Thousands of people are packing movie theaters across the country to see the new "Alice in Wonderland" in 3-D, and dozens of them will likely leave with headaches.

That's not a criticism of the film, but a fact: Doctors say those with less-than-perfect eyesight can suffer nausea, blurred vision and dizziness from 3-D movies.

"The 3-D technology taps into our depth perception," said Dr. Lawrence Tychsen, ophthalmologist at St. Louis Children's Hospital. "To fully appreciate depth in a 3-D movie, you need equally clear vision in both eyes. Even a small misalignment could contribute to those symptoms of discomfort."

Tychsen said relatively minor conditions such as nearsightedness, farsightedness or a lazy eye - if not treated with glasses or contacts - could trigger headaches and other side effects from 3-D visuals.

He estimated that up to 20 percent of the population could be affected - both adults and children.

 VISION FATIGUE

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The problem comes from so-called vision fatigue, caused when 3-D technology forces the eyes to make constant adjustments to focus on images that are simultaneously near and far away. Humans see in three dimensions, but the exaggerated imagery of 3-D movies can cause a strain in some, according to Jeffrey Anshel, a California optometrist who has researched vision fatigue in computer users.

Reports of vision fatigue popped up in recent months after the release of the 3-D blockbuster "Avatar," which has shattered box office records, raking in more than $2.5 billion worldwide and becoming the highest-grossing film of all time. Several theatergoers complained of motion sickness after watching James Cameron's epic sci-fi adventure, filmed with breakthrough digital 3-D techniques.

 MORE 3-D IS ON THE WAY

Despite causing discomfort in a small number of people, 3-D movies aren't going away anytime soon. Besides "Avatar," some of last year's other top-grossing films - "Up," "Ice Age: Dawn of the Dinosaurs" and "Monsters vs. Aliens" - were 3-D, and more than a dozen other 3-D movies are scheduled for release this year and next.