He did two crossword puzzles a day, sometimes more, working through the list of clues in strict order, as if to remember where he was.

And, perhaps, what he was doing.

Henry Gustav Molaison  known through most of his life only as H.M., to protect his privacy  became the most studied patient in the history of brain science after 1953, when an experimental brain operation left him, at age 27, unable to form new memories.

Up until his death in a nursing home in 2008, Mr. Molaison cooperated in hundreds of studies, helping scientists identify and describe the brain structures critical to acquiring new information. He performed memory tests; he filled out questionnaires; he sat for brain scans and performed countless research tasks, each time as if for the first time.

In between it all he did puzzles, books upon books of them, a habit he’d picked up as a teenager. Near the end of his life he kept a crossword book and pen with him always, in a basket attached to his walker. His solving opened a window on the brain, and demonstrated puzzles’ power, and their limitations, in stretching a damaged mind.