31 March 2011

Why everyone should know about the hippocampus, Part I

I’ve been thinking about how to tell you about the hippocampus.  I had never even heard of it before four years ago, but it’s now my favorite brain region.  I have no less than four pairs of seahorse earrings expressing my dedication to this brain region (explanation to follow).  I don’t want to lose your attention by talking about the hippocampus, so I decided a limerick would make it more palatable.  Here it is:

HM was a man in great pain –
Epilepsy was killing his brain.
With hippocampi removed,
His epilepsy soothed,
But his memory he never regained.

Ta da!  Did you like it?  If you remember any of it by the time you reach the end of this page, you can thank your hippocampus.

Henry Gustav Molaison (or HM, as he was known to science before his death in 2008) was arguably the most studied man in the history of brain science.  In 1953, at the age of 27, his epilepsy was so intractable and profound that it was ruining his life.  The focus of his seizures turned out to be the hippocampus – the two hippocampi on either side of his brain were affected.  So, after all other efforts to quiet his seizures failed, the decision was made to remove both of them.

This was not uncommon practice – well, removing one was not uncommon, and it seemed to have little detrimental effect.  So the tissue surrounding and including both his hippocampi were removed in the hope of relieving his epilepsy.

From the moment HM woke up from anesthesia, he could no longer translate any of his experiences into memories.  If he stayed engaged he could hold onto a thread for minutes at a time, but as soon as he got distracted he lost it.  From the time of his surgery until his death at the age of 82, he lived in increment spans of a few minutes.  And his memories of times before the surgery were partly lost as well, with a preference toward sparing of his oldest, farthest-back memories.

There’s a lot of argument about exactly what the hippocampus does, how it does what it does and what that means for our memories and our ability to retain and later recall them.  I won’t bore you with that (no matter how much I want to!).  So here’s what you need to have under your belt at cocktail parties (or at least the dork-fest kind of cocktail parties I prefer to attend):

-          The hippocampus is a seahorse-shaped structure embedded in the temporal lobe (the lobe on the side of your head, above your ears and behind your temples).  Because it is shaped like a seahorse (genus: Hippocampus; from the Greek hippos, horse, and kampos, sea monster), it is called a ‘hippocampus’.

Stolen straight from Wikipedia

-          The hippocampus is the brain structure that allows you to encode ‘episodic memory’ – your memory for events or episodes.  This kind of memory is what most people generally mean when they talk about memory.

-          Not surprisingly, the hippocampus is one of the first structures damaged in Alzheimer’s disease.

-          People with hippocampal damage generally experience (a) anterograde amnesia: the inability to form new memories; and to some extent (b) retrograde amnesia: the inability to recall episodic memories formed before hippocampal damage took place.

-         These individuals seem to have intact knowledge of any facts (‘semantic memory’) learned prior to hippocampal damage, but are impaired at – if not incapable of – learning new semantic information.  For instance, even after decades testing with the same experimenters, HM never learned their names or even recognized them when they entered the room.

-          People with hippocampal damage can learn new tricks, so to speak.  They have intact ‘implicit’ or ‘non-declarative’ learning – things like riding a bike or more readily seeing patterns one’s been shown before.  After a lot of practice HM became quite skilled at a mirror-drawing task in which he had to trace the outline of a picture he could only see through a mirror.  When asked how he was so good at it, he guessed it was just one of those things, maybe he just had a talent for it – he had no memory of having ever performed the task before!

As humans, we place a great deal of stock in our memories – so much so that it’s not uncommon to hear it said that our memories make us who we are.  The hippocampus – this tiny little seahorse-shaped brain structure resting so unobtrusively in our skulls – is no small part of what makes us who we are.  And that’s why it’s important to know about it.


  1. Cute limerick! Can I use it to introduce L and M to my kids on Monday? I always talk about what happened to poor HM. I promise to give you proper credit :)

  2. Oh, please do! I'd love that :). Everyone ought to know about HM!