You might imagine memory is a Santa’s sack of life events and the first half of jokes. You would be wrong. Neuroscientist Dean Burnett explains all in our new series, Use your head

We all have memories, as far as I can remember. But where do these memories come from and how do they get made?

People often compare the brain to a computer, but the brain doesn’t have USB slots that allow you to pick up new information by jamming a flash drive in your ear. That would be convenient, if a little painful.

So where do we get all this information sloshing around in our skulls? You might imagine human memory is a bit like a Santa’s sack filled with life events, song lyrics and the first half of jokes. But in truth, “memory” is not one single solid thing. It is a term covering lots of types of recollections that are surprisingly distinct, and used constantly in different combinations by a typical human.

Short-term memory – like writing your name with a sparkler

We’ve all heard about short-term and long-term memory. While people tend to use the phrase “short-term memory” to refer to our recall of things that happened recently – in the last hour or day – technically speaking, it’s actually far more fleeting. Short-term memory typically lasts between 15 and 30 seconds: it’s a bit like writing your name in the air with a sparkler. Any memory that can be recalled after that length of time is a long-term memory.

In computer terms, short-term memory is like the RAM – it holds the information we’re currently working with or using for cognitive tasks (thinking). This can be new information delivered by our senses, for example, or old information retrieved from the long-term memory. Neuroscientists theorise that all this thinking is supported by patterns of neuron activity in the prefrontal cortex (that bit at the front of your brain).

Long-term memory - information becomes a physical ‘thing’

Luckily, for memories we actually want to keep, there’s also long-term memory. If short-term memory is the RAM of a computer, long-term memory is the hard drive, which keeps everything from your failed screenplays to Minesweeper scores.

Unlike short-term memories, long-term memories have a physical presence in the brain, and aren’t dependant purely on specific patterns of activity. Neurons make new physical connections and synapses with each other when a new long-term memory is formed. This connection endures whether it’s being used or not.

Long-term memory can be split into explicit and implicit memory. Implicit memories include habits and skills that we can do automatically, such as rolling a cigarette, driving a car, forging your boss’s signature on expense forms.

Explicit memories are things we’re consciously aware of and are intentionally trying to remember. There are two kinds of explicit memory: episodic and semantic. Episodic memory is memory for things and events that happened to you. Semantic memory is for more general knowledge. Knowing Paris is the capital of France is a semantic memory, remembering being sick on your trip to Paris is an episodic memory.

Encoding – a terrifyingly complex tapestry in real time

When we actually want to learn something, it is long-term memories we are interested in. So how are they formed? The first step is to encode a piece of information – otherwise it quickly disappears, like breath on a mirror.

Information is channelled to the hippocampus, the brain region crucial for the formation of new memories and one of the only places in the brain where brand new neurons are regularly generated. The hippocampus links all of the relevant information together and encodes it into a new memory by forming new synapses. It’s basically like someone knitting a terrifyingly complex tapestry in real time.

But not all information is equal in the eyes of the hippocampus. “Important” things are encoded more readily and effectively than routine or incomprehensible things, like an uneventful daily commute, or the lyrics of a dance song in a language you don’t recognise. The hippocampus will prioritise those that have been rehearsed repeatedly in the short-term memory, or those with a strong emotional component. The hippocampus is selective because it is very busy.

Facebook Twitter Pinterest Long-term memories have an actual physical presence in the brain. Neurons make new physical connections and synapses with each other when a new long-term memory is formed. Illustration: Lauren Humphrey

Finding a home for your memories

Coding a memory is all well and good, but it is useless if it has nowhere to go. Finding a storage place is the next step.

Newer memories, once consolidated, appear to reside in the hippocampus for a while. But as more memories are formed, the neurons that represent a specific memory migrate further into the cortex. As a result, memories are stored throughout the brain. It’s a bit like the internet, which is made of information spread all across the planet and accessed via countless connections.

Similar memories tend to clump together – spoken memories near the language centres, visual memories near the visual cortex – and there’s a lot of redundancy too; you can have several memories for the same thing. Every time they are activated they are strengthened. Human memories aren’t stored like books in a library; they’re constantly being updated and tweaked.

Recalling memories you’ve forgotten you forgot

So how do you go about getting the bits you need out of this weird, ever-shifting library of information? It might seem as though lots of the so-called long-term memories have actually turned to dust because there are plenty of things you’ve forgotten: old addresses, passwords, deadlines for articles about the memory system that you promised to write.

The problem here is not that it has disappeared, but rather that you can’t recall it. It’s a bit like losing a glove – you still own a glove, it’s in your home somewhere, but you can’t use it.

Recall is a very impressive but slightly mysterious process. When we want to access a memory from the dark recesses of our brain, signals from our frontal cortex link to that memory via uncertain means, and the memory is reconstructed from the information available. The more often you use the memory, the easier it is to find.

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Revising and remembering is a crucial part of the learning process. And there are things you can do to make it easier – some stranger than others. Being in the presence of some of the elements from the original memory helps retrieval. For example, if you learned something while in a swimming pool, you’ll remember it better while in a swimming pool at a later date (they’ve actually shown this).

Is there such a thing as too many memories? Maybe. Ever tried to learn a new phone number and then found it impossible to remember the old one, even though you had it for years? Constantly updating memories can supposedly “supplant” existing ones, so you end up remembering things differently. This is known as “interference” and can lead to forgetting. As far as I can recall.

This blog is the first of a new series exploring the psychology and neuroscience of learning. Get involved with the Use your head series by joining the discussion on #useyourhead or pitching your ideas to natalie.gil.casual@theguardian.com

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