It seems like every other movie, video game, or television show episode features a soldier, homeless person, or a private investigator for hire who has suffered a bump on the head and can't remember who he or she is. This is amnesia, and we all think, because of TV, that we understand it. But do we really? This article should help to enlighten you on this mysterious disorder of the brain.

Amnesia is derived from a Greek word which literally means "oblivion." It is used to refer to the state of confusion that is caused by memory loss or memory alteration. There are many ways to bring about amnesia, including brain trauma, substance abuse, diseases, mental disorders, and post traumatic stress. A person could fall off a ladder installing alarm systems and bump his head. A woman could be attacked by a stalker and wake up to find that the whole night of the attack has disappeared from her memory. An elderly person might gradually lose the ability to recognize her own family.

In most cases of amnesia, the memories are still stored, uncompromised, in the brain. Rather, it is the memory retrieval system, which is part of the limbic system, that is compromised and refusing to access certain memories. This is why people with amnesia who can't even remember their names do not need to go back to school to relearn English or toilet train themselves all over again. Memories are stored all over the brain, so it's almost impossible for all memories on any given topic to be wiped out at once.

There are several different types of amnesia. You can usually tell which one a person has from his or her behavior. Anterograde amnesia is the inability to form new memories. People with this type of amnesia can't remember recent things, like where they put their car keys, but will remember their job driving an airport taxi twenty years ago in great detail. Retrograde amnesia is the loss of old memories. People with this type of amnesia may not remember who they are, but they can remember that they had chicken for lunch.

It is retrograde amnesia that is most commonly represented on your TV screen because it is an easy framing device to allow audiences into a story. However, in real life it is anterograde amnesia is most likely to come across, as it is caused by Alzheimer's disease and dementia.