Love looks different on Matt Frerking. His eyes struggle to stay open. His neck gives in to gravity. His body slumps.
During an attack of love, Frerking can hear and feel everything going on around him. He is not in pain. But his eyes are closed. And he cannot move.
With cataplexy, a disease estimated to affect at least 50,000 Americans, many of whom suffer for years before they're diagnosed, the slightest trigger can cause an attack of paralysis.
For Frerking, 39, a stepgrandfather from Portland, Ore., the trigger for cataplexy is love. Love for his wife, Trish, love for his stepchildren and their children. He can also have an attack after watching strangers show love.
Love, that basic human emotion, becomes his prison.
If Frerking wants to avoid an attack, he needs to avoid the things most people long for, including physical affection and giving and receiving love. Even thinking about his feelings for others or watching strangers express love can trigger an episode of paralysis.
"I have to limit those things very carefully,'' Frerking told "Nightline's" Cynthia McFadden, in the first episode of a new "Nightline" series, "Secrets of Your Mind: Why We Do What We Do," which begins Thursday.
"The brain is behaving as if it's asleep while Matt is awake," said Carol Ash, a sleep specialist at the Sleep for Life Center at Somerset Medical Center in New Jersey. Ash has not treated Frerking but has seen many cases like his.
Typically, when a healthy person is sleeping, a switch is turned on to inhibit motor neurons and muscles.
"[It] inhibits the muscles at night so that you can't get up and start running in your sleep, turns on during wakefulness and suddenly you're paralyzed,'' said Ash. "And so in someone like Matt, strong emotions are flipping a switch."
The faulty "switch" in question is located in the brain's hyphothalamus region. No one knows why or how, but somehow, increased blood flow to the brain centers where emotions are processed -- the amygdala and the cingulate gyrus -- triggers attacks.
For Matt Frerking, thinking about his past can cause problems. Anniversaries are especially tricky.
Matt and Trish Frerking met working across the hall from each other in a laboratory at the University of California at Davis.
Just one photo from their wedding 13 years ago hangs on a wall in a bedroom of their Portland-area home. Most of their other photos are stashed away, out of view, in photo albums.
"It was a good day, and I think that's about all I can really say about it, so … let's head downstairs,'' said Frerking, not wanting to trigger an attack by dwelling on the memory, as he toured his home with ABCNews as his wedding anniversary was approaching.
As his wife flipped through their wedding album, she said, "That was a quick ceremony. You had a mullet."
Even these everyday moments are landmines for her husband. The combination of the photos and his wife's little joke brought on an attack.
"I love him,'' said Trish Frerking, 51, who keeps a blog about what it's like to live with someone who has cataplexy. "Frankly, he loves me or he wouldn't put himself through it. And I'm thankful I'm the person who gets to be with him most of the time. I still feel lucky 13 years later."
They economize on the positive emotions, avoiding what Frerking calls the "warm and fuzzy" feelings, as if he had an emotional beaker that overflows almost as soon as it starts to fill.
"A lot of effort goes into keeping the fluid level in that beaker sufficiently low that even ... those moments when things get a little more full, there's some place for it to go," said Frerking, before he can't help but pass out.
The disease has not always intruded on the couple's life. Four years ago, it came on in an instant, and the two have been dealing with a new normal ever since.
Intimacy, for the most part, is off-limits.
"Holding hands in public is something that we can do for a few seconds at most, and that's about it," said Frerking. "Putting my arm around her is something that I don't do unless we're sitting down, and I know that it won't matter that much if I just flop over."
"It's tough,'' said his wife. "I miss touching him. ... I want to be touching him. ... I miss making out."
"Making out is hard to do," her husband agrees.
And it isn't only romantic love that causes the attacks.
Recently, at his stepgranddaughter's recent soccer game, just the sight of 3-year-olds in their uniforms caused him to have a massive attack before the game had even started.
"It's just too adorable for words, and he had to go sit down,'' explained his wife. "He was out for the entire soccer game and then some. He couldn't move, and so I'm thinking, yeah, Matt didn't really enjoy this very much 'cause he didn't get to enjoy any of it."
Movie trailers, because of the intensity of emotions they often convey in a short amount of time, also send Frerking into attacks, which means the couple rarely goes to the movies anymore.
Frerking's cataplexy used to be much worse. The attacks were more immediate before he began taking a potent cocktail of stimulants and sedatives twice daily, a regime he began two years ago and will likely have to keep for the rest of his life.
The medication slows the onset of Frerking's attacks, giving him a chance to find a resting spot before his muscle function shuts down.
Frerking rarely finds relief from his condition, though driving helps him stay focused and awake. And work is, for the most part, a refuge.
Ironically, Frerking is a neuroscientist who runs his own laboratory at Oregon Health Sciences University.
"My work is a very emotionally neutral kind of intellectually driven activity, so I'll spend a little time pondering over some problem that's bugging me at work,'' he said.
During an interview with ABCNews, Frerking made himself think about complex work problems to ward off an oncoming attack.
Frerking is also about to begin studying himself, to learn more about his baffling disease.
"So time will tell if anything comes of it," he said. "It would be pretty nice to contribute something."
But soon his wife called.
"Happy Anniversary honey," he said, as he answered a call on his cell phone from his wife, who wanted to thank him for the flowers he'd sent.
While on the phone he immediately slowed down and needed to support himself against a cabinet. "I need to stop thinking about it,'' he said.
Later that night, during an anniversary dinner at a restaurant during which Frerking struggled to stave off an attack, his wife told ABCNews, "I do see this long-term future for us. We're not gonna be the gray-haired couple holding hands as we walk down the street maybe, but we'll be, you know, telling jokes and walking."
Watch the full story Thursday on "Nightline: Secrets of Your Mind" at 8 p.m. ET