping the activity of the brain by measuring the faint magnetic fields it creates.

The technique, called MSI for magnetic source imaging, has proven especially useful for observing such forms of mental functioning as “silent speech,” and for locating the parts of the brain that respond to faces, music, and other perceptual data.

In the late 1970s, Williamson founded the New York University’s Neuromagnetism Laboratory, where he began to utilize superconducting quantum interference devices to detect magnetic fields generated by the brain when it processes information. According to one observer, the device looked like “a hair dryer from hell,” but it enabled Williamson to map neural activity in three dimensions.

Investigating the brain’s responses to auditory stimuli revealed one of the odder phenomena that came to light thanks to MSI: that different regions of the brain respond to soft and loud noises. An investigation of sound and memory found evidence that a tone produces an auditory memory which decays over time.

Before Williamson’s MSI research, brain activity was conventionally monitored via the electroencephalogram, which was a cruder instrument.

Williamson also investigated the possibilities of treating the blood disorder thalassemia through magnetism.

Williamson was born on November 6, 1939, in West Reading, Pa. He received a doctorate in physics from MIT in 1965, and worked there until he came to NYU, in 1971. He was named university professor in 1989.

He is survived by his wife, Joan, a retired Long Island University professor, and his brother, David.