When Henry Molaison (now widely known as H.M.) cracked his skull in an accident, he began blacking out and having seizures. In an attempt to cure him, daredevil surgeon Dr. William Skoville removed H.M.'s hippocampus. Luckily, the seizures did go away — but so did his long-term memory! Sam Kean walks us through this astonishing medical case, detailing everything H.M. taught us about the brain and memory. [Directed by Anton Bogaty, narrated by Addison Anderson].

Sam Kean: What happens when you remove the hippocampus?

What happens when you remove the hippocampus?

H.M.'s case unveiled procedural vs. declarative memory, showing our brain's complex systems for storing different types of knowledge.

Removing H.M.'s hippocampus stopped his seizures but erased his ability to form new memories, revealing brain memory processes.

H.M.'s case split memory into two types: procedural, handled by basal ganglia, and declarative, needing an intact hippocampus.