Nintendo itself raised questions about the health effects on children of using the 3DS, but the console also offers promise in treating visual problems, according to some experts.

The health effects are due to the fact that the 3DS uses a different technology than ordinary 3-D, the kind that requires glasses. But the same technology can also be used to diagnose problems, says Dr. Nathan Bonilla-Warford, an optometrist with VSP Vision Care, a non-profit that provides optometry care.

In systems with glasses, a 3-D screen produces two images, slightly offset from each other. They can be either red and blue or polarized slightly differently. IMAX theaters and televisions use the latter method, while the 3-D movies of the 1950s used the former. The illusion of depth happens because the images simulate the natural offset that comes from the separation of the eyes. By coloring the images differently (or using polarized lenses) each eye sees only one image, fooling the brain into thinking it sees things at differing distances.

But the 3DS - and the new glasses-free 3-D televisions - are different. In those cases, a liquid crystal display is also drawing two images. But the images are separated by a barrier (which would be perceived as dark lines across the screen) that separates the images so each eye sees a different one. This creates the 3D effect. As an added bonus, the visual thickness of the barrier can be adjusted so that the strength of the 3D effect can be changed as well.

Bonilla-Warford says while there are issues with eyestrain, by using a 3DS, one could see if someone has a problem with the mechanism that keeps eyes working together - for example, if someone had an undiagnosed lazy eye. There are a whole lot of people with these kinds of problems, he said. It affects reading and writing and copying from the board, and a lot of the time it isn't attributed to an eye problem.

It isn't a totally new idea - Bonilla-Warford says he uses a DS in therapies now for some kids who have visual problems.

But for all its potential as a tool, that doesn't mean there are no potential problems. The eye tries to automatically focus at a certain distance, by crossing (or uncrossing). Objects that are closer mean the eyes have to be aimed towards each other, while those further away force your eyes to point further apart. This can be tiring after long periods, and is the reason some people get headaches sitting too close to a screen.

When people wear glasses to view 3-D images, their eyes are 'fooled' into thinking the image is far away, so they focus accordingly. But the 3DS screens are smaller, and the eye will still focus on what looks like a close object, even though it looks like it has depth since people tend to hold them closer to the face.

Children, he says, are less self aware. When they get headaches they may not associate it with 3D viewing. They have a disincentive to tell their parents, he said. Because of course the first thing a parent will do is stop them from playing. So parents need to be much more on top of their children - watching if they are rubbing their eyes or squinting, he said.

Bonilla-Warford says that there is no evidence yet that using a 3DS is actively harmful, as extensive studies have not been done. But it is true, he says, that at about six years of age is when children develop the visual system that will look like the one they have as adults. We do know that anybody --whether child or not - with pre existing problems with the way eyes move and focus, they will have harder time viewing any 3D. So it may take subtle eye problems and exacerbate to where get nausea, headache and discomfort, he said.

The takeaway is that for most kids, a good rule is 20-20-20. Every 20 minutes, stare at something 20 feet away for at least 20 seconds. That helps prevent eyestrain, especially during those marathon gaming sessions.