The capacity to remember. It is a complex process and probably occurs in many areas of the BRAIN including the LIMBIC SYSTEM and the temporal lobes. There are three main steps: registration, storage, and recall.

During registration, information from the sense organs and the cerebral cortex is put into codes for storage in the short-term memory system. The codes are usually based on the sounds and words that would be used to describe the information (acoustic) but may use any of the five senses. This system can take only a few chunks of information at a time: for example, only about seven longish numbers can be retained and recalled at once – the next new number displaces an earlier one that is then forgotten. And if a subject is asked to describe a person just met, he or she will recall only seven or so facts about that person. This depends on attention span and can be improved by concentration and rehearsal – for example, by reciting the list of things that must be remembered.

Material needing storage for several minutes stays in the short-term memory. More valuable information goes to the long-term memory where it can be kept for any period from a few minutes to a lifetime. Storage is more reliable if the information is in meaningful codes – it is much easier to remember people's names if their faces and personalities are memorable also. Using techniques such as mnemonics (for example a list of initials which add up to make a phrase or rhyme) takes this into account.

The final stage is retrieval. Recognising and recalling the required information involves searching the memory. In the short-term memory, this takes about 40-thousandths of a second per item.

Most kinds of forgetting or AMNESIA occur during retrieval. Benign forgetfulness is usually caused by interference from similar items because the required information was not clearly coded and well organised. Retrieval can be improved by re-creating the context in which the information was registered. This is why the police reconstruct scenes of crimes, and why revision for exams is more effective if facts are learnt in the form of answers to mock questions.

Loss of memory or amnesia mainly affects long-term memory (information which is stored indefinitely) rather than short-term memory which is measured in minutes. Short-term memory may, however, be affected by unconsciousness caused by trauma. Drivers involved in an accident may be unable to recall the event or the period leading up to it. The cause of amnesia is disease of or damage to the parts of the brain responsible for memory. Degenerative disorders such as ALZHEIMER'S DISEASE, brain tumours, infections (for example, ENCEPHALITIS), STROKE, SUBARACHNOID HAEMORRHAGE and alcoholism all cause memory loss. Some psychiatric illnesses feature loss of memory, and AGEING is usually accompanied by some memory loss, although the age of onset and severity vary greatly.