Summary
Highlights
On September 1st, 1953, neurosurgeon William Scoville performed a risky surgery on Henry Molaison (H.M.) to alleviate his severe seizures that began after a childhood accident. Scoville removed portions of H.M.'s brain, including the hippocampus, a part of the limbic system.
Initially, the surgery seemed successful, drastically reducing H.M.'s seizures without personality changes or a drop in IQ. However, H.M. developed profound amnesia, losing most memories from the preceding decade and, critically, becoming unable to form new ones. He would forget events minutes after they occurred, unaware of recent conversations or repeated actions.
Neuroscientist Brenda Milner began studying H.M., whose case was brought to attention by Scoville. Through a series of tests, Milner discovered that H.M. could retain information in his short-term memory through constant repetition, but would forget it within minutes if distracted. This was the first empirical evidence distinguishing short-term from long-term memory and demonstrating that different brain regions govern them.
Milner's findings revealed that the hippocampus is crucial for memory consolidation, transferring temporary memories from the cortex to permanent storage. Without his hippocampus, H.M. could form initial impressions, but these could never be solidified into lasting memories, illustrating its vital role in creating long-term memories.
Milner also uncovered the difference between declarative (knowing 'that') and procedural (knowing 'how') memory. In an experiment, H.M. showed improvement in a mirror-tracing task over time, despite having no conscious memory of ever performing it before. This demonstrated that his unconscious motor centers remembered the skill, indicating that procedural memory relies on other brain structures like the basal ganglia and cerebellum, which were intact in his brain.
H.M. died at 82, having been studied by over 100 neuroscientists, making his the most researched mind in history. After his death, his brain was preserved, scanned, and meticulously sliced for digital mapping down to individual neurons. His unique case and the extensive research conducted on him fundamentally reshaped our understanding of memory and brain function, leaving an indelible mark on neuroscience.