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The curious case of patient HM and his memory loss


The curious case of patient HM and his memory loss

KAILAS ROBERTS outlines the ‘surgery gone wrong’ in the 1950s that provided researchers with a fresh understanding of how new memories are formed.

Scientific progress is fundamentally based on observation. A person must notice something about a situation or process and make an inference.

If it is observed that when A occurs, B also happens, this may lead the observer to surmise they are connected. An obvious case of this is that when someone smokes over a long period, they are more likely to develop cancer.

But the observation does not mean that one causes the other – just that there is an association. To prove so-called causality, you have to design an experiment that removes other influencing factors.

For example, smokers may also eat more poorly, and perhaps that increases the risk of cancer – meaning that you must ensure the participants in the study all eat the same food.

Sometimes, you can’t conduct an experiment like this, however, and have to rely on other things such as the magnitude of the connection between A and B, whether it is biologically plausible and whether one thing always precedes the other.

Interestingly, there has never been an experiment proving smoking causes lung cancer. This would involve asking one group of people to smoke, and another not to, and then measuring the rates of cancer in the two groups.

Given that observational data so strongly supports a link, it would be unethical to make any of the study participants smoke at all.

Despite this limitation, there is universal consensus that smoking does, indeed, cause lung cancer.

When it comes to understanding how we remember things, a critical piece of observation came from a surgical error in 1953. The victim, for many years known only as ‘HM’, suffered severe epilepsy, and underwent an operation to prevent him having seizures.

The operation was a success in this way (his seizures stopped), but a disaster in another. As he recovered from his operation, his doctors noticed he could not remember things.

Well, that’s not quite true. He could remember the bulk of things that had happened in the time before the operation, but his ability to remember anything new was obliterated.  Famously, he described his experience as “like waking from a dream … every day is alone in itself”. He could not learn new words, facts or faces, and he would even forget who he was talking to the moment he walked away.

This catastrophe occurred because his surgeon had removed 8cm of tissue from his hippocampus: a small seahorse-shaped structure in his temporal lobe.

Up until that point, the importance of this structure was not known, but this ‘surgery gone wrong’ created a fresh understanding of how new memories are formed.

Observations about HM led to huge amounts of research focusing on this part of the brain (and on HM himself), and the hippocampus has come to be widely recognised as critical in forming new memories.

HM was still able to complete tasks, and even get better at them over time, making it clear that the hippocampus is less important for these so-called procedural memories.

These include riding a bike, driving a car and other ‘auto-pilot things’ that we do.

We now know that the hippocampus is one of the first areas of the brain to be affected in Alzheimer’s disease, leading to the typical forgetfulness of recent events, and that other areas of thinking can be relatively well preserved early on.

HM went on to live for many decades, dying in 2008. Although there is, of course, a tragedy to his story, at least his legacy has been one that has enriched medical research and will hopefully in the end hasten our progress in finding a cure for this difficult disease.

Kailas Roberts is a psychogeriatrician and author of Mind Your Brain: The Essential Australian Guide to Dementia available at all good bookstores and online. Visit or

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