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What is long-term memory?
Our long-term memories define us
The ability to learn and to establish new memories is fundamental to our lives; we rely on our memories to do the simplest of things - remember who we are, recognise the words we read and the sounds we hear. More than any other aspect of the mind, our memories represent everything about us as entities, they are our heart and soul. But after more than a century of intense study how much to we really understand about how and where our long-term memories are stored and retrieved?
What turns a short-term memory into a long-term one?
When we talk about memory we're really talking about long-term memory. Our short-term or working memory capacity is about thirty seconds. Anything that sticks around for longer has already made it to at least the first stage of long-term memory which can, with regular servicing, last our whole lifetime.
Some of what is known about the human memory system is backed up with hard data, other pieces of the jigsaw are held together with educated speculation. What does seem evident though is that structurally, long-term memories are stored in a different place than short-term memories and almost certainly in a very different way. We know this through the observation of patients with memory deficits (see the page on the Hippocampus). For long-term memories to survive for decades it's thought we have recall them from time to time, otherwise they will fade as part of the natural process of forgetting. Studies have shown that every time we remember something we strengthen the connection to it, which helps us remember it even after a gap of many years.
Serious research into memory began with the pioneering work of German Hermann Ebbinghaus, who wrote a ground-breaking treatise called 'Memory: A Contribution to Experimental Psychology' in 1885. In it he made claims about memory which are still largely true today: "he who learns quickly also forgets quickly"; "relatively long series of ideas are retained better than relatively short ones" and as anyone who has observed or cared for a person with dementia will recognise, "old people forget most quickly the things they learned last". Ebbinghaus' work brought about the science of cognitive psychology which, since the invention of brain scanners such as PET and MRI, has become cognitive neuroscience. Today the largest neuroscience conference attracts well over thirty thousand delegates each year.
How do we make a (long-term) memory?
It's something of an understatement to say that memory is incredibly complex. Our memories have many facets to them and don't exist as complete chunks of information nor in one specific brain region. Memories are usually made up of all sort of ingredients - take a simple stroll in the park: who were you with? What was the weather like? Where did you go? What did you wear? What did you talk about, eat, drink and smell? Were you happy or sad? Our brains can store each facet of this one memory in a different place and then bring it all together again coherently, in a blink of an eye.
Where is memory?
Modern brain imaging research has shown that it takes more than one area of the brain to make and store a memory - the two most likely suspects are the prefrontal cortex and the temporal cortex. Within the temporal cortex sits the hippocampus, a seahorse-shaped collection of cells which seems to be crucial for helping us to process all the different bits of a memory and bind them together. It's also closely associated with our emotional memories and it's the place that helps us turn new information into a lasting memory (see the page on the hippocampus).
Remember the three Rs
Once something is committed to memory how is it stored? How do we get it back? A huge amount of research has been devoted to answering these questions. It's thought that how and where memories are stored depends on what sort of memory it is. Psychologists have many ways to classify memories. 'Declarative' or explicit memory is the sort that you consciously store and recall (a phone number). 'Procedural' or implicit memory can be thought of as memories for things we do that have been learned and we no longer need to consciously think about them (riding a bicycle). There are lots of subsets of these memories types like semantic (based on meaning), episodic and autobiographical (based on events that happened to you), sensory, spatial and so on. Each bit deals with a specific type of experience. Different types of memory may be stored in different ways and in different places.
How we remember can also be categorised.
Do memories leave traces in the brain?
Where memory research diverges and disagrees is on the question of exactly what might be going on in the brain, at a molecular level, when we create or recall a memory.
Any theory needs to take into account two essential properties of our memories - first, because a lot of the information needs to be maintained over a long time there has to be some degree of stability. Second, because the mind needs to learn and adapt, the system also needs to be highly flexible.
That's why so much research is focused on synapses - the connections between neurons. Synapses are the main site of information exchange and storage in the brain. They form an unimaginably vast but also constantly fluctuating network whose ability to change and adapt, called synaptic plasticity, may be the fundamental basis of learning and memory. But if this system is constantly changing, then how do our memories stay put? Perhaps, as the dominant theory suggests, whenever a new memory is formed then in a process called 'translation', a protein is synthesised at the synapse which represents that memory. New theories are challenging this all the time. Some focus more on what's going on inside the neuron whilst another suggests that our memories are created and simultaneously copied many times like back-up files on a computer, to help keep the memory alive in case one of the copies is erased or forgotten.