Henry M., referred to in the psychology community as "H.M.", was 27 in 1953 and suffered from epilepsy. He underwent an experimental treatment by Dr. William Scoville: to treat Henry's temporal lobe epilepsy, Scoville simply removed a fist-size chunk of the temporal lobes, which included the hippocampus, the amygdala, and the entorhinal and perirhinal cortices.
The epilepsy is cured, but an unforeseen side-effect made Henry the most important contributor to the modern theory of memory, even though he is completely unaware of that fact. After the surgery, Henry stopped being able to form any new long-term memories. He can hold short-term memory as well as the average human being, but those information cannot get committed to the brain for later recall, a condition which led to the currently accepted hypothesis that the hippocampus is sort of a gateway that moves information from short-term memory into long-term storage.