How memory works, learned from a person with half a brain

September 18, 2015

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How half a brain helped us understand remembering, and a magic number that determines how much we can hold on to.

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This article has been updated, but first appeared on our predecessor website, The Dirt Psychology.


In 1953, surgeons removed almost half of Henry Molaison's brain. Henry (or 'H.M.' as he was, and still often is, known) suffered tonic-clonic seizures (previously known as grand mal, an 18th century term for 'big illness') as a result of his epilepsy. Henry's brain was essentially dying as a result and one of the only remaining options was to experimentally remove the areas of the brain causing the seizures. We learned a extraordinary amount thanks to Henry's sacrifice. Unfortunately, he died soon after. Just kidding; he lived until 2008 at the age of 82. This is a rare operation, but one that is so successful that it still occurs today, albeit in another form.

Let's talk about memory without a brain.

If I remove half your brain, will you remember me?

Following the operation, Henry suffered significant anterograde amnesia (he couldn't form new memories). He also suffered quite severe retrograde amnesia (he couldn't remember the last 2 years or so, and had spotty memory for the last 11 years of his past). What made H.M. particularly interesting however, is that he had a fully functioning short-term memory (he could participate in tasks and activities), and in the exploration of that, scientists found that Henry could actually learn new physical skills even though he couldn't remember learning them! This was first noticed when Henry was asked to trace a design while looking in a mirror (so everything is back to front). He found this very difficult at first (as would you), but eventually got better and better over the weeks (despite having no recollection of the task - in fact it would have to be explained to him every time).

How to make new memories when you can't make new memories

More interesting is that Henry eventually learned how to make 'new' memories, by modifying his old ones. He was able to learn new information this way and thus, by the time of his death, happily spent his time doing crosswords (among other things). Henry taught us a great deal about memory and thanks to him we were able to learn and confirm many of our ideas about how our memory works. This article goes over the basics.

How memory works

Before we begin, let me preface this by saying that this 'multistore' model is only a very general way of looking at it and the more you want to know, the more complex memory gets. That said we can imagine roughly three 'clusters' of memory; the sensory store, short term memory, and long term memory.

The sensory store refers to the fact that at some point, for some time, our senses hold onto the information they are receiving long enough to figure out whether or not we need that information (more on that here).

Our short term memory is a fairly controversial topic because people have all sorts of ideas about how that works. But here, I simply refer to when we pay attention to stuff from the sensory store and thus 'bring it to the table' to work on.

Our long term memory refers essentially to our memory storage space. It's where we put stuff when we're done playing with it.

How does it all work? Well, stuff comes into our sensory store and we decide it's important. So we pay attention to it (again, read about what makes us pay attention here). Everything else eventually ‘decays’ from your sensory store, freeing it up for the new stuff that’s already coming in.

You can only pay attention to seven things at once (the memory ‘magic number’)

In the meanwhile, the stuff you paid attention to is in the short term memory. Evidence suggests that your short term memory can essentially only maintain to seven (give or take two) things at once. To take in any more, you have to ‘chunk’ items together. That’s why it’s easier to remember groups of numbers if you imagine them as ‘123’, ‘456’, ‘789’, instead of ‘1’, ‘2’, ‘3’, ‘4’, and so on. So, if you try to take too much on, the rest gets displaced and you lose it to the sands of time.

That said, these models only apply for conscious memory. Our unconscious memory processes are likely different again.

Your long term memory might be unlimited

If you rehearse those seven bits of information enough (and some things are easier than others), then your brain will store them for recall later on. This capacity seems to be unlimited, so long as the links to the information exist. Consider times where you have 'forgetton' all about your appointment on Wednesday, but as soon as someone brings it up, all of a sudden the time, date and other associated information flood back to you. It's impossible to tell how much information we can store for which the links have long been lost to time.
No memory is ever alone; it's at the end of a trail of memories, a dozen trails that each have their own associations" - Louis Dearborn L'Amour

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