HM and the Seahorse - a Story of Memory Discovery Content from the guide to life, the universe and everything

HM and the Seahorse - a Story of Memory Discovery

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The hippocampus in the brain

In 1953, a 27-year-old man entered a hospital for surgery that would cure him of the devastating fits that resulted from his intractable1  epilepsy. Little was he to know that he would become the key to unravelling one of the greatest mysteries about the human brain; how we form our memories.


As the medical profession is sworn to maintain confidentiality, the man in question is always referred to as HM2.

A bright young lad from Hartford, Connecticut, of above average intelligence, HM was an active teenager, who loved ice skating and had an uncanny ability to work out who the killer was in detective shows before the detective. At the age of 16, while out with his parents celebrating his birthday, he suffered his first 'grand mal' fit; the most severe type of fit that anyone can have.

These fits became more frequent, to the point when in 1953 he was having up to 11 fits a week. The drugs available at the time couldn't control them. For a young person, this was devastating. Without any intervention, there was no chance that he would be able to apply for a job, let alone leave the house. It was then that the idea of surgery was floated.

This was not going to be any run-of-the-mill surgery. Then again, this was no normal case. The surgeon in question, Dr William Scoville, presented the family with a radical method.

The Surgery

Scoville's reasoning was that the epilepsy was caused by a small focal point in the brain 'short circuiting', and the electrical disturbances spreading throughout the brain, causing the devastating fits that HM was suffering from. We know now that Scoville was right, and that although we are not really any closer to finding out what really causes epilepsy in the first place, this is the pathway by which people have such violent fits.

For HM, he had the most common form of intractable psychomotor epilepsy, that which is localised in the temporal lobes3. So, to stop the fits from continuing, the only option was remove parts of the these lobes.

In 1953, an apple-sized chunk of his temporal lobes on both sides of his brain were removed. The fits never returned. However, something else, something quite extraordinary, yet equally saddening, happened.

The Seahorse

The temporal lobes aren't just formed out of the cerebral cortex - that white wrinkly part of the brain that is commonly associated with any model of the brain. Peel that away, and you can see some very different structures.

Positioned just underlying the temporal lobes is the hippocampus4; a strange, curved structure which looks like a seahorse in cross-section, hence the name. It was never really known what it was for, until this point. When Scoville removed parts of HM's temporal lobes, he would have had no option but to disturb the hippocampus too. The effect of this on HM was marked.

From 1953 onwards, he couldn't remember anything you told him for any reasonable length of time. Every time a doctor who was assigned to his case came to chat to him, they had to reintroduce themselves every time they met because he couldn't remember who they were. If you talked to him, and a loud noise, say a slamming door, distracted him for a moment, he would have no recollection of what you said to him, moments before.


As word began to spread about the case of HM, he became somewhat of a celebrity among psychologists. After a few tests, what he had lost was whittled down. It wasn't his long-term memory, as he could still remember aspects of his youth. It wasn't his short-term memory either, as he could memorise eight-digit numbers and retain the information for about 30 seconds. Ask him that number in a minute's time, and he had no memory of it. Indeed, he had no memory of the psychologist telling him to memorise it.

What it seemed that he had lost was the ability to convert the short-term memories to long-term memories.

What was finally deduced was that:

  • Short-term memories are processed very differently from long-term memories.

  • The hippocampus is not involved in the retrieval of long-term memories.

  • The hippocampus is not involved in 'procedural memories', those things that you learn by practice, ie learning how to ride a bike.

  • As there was no effect on HM's personality, IQ, or his knowledge of the world, the hippocampus must logically not be involved in those aspects either.

  • Therefore, the hippocampus is involved in the formation, or at least, making sure that the long-term memories are stored in its correct place, the parietal lobe.

As HM had lost his hippocampus, he could form no more new memories.

HM Today

Right now, I'm wondering, have I done or said anything amiss? You see, at this moment everything looks clear to me, but what happened just before? That's what worries me. It's like waking from a dream. I just don't remember.
- HM

At the time of writing, HM is still alive, and stands as living evidence of a biological seat for memory - that indeed it is a concrete, real thing, rather than a philosophical or theoretical aspect of the human psyche.

He still likes detective shows. He likes doing crosswords, and watching TV. However, it is impossible for him to make new friends as he cannot remember a person for any longer than ten minutes. He lives in a world where, for him, Truman is still President5. News of his mother's death evokes the same painful grief for a short period of time, and then, it is gone. He never really knows exactly how old he is, but reckons that he is about 30. When he looks into a mirror, he is shocked by the reflection.

Even with this, he is quite happy, if slightly confused, and quite unaware of the unexpected sacrifice he made that provided the groundbreaking evidence of the link between memory and the brain. As he says:

... what I keep thinking is that possibly I had an operation. And somehow the memory is gone... and I'm trying to figure it out... I think of it all the time. I don't remember this, and why I don't remember that... it isn't worrisome in a way, to me, because I know that if they ever performed an operation on me, they'd learn from it. It would help others.
- HM
1Intractable means incredibly severe with no chance of improvement, even with drug treatment.2He is not a case documented by the psychologist Oliver Sacks in the book, The Man Who Mistook His Wife for a Hat (ISBN: 0330294911). However, the case of 'The Mariner' is a case in the book - and the patient does display some similar symptoms to HM3Here, you can find a diagram of all the lobes of the brain. Epilepsy here would have not only given him writhing grand mal fits, but also hallucinations of smell, sight, taste and hearing, hence the use of the word psychomotor to describe the type of epilepsy he had.4Here, you can find a diagram of where the hippocampus lies in the brain.5Harry S Truman was the 33rd President of the USA from 1945-1953, who was, among other things, instrumental in establishing the Central Intelligence Agency - better known as the CIA, in 1947.

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