A New York Times profile earlier this week on Brenda A. Milner, Sc.D., credited her with changing “the course of brain science for good as a newly minted Ph.D. in the 1950s by identifying the specific brain organ that is crucial to memory formation.”
Milner, a Dana Alliance for Brain Initiatives member, identified the hippocampus and other areas of the brain that process memory while working with Henry Molaison, more commonly known as H.M., who developed amnesia at age 29 from the removal of tissue from both his medial temporal lobes, a surgery that was supposed to alleviate his epilepsy.
With the help of H.M., Milner discovered that memory was processed in the medial temporal lobes, which was why he could no longer form new long-term memories. He was, however, able to learn new tasks, meaning he must remember the actions in another part of the brain. His ability to acquire new skills proved to Milner there are two types of memory that occur in different parts of the brain – explicit memory, which recalls describable details like facts and events, and implicit memory, for unconscious memories such as actions and procedures. Milner described this finding in a 2010 Dana Foundation interview:
I went to the McGill psychology department and borrowed learning tasks to give him [H.M]. I took down a maze task, which I was sure he wouldn’t learn, and he didn’t. It was a nice control test, because he showed absolutely no progress over three days. Then I gave him the mirror drawing task. H.M. did 30 trials over three days and at the end of the last trial, his performance was absolutely perfect. I can still remember him looking at what he had drawn, saying: “This is strange. I thought this would be difficult, but it looks as though I’ve done it rather well.” I was very excited because it showed that he could have this excellent performance without any awareness that the reason he was doing so well was that he had had the chance to practice the task over three days.
When I saw that H.M. had this beautiful learning of something he had no memory of having acquired, I then speculated that this task, which involved motor learning, depended on a different system in the brain. His surgeon had damaged his medial temporal system, but this was a kind of learning that was unaffected by this operation, so therefore it must involve other structures.