On Tuesday evening Henry Gustav Molaison — known worldwide only as HM, to protect his privacy — died of respiratory failure at a nursing home in Windsor Locks, Conn. HM was the famous patient of McGill neuroscientist Brenda Milner. In the 1950s, he had epilepsy surgery and could not form new memories but he could learn new tasks. Her work with him delineated memory formation. “The study of HM by Brenda Milner stands as one of the great milestones in the history of modern neuroscience,” said Dr. Eric Kandel, a neuroscientist at Columbia University. “It opened the way for the study of the two memory systems in the brain, explicit and implicit, and provided the basis for everything that came later — the study of human memory and its disorders.” Henry Gustav Molaison, born on Feb. 26, 1926, left no survivors. He left a legacy in science that cannot be erased.