Memory is the ability to recall facts and knowledge. It is a function of the brain. It's independent of recognition, learning, and skill, all of which may be retained when memory is missing.
There are two distinctly different types of memory, each of which is independent of the other:
- Short term memory - The ability to remember what has happened in the immediate past (generally the previous 24 hours). This allows you to remember the content of conversations, transfer knowledge to writing, have a rough idea what time of the day it is, to remember whether you have eaten recently, and so on. Short term memory is very limited, which is why phone numbers used to be limited to 7 digits (the maximum number most people can remember when told to them once).
- Long term memory - The ability to remember long past events. Most people do not remember anything that happened to them before the age of about four, but can recall the names of people they haven't seen in a while, when major events took place, remember the names of people they have not seen in some time, and so on. Long term memory can be very impressive - even average people can be taught to remember very long passages (Such as House's ability to play many songs on the piano without any written music).
Short term memory appears to be a function of the brain's electrochemistry. Long term memory appears to be a function of the brain's actual physical form. Recent research has indicated that during sleep, the brain undergoes the physical transformations that form long term memory.
Memory problems are an indication of several diseases and can also indicate brain damage. They fall into three general types:
- Loss of short term memory - This is typical of many illnesses of the brain, most notably Alzheimer's disease. People with no short term memory do not form new long term memories and have difficulty learning new skills. People who suffer from short term memory loss often fail to realize that anything is wrong, and the symptoms are often subtle. For example, in early stages, a person may keep forgetting answers they have calculated or facts they have just looked up. They also develop coping behaviors to cover the problem, such as not engaging in conversation and avoiding direct tests of their memories.
- Failure to form new long term memories - This is a rare condition. Individuals with this disorder do have some short term memory, but have no memory after the time they first lost their ability. It's usually the result of trauma or serious brain damage. The movies Memento and 50 First Dates illustrated people with such a condition.
- Loss of long term memory - Also called amnesia - Long term memory is very resilient and loss of long term memories is very rare, although not unknown. It is usually the result of severe brain damage. It may be temporary in some cases (such as during a transient ischemic attack) or could be permanent.
Forgetfullness is a normal process and does not indicate any underlying neurological condition. Moreover, memory is imperfect and malleable. Experiments have shown that memories can be implanted into the mind.