why Coraline 3D gave me a headache

Coraline 3D gave me a horrible headache. I thought it was the glasses (worn over my glasses), but it turns out that the 3D cinema experience generally leads to dolorous cerebrums.

Something different happens when you're viewing three-dimensional motion projected onto a flat surface. When a helicopter flies off the screen in Monsters vs. Aliens, our eyeballs rotate inward to follow it, as they would in the real world. Reflexively, our eyes want to make a corresponding change in shape, to shift their plane of focus. If that happened, though, we'd be focusing our eyes somewhere in front of the screen, and the movie itself (which is, after all, projected on the screen) would go a little blurry. So we end up making one eye movement but not the other; the illusion forces our eyes to converge without accommodating. (In fact, our eye movements seem to oscillate between their natural inclination and the artificial state demanded by the film.) This inevitable decoupling, spread over 90 minutes in the theater, may well be the cause of 3-D eyestrain. There's nothing new about the idea—an article published in the Atlantic in 1953 refers to the breakdown of the accommodation-convergence ratio as a "difficulty [that] is inherent to the medium." And there's no reason to expect that newfangled RealD technology will solve this basic problem of biomechanics.