Brain facts

Why is the hippocampus important in memory formation?

In my last week’s post (if you missed it, have a look here), while I was talking about how the first studies on memory started and the animal model used, I mentioned a particular structure of the brain called hippocampus. Since I have been mentioning different areas of the brain also while I was presenting to you the neuromodulators used by the brain to adjust the electric impulse our neurons use to communicate (have a look here in the brain facts area if you are interested in knowing more about them), I thought it would be useful to start describing a bit these areas before moving on with the story of my studies and finally leaving the Italian University to start moving around European cities for my studies.

So let’s start talking about the hippocampus.

The first that identified this cerebral structure was the Venetian anatomist Arantius in the XVI century and he gave it the name of hippocampus (from the Greek ἱππόκαμπος, “seahorse”) due to its particular shape: a curved and convoluted shape that inspired a comparison with the shape of the folded back forelimbs and webbed feet of the mythological hippocampus, a sea-monster with a horse’s forequarters and a fish’s tail (have a look at the picture below for a comparison of the hippocampus with a seahorse).

The human hippocampus and fornix (left) compared with a seahorse (right). From wikipedia.com

The hippocampus is a cerebral structure included deep into the temporal lobe of the cerebral cortex. It belongs to the limbic system, an area of the cerebral cortex that regulates motivation, emotions and learning and memory. Today, the role of the hippocampus is well recognised, and it is considered the main structure implied in memory formation. However, it has been demonstrated how the hippocampus plays a pivotal role in other processes, such as spatial orientation. Nevertheless, today I would like to focus on how researchers discovered that the hippocampus is important in memory formation and, to do so, I have to introduce the patient H.M., the most studied medical case in the history of neurology.

H.M (Henry Molaison), is an American patient that, according to what his mother reported, banged his head hard after being hit by a bicycle rider in his neighborhood near Hartford at age of 9. When he woke up, he started to suffer from a pharmaco-resistant epilepsy that got worse over the years, up to the point of becoming a generalized epileptic crisis during adolescence, making him unable to work. After spending 20 years with such a condition, in 1953 he got reported to the neurosurgeon William Scoville. On the 1st of September he underwent a surgery that removed a deep portion of his temporal lobe bilaterally: the extracted area comprised the hippocampus, the amigdala and the adjacent areas, including the entorhinal cortex (see picture below for a schematic of the brain areas removed). Indeed, the surgery was successful! He did not have any more epileptic crisis, but the psychological exams revealed an alarming effect: when asked what day it was (26th of April 1955) he replied “March 1953” and he reported to be 27 years old, instead of 29. Moreover, he did not have any recollection of his surgery and every time he was starting a new action, he was forgetting what he was doing before. However, he remembered all the events of his life before 1953 and his mental abilities slightly improved.

Schematic of H.M brain after surgery and bilateral removal of the hippocampus, amigdala and adjacent areas, versus a normal brain. Modified from mikeclaffey.com

Back then, there was the theory that memories formed and stabilized diffusely in the cerebral cortex. Nevertheless, today we know that the hippocampus and the hippocampal formation (that is the area surrounding the hippocampus that includes the entorhinal cortex) are the areas involved in sorting out the short-term memories and their chance to become long-term ones. More specifically, the complex formed by the hippocampus and the entorhinal cortex play the role of forming and consolidating explicit memories (such as autobiographic -episodes recollected from an individual’s life- and episodic memories -personal experiences and specific objects- ) and also in acquiring the meaning of new words. On the other hand, the procedural memory (the memory involved in motor abilities) is stored and ruled by other areas: indeed, Molaison was able to learn new and complex motor abilities, even though he did not have any recollection of how nor when he learned them.

Henry Molaison died in Hartford, Connecticut, on the 2nd of December 2008 in a retirement home, after being studied for over 50 years. He was also studied by the Canadian neuropsychologist Brenda Millner, who thanks to him was able to distinguish, and later on define, two different systems of memory (the episodic memory and procedural one). Even though Henry Molaison was clearly not able to live autonomously, he was described as a pleasant and patient man, despite the inability of creating a real relationship with him. From 2009, a 3D digital map of his brain his available for free (you just have to register) here.

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