Here's an excerpt from Healing the Brain: Memory. Learn more about the brain in this special offer: 6 brain books for a buck.
Emotions and “flashbulb” memories.
Your emotional state when an event occurs can greatly influence your memory of it. Thus, if an event is very upsetting, you will form an especially vivid memory of it. For example, many people remember where they were when they learned about President Kennedy’s assassination, or about the attacks of September 11, 2001. The processing of emotionally-charged events in memory involves norepinephrine, a neurotransmitter that is released in larger amounts when we are excited or tense. As Voltaire put it, that which touches the heart is engraved in the memory.
Location, light, sounds, smells...in short, the entire context in which the memorizing takes place is recorded along with the information being memorized. Our memory systems are thus contextual. Consequently, when you have trouble remembering a particular fact, you may be able to retrieve it by recollecting where you learned it or the book from which you learned it. Was there a picture on that page? Was the information toward the top of the page, or the bottom? Such items are called “recall indexes”. And because you always memorize the context along with the information that you are learning, by recalling this context you can very often, by a series of associations, recall the information itself.