What if being really scared – the kind of scared that comes from jumping from an airplane at 9,000 feet – could enhance memory by cementing details we might otherwise forget?

For the past few months, UC Davis researchers have turned to novice sky divers to explore how stress affects memory.

The question is part of a simmering scientific debate. Some evidence suggests that short-term stress might help us better remember disturbing events. Other studies suggest it doesn't.

Wading into the fray is Andrew Yonelinas, acting director of the UC Davis Center for Mind and Brain, whose current attempt to help untangle the memory mystery began with a quiet country bike ride.

Yonelinas studies memory and amnesia. When he wanted to look more closely at the relationship of stress, fear and memory, he faced a challenge: how to get really stressed-out people – without violating ethical guidelines.

Scientists who study stress and learning in mice or rats can pen a rat in a cage with a couple of aggressive males. They can put a mouse in a basin of water to sink or swim. Not cool for humans.

Researchers traditionally have used milder stressors, like asking someone to give a speech or hold their hand in a bucket.

Yonelinas was bicycling along rural roads northwest of Davis, pondering how to badly rattle a bunch of people, when inspiration came from above. He saw sky divers drifting toward their landing patch. Those people had to be pretty shaken up, he figured.

When Yonelinas tested levels of the stress hormone cortisol in sky divers before the plane took off and after landing, "it was maybe double" what standard lab stress experiments produce, he said. The effect was broader, too, with everybody he measured feeling some stress.

That didn't surprise Neal Wathen, chief instructor at SkyDance SkyDiving in Davis, who has been teaching people to sky-dive for 12 years.

"The very first jump … typically they get so nervous, it's surreal," he said. Wathen has had students grab at things and scream. He has a video of his own first jump that shows his face turning white.

Sky divers can become so frozen with fear they fail to deploy their parachutes, he said.

"They don't do anything. They can't perform at all," Wathen said, which is why SkyDance sends up novices mostly on tandem jumps, attached to an instructor who opens the chute and steers to a safe landing. The other option is to go unattached, but with a full day of coaching and two instructors hanging on until the new jumper's parachute opens.

Even after about 5,000 jumps, the sport hasn't completely lost its edge for Wathen, who still gets butterflies.

Every sky diver plunging earthward knows, "if I don't do something to change the course of events, I'm dead," he said. "That's what makes it better than golf."

Or at least scarier.

Other scientists hoping to understand extremes in emotions have studied sky divers, including a team that theorized memory problems might explain some fatalities.

Yonelinas is approaching it from the opposite direction, asking "can a stressful event reach backward in time and rescue memories that otherwise would be forgotten?"

Research into how people remember suggests that initially our memories are fragile. Something happens in our brains in the first couple of hours to preserve some memories and discard others.

It would make sense, Yonelinas said, for the human species to have evolved a way to lock in strong memories of what happened just before a frightening situation. Some studies suggest that when people are shown a series of photos, they will remember disturbing or violent images better after being briefly stressed. That's entirely different from chronic stress, which has been shown to impair memory.

To figure out what's happening after an extreme, short-term stress like sky diving, Yonelinas and Julie Jorgenson, manager of the UC Davis Human Memory Lab, set out to study 50 people preparing to take their first or second tandem sky-dive.

Half would be given a set of tests, and then sky-dive and be grilled about what they remembered. The other half would take the same tests, then wait for about the same time without sky diving before having their memories checked. That "control" group wouldn't dive until the memory test was over.

That way, researchers could measure differences in recollections between those who went through the jolt of jumping from a plane and those waiting to jump. Everyone studied gave repeated saliva samples so their cortisol levels could be measured before and after sky diving.

Megan Thomson, who participated in the study partly because she wanted to sky-dive to celebrate her recent 26th birthday, has a hunch that she remembers better when she's not anxious.

Thomson, of Davis, heard about the study because she sometimes baby-sits Yonelinas' children. Like everyone else who took part, she wasn't told beforehand that her memory would be checked, so the picture recall test was a surprise.

"When I'm stressed, I don't always listen or pay attention as well as I could," said Thomson.

On her tandem jump Thursday from SkyDance's red-and-yellow plane, she said, the scariest moment came right when she was at the plane's door.

"It felt like a roller coaster at the top," just at the instant before it plunges down.

Thomson won't know how well her group remembered for a while. Yonelinas and Jorgenson still have a few sky divers to test, and they expect it will take months to analyze the results.

Call The Bee's Carrie Peyton Dahlberg, (916) 321-1086.