Philosophers have been thinking about memory for thousands of years. Plato in the 4th century BC suggested the Wax Tablet Hypothesis. This theory assumes that the mind accepts impressions as if it were a wax tablet drawn on with a sharp object. Once the impression is made it remains but it wears away over time leaving a smooth surface and hence no memory. Therefore, memory and forgetting are just opposite aspects of the same process. This explanation was accepted with only minor variations until relatively recently but it is now generally accepted that they are two different processes.

Aristotle had a theory about the physical location of memory. He realised that part of the hearts function was to move blood around and thought that memory was based on the movement of blood. Therefore forgetting could be attributed to a gradual slowdown of these movements.

The physician Galen in the 2nd century AD did a great deal of work with the various biological processes of the body and concluded that memory resided in the brain, not the heart. The question then becomes where and how does the brain encode memory?

It is now generally agreed that memory is located in the cerebrum, a large portion of the brain covering the surface of the cortex. But exact localisation is still difficult.

David Bohm (1917-1992), a physicist by profession, was one of a number of scientists that suggested the brain can work in a similar way to a hologram in one very important aspect. If a holographic photographic plate, which is simply a piece of glass, is smashed into pieces, each piece can still be used to show the entire hologram, albeit slightly blurred. Just as each part of a holographic plate contains a minirecord of the overall picture, so too every part of the brain may include all memories.

As the brain has no nerves to carry pain impulses, it is possible to open up the top of the skull, hopefully under local anaesthetic, stimulate various parts of the brain and ask the patient about it. Dr Wilder Penfield did some experiments in the 1930s and found that stimulating various areas of the cortex elicited a range of responses but stimulating the temporal node could bring back memories in all their glory, including even the emotional content. Some of these memories, such as childhood memories, couldn't be recalled in the normal way. It was Penfield's belief that the brain records everything – or at least everything we pay conscious attention to - and the main reason we forget is therefore a breakdown in the recall mechanism.

As a side note, there is evidence to support the idea of that some forms of memory are not necessarily stored in the brain but can be encoded in RNA (which is produced by our DNA). In one experiment for example, when the RNA from one rat was taken and injected into another, the second rat remembered things the first rat had been taught. While this is not conclusive evidence for the concept of genetic memory (as the RNA was not a copy produced by DNA passed on to the next generation) it is an interesting idea.

Why do we forget?

With these modern theories of how memory works, the main idea is that once memories are stored they are there for the rest of our lives (or even longer in the case of memories carried in RNA) and forgetting is equated with simply being unable to recall the correct memory.

Remembering can be broken down into 3 parts:

  1. Recording
  2. Retaining
  3. Retrieval

This can be illustrated by comparing memory to a filing cabinet. First you put the information on a piece of paper (Recording). Then put it in the file cabinet under the appropriate heading (Retaining). Later, if you want it, you go back and get it out of the filing cabinet (Retrieval). Looking at this, it can be seen that to help us remember it would be good to have an efficient filing system. This corresponds to memory aids.

But there are also other problems that may contribute to poor memory that come into play at the one or more of the three stages of remembering. Some of the major ones are:

  • Emotional Blocks – A mental barrier that we use to protect ourselves from things that we perceive could cause us harm, error, or ridicule. Obviously there are many valid reasons for such blocks and various kinds of therapy if you have a serious emotional block. One example of a less serious emotional memory block is a person who as a child didn't have any playmates and his closest friend was probably the housekeeper. However, as he moved around a lot such friends came and went. Now, as a lecturer he can remember the names of most of his students except for the ones he cares about the most. This is possibly because in his childhood he found that missing people was less painful if you didn't know their names and it is in anticipation of his students leaving that he didn't remember their names. In such cases, once the emotional block is realised it can be discarded, or strategies made to work around it. But it is important not to discard a block until you are sure you can live without it; after all, it is there for a reason.
  • Diet – With a 'well balanced' diet you will be able to remember more than if you had sub-standard diet. Unfortunately, brain chemistry is so complicated that it is hard to say what a well balanced diet actually is.
  • Illness and injury, – The most obvious illnesses and injuries that affect memory are various head injuries, amnesia, Alzheimer’s disease.
  • Allergies – Generally allergies only play a very small part in memory problems but anything that disrupts our physical or emotional balance can make us forgetful.
  • Drugs – Specifically recreational drugs but any drug that affects mood, perception or alertness will also affect how you process information.
  • Alcohol – See above. In extreme circumstances people can experience state dependency where things learned sober are hard to recall when drunk and vice versa.
  • Smoking – Tests have been carried out that showed non smokers could recall better than smokers. It has been suggested that this is because smoking may impair the amount of blood supplying oxygen getting to the brain and the lower oxygen levels result in decreased memory efficiency.
  • Lack of exercise – While this does not necessarily impair memory function it does slow down response time and good general health and physical fitness is a powerful aid to memory.
  • Fatigue – Occasionally fatigue can be an aid to remembering as we float in free association mode but it can also inhibit the actual creation of new memories.
  • Tension – This can be a vicious cycle. You are in a situation and don't remember an important detail. The more you try to remember it, the more tense you become and less able you are to remember. Tension can have the opposite effect of fatigue; while you cannot remember previously known information there is evidence that adrenaline and other hormones produced when you are excited or under stress help lock new memories in, possibly simply as a byproduct of making your senses more acute.

Old age is not a reason for memory loss. It can be seen that there are a lot of factors that can contribute to memory loss of a greater than normal degree. With the elderly memory loss can come as a side effect of being unable to adequately look after themselves, such as substandard exercise and diet, or as a result of illness, injury, or medications.

Update: Thanks to dscotese who pointed out that studies have shown that adrenaline improves memory up to a point, and then rapidly and severely degades it, one explanation of traumatic blockage.

Buzan, Tony "Use your Memory" Revised Edition, 1989

Higbee, Kenneth "Your Memory: How it Works and How to Improve It", 1990

Minninger, Joan PhD. "Total Recall: Successfully Boost your Memory Power", 1984