... When wasteful war shall statues overturn,
And broils root out the work of masonry,
Nor Mars his sword nor war's quick fire shall burn
The living record of your memory...
- William Shakespeare, Sonnet LV

Memory is certainly a very large part of the complexity of the brain; one may even argue that it is one of the most important facets of consciousness and indeed humanity. Encompassing not only 'learnt' facts, it involves many other aspects, to name two; memory of the self, and, as with Shakespeare, the memory of other people in life. As a result, it is certainly a large and exquisitely confusing topic.

What is Memory?

memory n. 1 ability to remember things 2 things remembered 3 computer store for data etc
- The Oxford English Dictionary

Although the dictionary may have no more than a throwaway two-line definition, invariably, human memory is not as simple as that. In fact, just like its equally confusing and complex cousin, intelligence, there has been shown to be several types of memory, each with its own distinguishing features and unique characteristics.

Short-term Memory

This is divided into two types:

Sensory Memory

This is defined as a brief (below two seconds) storage of perceptual experience (for example, sounds or pictures). This kind of memory is unprocessed and is rapidly overwritten by other bits of information.

Working Memory

This is what is mainly regarded as 'short-term' memory, and indeed, is defined as a short-term memory store. For instance, if you were to say the phrase 'There's a fish on top of the TV' to someone, and then ask them to recall it about ten seconds later, they will have no problem remembering it. Ask them in ten years time, and they won't recall it at all.

This type of memory has limited storage space in terms of information. That is, you may only be able to remember a maximum of five to nine items at a time. This is why telephone numbers cannot really be any more than about eight digits long, because none of us would be able to remember them.

Confusingly enough, the length of the working memory is not governed by time. What happens is that as the brain is very plastic, and there is only a finite storage space for working memory, new information pushes out old. You may find yourself in the common situation where you have been sent upstairs by your boss to get something, walk up the stairs, stump your toe, and reach the top with the thought 'Why did I just come up the stairs?'. The distraction of stumping your toe pushed out the information which sent you to the top of the stairs in the first place. It also may account for someone in conversation being interrupted in mid flow, addressing the other person's point, and then returning to the original conversation wondering what they were talking about in the first place.

Working memory is itself divided into two sub-sections, one dealing with words called the Phonological Loop, such as the above example. There is another type called the Visiospatial Sketchpad, which is explained in more detail here. Both types of working memory are controlled by what is called the Central Executive, which integrates and processes the information.

Long-Term Memory

There seems to be a sharp dividing line between long and short-term memory, as demonstrated by H.M and the Seahorse - a Story of Memory Discovery. Like the Working memory, Long-term memory is divided into two types:

Declarative Memory

This can be thought of as knowing what, where and why about the world. It is also sub-divided into two types:

Episodic Memory

This type of memory carries information about specific events, and the individual's own autobiographical memory. Although termed as long-term, information about events can be both short-term and long-term. It would be unreasonable to suggest that a person can remember every single detail about their youth - hence some episodic memory must have a very short duration - perhaps up to 30 seconds.

On the other hand, most people can remember aspects of their childhood in great detail - when they first broke a bone, or passed an exam. This kind of memory is termed as autobiographical - as the name suggests, memories about our own lives.

Semantic Memory

Often described as memory for facts, which suggests verbal knowledge, it is in fact the memory which enables your knowledge of the world - this would include, but not be exclusively, verbal knowledge. Aspects like the scent of a rose, the deceivingly unpotent taste of a cocktail, or the feel of silk, are stored here.

Procedural Memory

This is a bit different from Declarative memory, as it is mainly concerned with the learning of skills, such as riding a bike, habits, and also what is termed as implicit memory - what you learn unconsciously by experience, for instance, that cats really don't like having their tails pulled and other appropriate social behaviour. Associative behaviour - that of the linking of emotions to events - may reinforce the memory. For example, say you had a bad experience with a banana when you were younger, you will remember it quite clearly; despite having being exposed to it only once. On the other hand, you will struggle to remember the faces of your primary school friends, even though you saw them every day.

Important differences between the specific types aside, they all have one thing in common - that of the encoding of the memory, its storage, and most importantly, its eventual retrieval.

Forgetting

No entry about memory can really be complete without forgetting. We forget for different reasons because the different memory stores decay different reasons.

With Sensory memory, it is the fault of time, there is a 'trace delay', and also, it is rapidly overwritten by other information.

With Working memory, it is mainly due to the limited storage space which forces substitution of information.

In Long-term memory, there must be more direct interference - that of a problem in the retrieval process, in the actual storage area, or indeed, in its conversion from the short to the long-term. To understand the forgetting in this memory store, let's look more closely at how long-term memories are formed.

The Long-term: Formation

At the time of writing, current theory is that all long-term memories start off as being short-term memories. With all brain functions, the information from the outside fires off neurones in the periphery (the nervous system, excluding the brain and spinal cord), which gets integrated in the central nervous system (the brain and spinal cord only). This integration also involves more firing of neurones. This creates paths by which the memories can be retrieved.

So let's imagine that you are revising. Once you read through your notes once, they are integrated in the brain. So the neurones involved fire. No doubt you are a conscientious student, so you have planned out several revision periods where you re-read your notes at several points throughout the course. Every time you read through them again, the same neurones fire. Hence, they are being increasingly used. The body then does what it does best and adapts to the environment. As the neurone in question is firing more and more, it adapts to become more efficient for the job in hand, growing and altering its intracellular chemistry so that it can retrieve information much better. This process is called Long-term Potentiation, and is partly the reason why students are told to take their revision in short periods, often repeated.

If a once well-used neurone isn't being used so much, then the opposite happens. This neurone can then be used in the retrieval of other memories. This all contributes to the plastic ability of the brain and the body's ability to adapt to most situations.

However, even at this stage, they are still short-term. The conversion of short to long-term occurs via a strange structure called the hippocampus. The process by which it does this is excruciatingly complex and infuriatingly difficult to understand. Any explanation of how it does this would be outside the scope of this entry.

Long-term Storage

The Long-term memory retrieval pathways are currently thought to be stored in an area of the brain called the Parietal lobe (here is a diagram of where all the lobes are in the brain), and so if this area is damaged, then there will be a problem, as the storage area is damaged. This is what most people imagine happens when there are problems with long-term memory. This is not the case.

It's worth taking a small detour to explain what is being shown from recent evidence. Although the 'brain=computer' analogy of the way that memories are integrated in our brains is becoming outmoded¹, and is being replaced by an 'ensemble' theory, where an image is not stored in the whole, but the manner in which the image is retrieved is stored. If this is difficult to imagine, think of how we listen to music. It may not be the sheet music with all the musical notes on their respective ledger lines which we listen to, although that is the manner in which it is notated to the respective musicians, but the manner in which it is conveyed to our ears, with all the instruments working in harmony to create the overall picture of the piece.

Retrieval of the Long-term

In fact, it is problems with the process of retrieval which is the most common reason why long-term memories are forgotten.

There are several 'circuits' in the brain which facilitate the retrieval of information. One is called the 'Papez Circuit', and involves several structures such as the aforementioned hippocampus, the fornix, the mamillary body², the anterior thalamic nucleus and the cingulate gyrus. Confusing and alien-sounding names aside, damage to these structures within the circuit will have a considerable effect on long-term memory retrieval.

How is all this Known?

As with most things in the brain, the reason why this is known is due to advances in scanning technology (the details of which can be read here) and sadly, memory disorders - particularly two groups, distinguished by the areas of the brain which they are involved in.

Understandably, due to the very nature of the brain, studies and experiments are fiendishly difficult to perform on a quantitative basis. Hence why a compromise is made, and that much of the studies becomes 'scientific surmise', with similarities being noted of many patients with the same syndrome.

The Amnesic Syndrome

This typically occurs when the temporal lobes, the hippocampus and/or other aspects of the brain called the thalamus and the hypothalamus, are damaged. Depending on what is damaged, several disturbances of memory can occur. Probably the most well-known of these cases is the intriguing case of HM, of whom more information can be found here.

Damage to the temporal lobes results in the inability to form new long-term memories, called anterograde amnesia, and long-term memories formed over the past few years, called retrograde amnesia, whilst sparing short-term (stored in a completely different area - the frontal lobe cortex) and semantic memory.

Dementia

dementia n. a chronic or persistent disorder of behaviour and higher intellectual function due to organic brain disease.
- The Oxford Concise Medical Dictionary

Whilst pure disorders of short-term memory and semantic memory are very rare, disorders of both short-term, semantic and long-term episodic memory are found together in dementia.

The most well-known condition which falls into this category is Alzheimer's Disease. The characteristic lesions are known as amyloid plaques, and it is their distribution throughout the brain which accounts for their clinical signs. They are quite sparse in the primary areas of the brain ie, the Primary Visual Cortex, or the Primary Motor Cortex, which accounts for the lack of problems in vision and movement. However, where the plaques are most dense are in the temporal, frontal, parietal and occipital lobes. This would lead to effects in memories formed over the last few years, some aspects of memories formed further back in time and also short-term, semantic and procedural memories. As the Amnesic Syndrome concentrates on lesions in the temporal lobes, then logically, the other lesions in the different areas must account for the other effects, if not in storing the information, then being part of the mechanism of formation or retrieval.

A condition which occurs in the frontal lobe is that of Dysexecutive Syndrome. In this case, intelligence and cognitive ability may only be slightly impaired. However, there is a greater change in personality. So why mention this in the context of memory? Our personalities must be encoded somewhere, in effect, being a memory somewhere in the brain. The Dysexecutive Syndrome demonstrates quite a marked difference in personality change, especially in mood, social awareness and behaviour. Even then, it is also divided into two types, depending on where the lesions are in the frontal lobe.

Dorsal - that is to the rear of the frontal lobe - lesions lead to a number of effects, including that of apathy and a lack of spontaneity. Ventral - the opposite of dorsal - lesions lead to various effects, including poor social judgement and acquired psychopathy³.

Ad Memorandum

Just as with the ever-complex brain, memory continues to both surprise and infuriate research scientists. It is - as with many seemingly simple aspects - invariably far more fascinating and complex than any two-line dictionary definition can ever convey.