There are two main types of amnestic disorder, or amnesia: retrograde amnesia and anterograde amnesia.
Anterograde Amnesia
Anterograde amnesia is caused by a injury to the brain. This results in a person being unable to learn new information. The memories from before the injury is will still be intact, but the ones from after can be lost. The short term memory is also intact but if a person is distracted in a conversation, the memory will fade. Memories of fact and events are lost but any skills or habits will remain. If a person learns how to play a game, the next day, they will be unable to remember learning how to play but they will be able to play better than another first-time player.
Anterograde amnesia can occur with damage to the hippocampus or the association areas of the temporal lobe. Any new information must pass through the hippocampus before it can be made into a permanent memory. If this area is damaged, no new information can become memory. Damage can happen from a stroke to one of the arteries that gives blood to the hippocampus or the temporal lobe's association areas. Damage can also come from the normal aging process.
Anterograde amnesia can occur with damage to the hippocampus or the association areas of the temporal lobe. Any new information must pass through the hippocampus before it can be made into a permanent memory. If this area is damaged, no new information can become memory. Damage can happen from a stroke to one of the arteries that gives blood to the hippocampus or the temporal lobe's association areas. Damage can also come from the normal aging process.
Retrograde Amnesia
Retrograde amnesia is a result of a brain injury where a person is unable to recall memories from before the injury. The amount of memories lost can vary from a couple minutes to a few years. Usually the older the memory, the less likely that it will be affected by the amnesia. Some people can have short term amnesia, which can last a few minutes to a few hours. A person with short term amnesia will usually have anterograde amnesia.