Thousands of moviegoers were no doubt clutching their seats while watching "Cloverfield," last weekend's No. 1 film at the box office. At least a few of them were clutching their stomachs as well.

Thousands of moviegoers were no doubt clutching their seats while watching "Cloverfield," last weekend's No. 1 film at the box office. At least a few of them were clutching their stomachs as well.

Since the movie opened, some patrons say they felt nausea and dizziness while watching the horror flick, much of which was filmed in a jerky motion with a hand-held camera.

Erika Hasegawa, 32, was watching "Cloverfield" in Los Angeles, when she had to leave in the middle.

"I'm really nauseous right now — just hold on for a second," she said, before retching into a trash can. "I wish I could get my money back."

It's unclear how many people have felt ill while watching the movie, which follows a group of young hipsters filming themselves with a camcorder while they flee a reptilian monster destroying New York City. The movie did set box office records on its opening weekend, earning about $46 million.

Reports of illness while watching "Cloverfield" started popping up on Internet bulletin boards, with some writing that they had to leave a few minutes into the movie, while others said they tried to stare at a wall as the movie continued.

Some experts were not surprised, given the film's use of hand-held cameras that were jerked around to boost suspense.

It's a technique that has been used in other movies — notably the "Blair Witch Project," a film released in 1999 that also prompted viewer complaints about nausea and vomiting. Both movies were filmed from the perspective of young adults holding camcorders while trying to escape from a mysterious, terrifying force.

The problems may be more pronounced at theaters with oversize movie screens. The larger the screen, the harder it is for the viewer to keep a visual frame of reference, said John Risey, an audiologist at the Ochsner Clinic in New Orleans.

"To your brain, it does appear that the entire world is moving," Risey said. That problem is not an issue when viewing the same film on a small TV screen, he added.

"What makes you dizzy is that the visual environment is moving beside you, and yet you are still," Risey said. The brain becomes confused, he continued, and can trigger reactions in other parts of the brain that cause nausea and vomiting.

What the eyes process visually has enormous impact on how the body reacts, according to Dr. Dennis Maceri, an associate professor at the University of Southern California School of Medicine who specializes in the care of the ears, nose and throat. The body reacts to what we see, even if it's not real.

In the 1968 Steve McQueen film "Bullitt," for example, viewers follow a camera filming a car chase set along the rolling hills of San Francisco.

"He goes down the big hill, and you can feel your stomach move ... even though your body is stationary," said Maceri.

"The eyes can fool you," Maceri said.

It's similar to the feeling some people get while they try to read in a moving car, he said. To the eyes, the words on the page appear to jump around, and "you can't stabilize your gaze."

Experts suggest those who feel motion sickness try to stare at a fixed point that doesn't appear to move, such as the head of someone sitting in front of you in a theater or at a nearby seat, Risey said.

Dramamine, a medication taken to relieve nausea, might also help, he added.

Peter Bohlen, an 18-year-old high school student from Glendale, Calif., said he felt nauseated after he watched the movie with four of his friends, and still wasn't feeling well a day later.

"They're trying to go for the home video feel, so it's constantly shaking around," Bohlen said. "My brain tried to make it work, but it couldn't. I got a pretty heinous headache afterward."