His doctor, the prominent brain surgeon William Beecher Scoville, suctioned out two slug-sized slivers of tissue, one from each side of the brain. The operation controlled the seizures, but it soon became clear that the patient could not form new memories.

“He loved to converse, for example, but within 15 minutes he would tell you the same story three times, with same words and intonation, without remembering that he’d just told it,” said Suzanne Corkin, a neuroscientist at the Massachusetts Institute of Technology who studied and followed Mr. Molaison in the last five decades of his life.

Each time he met a new acquaintance, each time he visited the corner store, each time he strolled around the block, it was as if for the first time.

Before H.M., scientists thought that memory was widely distributed throughout the brain, not dependent on any one area. But by testing Mr. Molaison, researchers in Montreal and Hartford soon established that the areas that were removed — in the medial temporal lobe, about an inch deep in the brain level with the ear — are critical to forming new memories. One organ, the hippocampus, is especially crucial and is now the object of intense study.

In a series of studies, Mr. Molaison soon altered forever the understanding of learning by demonstrating that a part of his memory was fully intact. A 1962 paper by Dr. Brenda Milner of the Montreal Neurological Institute described a landmark study in which she had Mr. Molaison try to trace a line between two five-point stars, one inside the other.

Each time he tried the experiment, it seemed to him an entirely new experience. Yet he gradually became more proficient — showing that there are at least two systems in the brain for memory, one for events and facts and another for implicit or motor learning, for things like playing a guitar or riding a bicycle.

In the new brain-mapping project here, set to catalog many donated brains, scientists will have the ability to study areas of Mr. Molaison’s brain at a level of detail that imaging cannot reveal, to solve lingering mysteries about the man and the brain.