Lisa and Peter Marshall were watching a wedding scene on a TV show late last year when he suddenly looked at his wife and exclaimed, “Let’s do it!’ ” “Do what?” she asked. When he pointed to the TV, she asked: “Do you want to get married?” He said yes with a big smile. “Well, okay — we should get married then,” Lisa Marshall recalled telling him. The next day, she wondered whether her husband would remember asking her, and he didn’t. He also didn’t remember their first wedding, said Marshall, and for the past six months he has simply thought of her as his favorite caregiver.