Retrograde Amnesia

Retrograde amnesia is caused by trauma that results in brain injury. Critical details of the physical changes in the brain that cause retrograde amnesia are still unknown. Retrograde amnesia is often temporally graded, meaning that remote memories are more easily accessible than events occurring just prior to the trauma (Ribot's Law).Events nearest in time to the event that caused memory loss may never be recovered.

A person who has suffered this injury will often feel as if the time (1–4 hours) before the injury were a dream. If someone informs the injured person of the events just before the trauma, he or she will most likely recollect some of the happenings.

The memory loss may just affect specific “classes” of memory. For instance, the victim, a concert pianist before, may still remember what a piano is after the onset of retrograde amnesia, but may forget how to play. The relearning rate for often used skills such as typing and math is typically faster than if the victim had never learned these skills before. While there is no cure for retrograde amnesia, “jogging” the victim’s memory by exposing the victim to significant articles from his or her past will speed the rate of recall.

http://en.wikipedia.org/wiki/Retrograde_amnesia