In my all-time favorite episode of Radiolab, “Finding Emilie,” a young art student named Emilie Gossiaux gets into a terrible accident while riding her bike and, rendered blind and deaf, is unable to communicate with her loved ones until she makes an incredible breakthrough. Listening to it on my drive home only got me to the middle of the episode, so I sat in my parked car staring at the garage until it was over. I was captivated by the voices of Emilie and her family. I’ve been an audio convert ever since.

It’s likely that thousands, if not millions, of others had the same experience last year when they discovered Serial, the This American Life spinoff considered to be the most successful podcast of all time (5 million downloads and counting) that launched the medium back into the spotlight.

As a New York magazine piece noted last year, the increasing popularity of audio storytelling owes a lot to technology, as smartphones allow people to consume shows on demand anywhere, and cars increasingly come equipped with satellite radio and Internet-friendly dashboards. A recent report by Edison Research estimated that 64 percent of 12- to 24-year-olds and 37 percent of 25- to 54-year-olds in the United States listened to online radio weekly in 2014. The same year, 30 percent of respondents reported that they had listened to a podcast at least once, with 15 percent indicating that they had listened to a podcast within the last month.

Beyond the obvious convenience factor of listening on the go, what is it that makes some audio storytelling so engaging? And what happens in the brain when someone hears a really compelling story?

“A good story’s a good story from the brain’s perspective, whether it’s audio or video or text. It’s the same kind of activation in the brain,” says Paul Zak, the director of the Center for Neuroeconomics Studies at Claremont Graduate University. Zak has studied how watching and listening to stories influence our physiology and behavior.

In a study published in The Annals of the New York Academy of Science in 2009, Zak and his colleagues had participants watch short video clips featuring an emotional or unemotional scene. Afterwards, they filled out a survey about their emotions, played a game designed to test their level of generosity toward a stranger, and had their blood drawn. Those who reported feeling empathy for the characters in the clip were found to have 47 percent more of the neurochemical oxytocin in their body than those who didn’t feel empathetic toward the characters.

The researchers reason that experiencing tension in a story makes people feel stressed, which makes their bodies release the hormone and neurotransmitter oxytocin. Since oxytocin has been shown to increase empathy in some experiments, when things get tense while listening to a story, reading a book, or watching a TV show or movie, you may begin to empathize with the characters and get “transported” into the story.

Which means, according to Zak, that the best stories will always have an increasing level of tension, and that there exists a type of universal story structure—one in which a protagonist faces some sort of stressful challenge or conflict—that draws attention because it’s engaging emotionally and intellectually.

“What we have found in our research is that people require some sort of stressor, some sort of arousal response in the brain to have this type of narrative transportation where we begin to share the emotions of the characters in a story,” Zak says. “It makes sense that we need some sufficient reason to have that response. Our brain is trying to save resources and energy and having this arousal response is costly. Therefore we only want to give attention to something when it matters, when there’s something going on.”

He describes transporting into a story as a “neuro ballet” in which the reader, viewer, or listener knows she's not physically part of the story, and yet she still physically responds to it in a way that can change her behavior in the future.

Since oxytocin has been shown to make people more sensitive to social cues, Zak says that stories that keep people’s attention have to be character-driven. “You can tell a war story or something with a lot of action that will grab your attention,” he says, “but you still need a personal story, someone to empathize with. We need to have that social aspect for it to resonate with us.”

Podcasts and audiobooks benefit from the advantages of any character-based story. But some research, like a recent study conducted at the University of Waterloo, has shown that people who listen to the narration of a passage, like the audio storytelling found in traditional audiobooks, remember less information, are less interested in the content, and are more likely to daydream than those who read the same book out loud or silently to themselves.

But anyone who has gotten hooked on a podcast knows that audio can be much more than just narration. Emma Rodero, a communications professor at the Pompeu Fabra University in Barcelona, studies how audio productions retain people’s attention. Her work has shown that a dramatized audio structure, using voice actors who tell the story exclusively through dialogue, stimulate listeners’ imagination more than a typical “voice of God” narration. Participants who listened to the dramatized structure reported that they generated more vivid images in their minds, and conjured the images more quickly and easily than those in the narration condition. They also reported being more emotionally aroused and interested in the story.

Another study illustrates the importance of using sound effects, sounds that represent objects and/or environments and sound shots, an effect that gives the listener a sense of space by recording a sound that’s far away. Rodero found that the use of sound effects and sound shots in an audio drama increased the level of mental imagery that listeners reported, and also caused listeners to pay more attention.

Audiobook producers are catching on, and have started rolling out new types of “audio entertainment.” A novel by best-selling crime writer Jeffrey Deaver, called The Starling Project, has only been released as an audiobook, and features characters brought to life by 29 voice actors. Adapted by famed sci-fi author Orson Scott Card, Ender’s Game Alive, released by Audible in 2013, tells the Ender's Game story entirely through the use of dialogue and sound effects. And companies like Graphic Audio are creating audio dramas exclusively in the style, calling it “a movie in your mind.”

The tagline captures one of the best things about audio storytelling, according to Rodero. She says that, like reading, listening to audio allows people to create their own versions of characters and scenes in the story. But she thinks listening, unlike looking at a written page, is more active, since the brain has to process the information at the pace it is played.

“Audio is one of the most intimate forms of media because you are constantly building your own images of the story in your mind and you’re creating your own production,” Rodero says. “And that of course, is something that you can never get with visual media.”