How memory works: why you can be sure of something that never happened
The hippocampus: why one man could not keep a single new memory
In 1953, a surgery stopped the seizures of a young man named Henry Molaison. He kept his past, and lost something nobody expected.
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1953. Henry Molaison is 27, and his seizures are so strong that no medicine stops them. A surgeon takes out a small part, deep inside each side of his brain.
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The seizures ease, but something else is gone. For years he meets the same doctor again and again, and each time he says hello to a stranger.
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Yet he still knows his parents and the house where he grew up. Most of his old memories are still there. Only new ones do not stay.
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The part he lost is the hippocampus, one on each side of the brain, named after the seahorse for its shape. Without it, a new memory cannot hold together.
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Your weekend by the lake is kept in pieces, in different places of the outer layer of the brain, the cortex. At first, the hippocampus is what ties them together.
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Over the weeks, the pieces build their own links and need the hippocampus less and less. This settling is called consolidation, and Henry's old memories had settled long before 1953.
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A year later, your friend Miva tells the story of the weekend, sure of every piece: the campfire, the storm, the snapped poles. Rono's video shows the storm folding his tent flat, but no campfire and no broken pole. So how did pieces that never happened get into a memory that had settled?
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Quick check
A man hurts both sides of his hippocampus in an accident. Which is he more likely to lose: his wedding day ten years ago, or the dinner he eats a week after the accident?
Show the answer
The dinner. A new memory needs the hippocampus to hold together, and the wedding settled in the cortex long ago.
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Go further
How Henry was known for fifty years, and the scientist who learned the most from him.