3-D Movies Like 'Alice in Wonderland' Are Just a Headache for Some

March 2, 2010— -- Movies and television shows in 3-D have become trendy, but they spotlight the problems some people have watching them. Some people feel discomfort; others can't see the 3-D effect at all.

Johnny Depp, who plays the Mad Hatter in the upcoming 3-D feature "Alice in Wonderland," says he avoids watching all his films, but in an interview with Entertainment Weekly he said he had a special excuse this time, since he can't enjoy the 3-D effects.

"I've got a weird thing where I don't see properly out of my left eye, so I truly can't see 3-D," he told the magazine.

Depp is far from alone in his troubles with the third dimension. Exact numbers are hard to pin down, but studies have estimated that between 5 percent and 12 percent of Americans have some degree of stereoblindness, which is the inability to see depth properly.

"I don't sense that I'm looking at a 3-D image," said Rafe Needleman, an editor at the technology Web site CNET. "It's far more tiresome than convincing, because mostly, I see double images or ghosting. Either it doesn't work and I don't see anything, or I see two images where I know I should see one."

Three-dimensional films are constructed by using two cameras set slightly apart from each other. That's why, if you look at a 3-D film without the glasses you receive at the theater, you can see two separate images. Using the glasses, however, helps the brain consolidate the images, giving the picture depth.

"In life, that's what's happening. That's what fusion and stereopsis are," said Dr. Lisabeth Hall, a pediatric ophthalmologist at New York Eye and Ear Infirmary, naming the process by which the brain consolidates images from the two eyes to create depth perception.

The fine depth perception used for 3-D films, as well as tasks like flying a plane or constructing a top-quality Swiss watch, is not the only depth perception the brain can generate from information the eyes give it. Even with one eye, the brain can figure out how far away certain objects are.

"The brain can make judgments about depth from many different clues," said Dr. Joseph Rizzo, a neuro-ophthalmologist at the Massachusetts Eye and Ear Infirmary and Harvard Medical School.

There are a number of reasons why someone might not be able to enjoy the full effect of a 3-D film. Doctors told us that for optimal stereo vision, two factors have to come together: