Like any medical condition, amnesia's causes and effects can be hard to pin down. For one person, it may erase a few minutes' worth of memory. For another, like Clive Wearing, a whole lifetime might disappear. So in addition to defining amnesia by the cause -- brain damage or psychological trauma -- doctors characterize amnesia by the type of memories lost.
Retrograde amnesia means you have trouble remembering the past. Oppositely, anterograde amnesia means you have difficulty making new memories and absorbing new information.
Anterograde amnesia, the more common of the two, is associated with injury to the hippocampus. With it, you cannot convert new sensory information into long-term memories. For instance, alcohol-induced blackouts are a type of anterograde neurological amnesia. Drinking too much alcohol blocks the neural pathways in the brain from forming new memories while intoxicated. People experiencing a blackout may talk and interact normally, but the next morning all will be blank.