Ever wondered why some things you never forget, while others you can never remember? The answer may lie in the emotions connected to your memories.

If you saw a tornado coming your way tomorrow morning, chances are you'd have a very accurate memory of it, for a very long time. So why can't you remember where you parked your car?

One of the keys to locking in a memory is how much emotion is attached to it.

"I think it's fascinating how some memories stick and others seem to disappear into thin air," says Stephan Hamann, an assistant psychology professor at Emory University who researches this very phenomenon.

Hamann uses functional magnetic resonance imaging, a technique that builds on standard MRI hardware, to chart activity in people's brains as they are shown different pictures and words.

Half of the stimuli are meant to evoke emotion, while the other half are neutral. Examining the data, Hamann is then able to "see what areas are more active at that particular time" when a picture or word is shown.

Results indicate that when items with high emotional content are shown, a specific area of the brain's temporal lobe called the amygdala lights up. The amygdala is the center of emotion in the brain and, it is becoming clear, a very strong tool for solidly hammering in a memory.

"When the amygdala detects emotion, it essentially boosts activity in areas of the brain that form memories," says Hamann. "And that's how it makes a stronger memory and a more vivid memory."

These can range from painful or fearful memories to ones that are slightly more pleasant, such as "the birth of a baby or a wedding."

In Hamann's experiments, test subjects are able to remember twice as many emotional words and pictures as neutral ones.

Total Recall

Although emotion can give weight to specific memories, it does not necessarily improve memory across the board. In fact, the very emotion that locks in one memory can often wash out others.

University of South Florida neuroscientist David Diamond devised a simple experiment to demonstrate this behavior in a rat.

First, the rodent swims its way through a water maze, searching for a safety platform. After a few tries, it remembers the location well.

Then, emotion is introduced. For a rat, this entails the ultimate stress: close proximity to a cat. As stress hormones flood the rat's brain, its amygdala locks in a memory of the cat.

But those same hormones also wash out the rat's other recent memories, stored in other parts of the brain. The next time it is placed in the maze, the rat is at a loss. Where is that platform? It has forgotten.

So was what the rat learned before blasted out of its memory by the shock of seeing the cat?

"Exactly. It's completely gone," says Diamond. "What we can also say is that the rat probably remembers the cat very well."

In this respect, he believes, rats and humans are very much alike."It appears to be pre-wired that we remember important events very well, and in the process of remembering events extremely well, other memories basically get kicked out."

Diamond offers this analogy: "After you see a bank robbery, you come back to the parking lot and you can't remember where your car is."

Remembered Lessons

These studies take us a step closer to understanding the complexities of memory — an intricate process of brain biochemistry that affects us every day, at the most basic level.

One day, they may also show us how to unlock its mysterious power. "We may be able to learn how to improve our memories if we study how extremely strong memories are formed," says Hamann.

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