Amnesia is a common theme in primetime dramas. In season four of "Lost,"Desmond experiences amnesia after a helicopter crash and doesn't recognize anyone [source: ABC]. He apparently has post-traumatic amnesia. But that may not be accurate. There are two main types of amnesia that occur after major trauma (usually a head injury): post-traumatic amnesia and retrograde amnesia. In retrograde amnesia, people don't remember what happened before the injury occurred. But they may have a sense of self, time, and place. In post-traumatic amnesia, the person is confused and doesn't know where or whom he is. From what you see on TV, post-traumatic amnesia can show up in someone who's never even been in a coma, or in a person who was in a coma for a very short time. In reality, post-traumatic amnesia is a normal part of the recovery process for patients emerging from a coma [source: Synapse].