The patient who could learn skills but not remember facts

In the 1950s, a man researchers called H.M. had surgery that took out his hippocampus. The hippocampus is a key structure in the brain that helps shift information into lasting storage.

The researcher who studied him was Brenda Milner. After the operation, he struggled to retain recent information for even a short time, and his capacity to build enduring memories was impaired as well. But he could still pick up movement based tasks by practicing them repeatedly, and he got better with practice.

Forgetting is fast and common. In the 1800s, the psychologist Herman Ebbinghaus showed that people typically lose 40 percent of fresh lessons during the opening 20 minutes, a pattern called the forgetting curve. That fading can be held back when memories are stabilized, the shift from fleeting short term storage to more durable long term storage.

What the study of H.M. revealed was a split in how that stabilizing works. The hippocampus played a particular role in cementing long term declarative memory, the facts and concepts you might need for a test, rather than procedural memory, the finger movements you might need to master for a recital.

Milner's findings, along with research by Eric Kandel in the 1990s, gave scientists their current model of the process. Sensory information is first copied and held briefly in nerve cells as short-term memory. Then it is sent toward the hippocampus, which bolsters and enriches nerve cells in the cortex, the outer layer of the brain.

Thanks to neuroplasticity, the brain's ability to rewire itself, new connection points called synaptic buds form. Those buds allow new links between nerve cells, and strengthen the web where the material is eventually kept as enduring memory. In H.M., without the hippocampus, that strengthening could not fully happen for facts and ideas, while learning by repetition still worked.