"Sometimes you get a feeling like, 'I can't believe I just did that,' " he said. "Once you get off it, you're like, 'Oh, I've got to get back on this. It was great.' "
As it turns out, some people's brain might enjoy a little fear, according to research from Georgia Health Sciences University and the Shanghai Institute of Brain Functional Genomics in China. Their research, published last week in the journal PLoSOne , focused on dopamine-producing neurons in the ventral tegmental area, or VTA, in the brain.
"In the textbook version, the VTA is a reward center or intimately engaged in drug addiction," said co-author Dr. Joe Z. Tsien, a co-director of the Brain and Behavior Discovery Institute at GHSU. It was previously thought all it did was respond to and reinforce a response to good things.
"What our paper will show is this is not the case," Tsien said.
The researchers worked with mice whose brains were wired with electrodes to record the real-time firing of neurons. They were then subjected to positive stimulus, such as receiving a sugar pellet, and fear-inducing stimulus, such as shaking the box the mouse was in. Nearly all of the dopamine-producing neurons in that brain area responded to the fear events, Tsien said.
Those neurons react "not only to the reward but also very, very robustly to essentially negative events," he said. Though the majority of the neurons were suppressed or shut down in response to fear, they had a significant "rebound" in excitation after the event ended, Tsien said.
"These neurons may provide some kind of mechanistic explanation for driving the thrill-seeking behavior," he said. "Those are supposedly fearful events, but we can see a huge rebound excitation that should lead to the release of the dopamine, which may explain why some people -- not all people, some people shy away from it -- feel attracted to such very risky behavior."
In fact, the researchers were able to locate a subset of neurons, about 25 percent in that brain area, that were excited by the fear events, Tsien said. In light of previous dogma that the area of the brain preferred rewarding stimuli, that was "very, very surprising," he said.
"That can also be part of that adaptation or the thrill-seeking behavior processing," he said.
The stimulus was often paired with a tone beforehand, and those signals also triggered a response, but often not when the animal was placed in a different box, showing the responses were highly contextual.
That "may help explain why environments play such a dominating role in eliciting craving or reinforcement of habits," the study noted.
It also shows the relationship between reward and punishment is not so cut-and-dried, Tsien said.
"They are relative," he said. "If you get a bonus every day, then after a while you don't feel this is a reward because it is expected. On the other hand, if every day you get a punishment and one day you did not get it, you feel that is a reward. That's why I think this will help us to understand why our brain continues to have this very adaptive mechanism able to deal with a very broad spectrum of information," both positive and negative.
To Rodriguez, it explains why she keeps watching scary movies and racing.
"You want it back again," she said. "You want to run back and get on the roller coaster. You get a little high out of it. It feels good."