What did Henry Molaison’s surgery reveal about the hippocampus and memory?

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Patient H.M. and short-term memory loss explained — hippocampus Learn about Henry Molaison (better known as “patient H.M.”), whose short-term memory loss following a lobotomy helped researchers to understand how brain structures are related to brain function.

In 1953, Henry Molaison (known as "patient H.M.") underwent surgery to remove portions of his medial temporal lobes, including the hippocampus, to treat severe epileptic seizures.

This procedure provided critical insights into the role of the hippocampus in memory:

Anterograde Amnesia: Following the surgery, Molaison retained most of his cognitive functions but lost the ability to form new declarative memories (facts and events). This condition, known as anterograde amnesia, demonstrated that the hippocampus is essential for the creation of new long-term memories.
Memory Consolidation: The case supported the hypothesis that while the hippocampus is vital for forming new memories, it may not be the permanent storage site for all types of information, as Molaison’s older memories remained largely intact.

These observations, which gained prominence in 1957, remain a cornerstone in the study of how the brain processes and stores information.1