I first met Henry Molaison more than half a century ago, during the spring of my third year in graduate school. I have tried to resurrect the details of my interactions with him that week, but human memory does not allow such excursions. The explicit minutiae of unique episodes fade as time passes, making it impossible for us to vividly re-experience the details of events in the distant past. What I do know is that I was very excited to have the opportunity to study such a rare case as Henry, and I had spent months preparing. Looking back at the results of all the tests he did that week, it was clear even then that the consequences of the operation carried out on him in 1957 – an experimental procedure to cure his epilepsy – had been catastrophic. Henry was left in a permanent state of amnesia, unable to retain any new information.

At the time of Henry’s operation, little was known about how memory processes worked. The extensive damage to the inner part of the temporal lobes on both sides of Henry’s brain made him a vital case study for memory researchers then and now. As the years passed, his fame grew and eventually spread to countries outside North America – and all that time Henry was stuck in the same moment. From time to time, I would tell him how important and well known he was, and he would smile sheepishly, as the praise was already slipping out of his consciousness. In his lifetime he was known as HM; only after his death, in 2008, was his identity revealed to the world.