Patient H.M.

Today’s blog post is about one of the most studied individuals in psychology and neuroscience. By studying him, scientists were able to massively expand their knowledge of how to human brain is structured, and how different abilities (or “functions”) are located in different cortical areas.

H.M. was born in 1928, and was 10 when he first started having epileptic seizures. These were extremely debilitating, and although several medications were tested, none had any affect. When H.M. was 27, a pioneering neuroscientist called William Scoville worked out where the seizures in H.M.’s brain were coming from, and decided that the best way to stop them was to operate, and cut out the parts of the brain that was responsible.

In terms of removing his seizures, this surgery was largely successful. However it came at a cost: H.M. could no longer form new long term memories, nor remember anything from X years before his operation. The image below is a scan taken of H.M.’s brain, and shows the lesions made during surgery, and how this differs from a normal brain scan.

As you can see from this image, H.M. was left with extensive damage to the central parts of his brain – this area is known as the medial temporal lobe. By analysing the brain damage, neuroscientists were able to make inferences about where certain brain functions are located. As H.M.’s memory was impaired, but other cognitive functions such as language were not, the medial temporal lobe was identified as being important in the formation of long term memories.

Perhaps unsurprisingly, things aren’t as simple as this. H.M.’s long term memory was affected, so severely that he was unable to remember things that happened a few minutes ago. However his short term memory was intact, with a normal digit span (a string of numbers that you can keep in your mind at once) of 7 +/- 2. Therefore, the structures damaged can’t be involved in short term memory.

There are also distinctions that can be made within long term memory. This can be divided into 3 different types of memory: semantic, which is general knowledge about the world; episodic, which are memories about ourselves and our lives; and procedural, which are learned physical movements e.g. riding a bike. Only H.M.’s episodic and semantic memory were damaged, which shows that our procedural memory must be located elsewhere. Other brain areas such as the cerebellum have been identified as involved in this. Not only was his procedural memory intact, he could also improve it by practicing new movements over time. The image below shows a mirror drawing task, where participants have to trace an image by only looking at its reflection in a mirror.

H.M.’s performance improved each time he did this task, even though he had no recollection of ever doing the task before! This illustrates nicely the different between the automatic, learned ‘procedural’ memories, and the episodic memories about previous experiences.

One last question remains – why did H.M. lose his episodic memories from the years before the operation, but not ones from when he was much younger? There are several theories for this, with one being that older memories are ‘consolidated’ into the rest of the cortex – only newer memories remain in medial temporal structures such as the hippocampus. Therefore, when this area of brain was destroyed, so were the newer long term memories.

When H.M. died in 2008 aged 82, scientists were able to reveal his real name – Henry Molaison. He made a massive contribution to the field of neuroscience, and is thought to have been one of the most tested patients in medical history.

For those interested in reading more about H.M., I would recommend this article, written while he was still alive: Corkin, S. (2002). What’s new with the amnesic patient HM?. Nature Reviews Neuroscience, 3(2), 153-160.

And finally, I’ll leave you with a quote from H.M. himself, when he was asked “Are you happy?”