The amygdala kicks in when we encounter something scary, for instance, such a menacing stranger. That’s why it used to be called the brain’s “fear center.”

But scientists now know that all sorts of stimuli — including things that disgust us or sexually arouse us – also activate this area, Nitschke said. And once turned on, the amygdala triggers a host of other responses in the brain and body that contribute to our excitement.

Why some people crave this type of stimulation isn’t clear. Perhaps certain individuals simply need a more active amygdala for some reason, Nitschke said. It’s also possible that those who routinely watch horror flicks or play violent video games get habituated, and consequently “need a bigger and bigger boost to get the amygdala going,” he added.

It’s known, for example, that showing a person a horrifying picture elicits a huge brain response the first time. But by the tenth time the response is tiny. “It’s like the amygdala is saying, ‘OK, I’ve done this, seen this before,’” Nitschke said, “even though the (tenth) picture is just as horrible as the first one.”