In the early 1950s the parents of Henry Molaison (who to protect his identity was long referred to in the scientific literature as HM), desperate to help their son who had been suffering from epileptic seizures, brought him to a neurosurgeon. The surgeon convinced them that he should perform a radical new surgery in which he would remove HM’s hippocampus and part of his medial temporal lobe. HM was 27 at the time. The operation was a “success.” It eliminated HM’s seizures but it had some drastic unintended consequences. From that point forward Henry suffered from retrograde and anterograde amnesia. He couldn’t remember much of the recent past (retrograde amnesia) and he couldn’t form new memories (anterograde amnesia). Up until that point no one had any idea that the hippocampus had anything at all to do with memory – but because of HM it was realized that it played a critical role in memory formation.
HM, to his credit, allowed himself to become a study subject. What scientists found was fascinating, with strong implications for learning:
- HM’s temporally graded retrograde memory loss extended in some cases up to 10 years. But beyond 10 years he had memory of the past. We now know that memories are continually formed, processed and re-formed until they are consolidated. Full memory consolidation can take up to 10 years! Think about that the next time you are sure your learners have fully remembered something. Reinforcement over time is critical.
- HM could learn to perform certain tasks but couldn’t remember that he had learned to perform them. It turns out that there are two major types of long-term memory: declarative (what you know) and procedural (what you know how to do). So knowledge and skills are processed and stored in different ways and probably in different places in the brain.
HM died in 2008 and his brain, which he left to science, continues to be studied today.