The effects of video-game playing on your brain have been studied for a quarter-century, but the latest research reveals that there are deep puzzles yet to be solved.

One of the earliest and most noted studies in the field was conducted back in 1992 by neuroscientist Richard Haier at the University of California at Irvine, who looked at how frequent sessions with the Tetris video game changed the players' brains. The game requires players to fit colorful puzzle pieces together at a quickening pace as they fall from the top of the screen.

Back then, Haier used brain scans to discover that some parts of the brain actually used less glucose as the players became more skilled at the game. The "Tetris effect" illustrated how video-game training could make brains work more efficiently - an idea that eventually led to a whole host of brain-training games.

Now Haier serves as a consultant to Blue Planet Software, the company that markets Tetris, and he was asked to follow up on his 17-year-old research using the new tools available to neuroscientists.

Haier recruited three colleagues - Sherif Karama from the Montreal Neurological Institute, Leonard Leyba from the New Mexico-based Mind Research Network and Rex Jung, a clinical neuropsychologist at the University of New Mexico. They came up with an experiment that budgeted out at "under $100,000," with the expense picked by Blue Planet, Haier said.

The company had no say in how the experiment was conducted - and it didn't get an advance look at the resulting research, which was published online today in BMC Research Notes, a peer-reviewed, open-access journal. "This was kind of a labor of love," Haier told me.

The researchers recruited 26 girls, aged 12 to 15. Adolescents were selected because their developing brains were more likely to reflect changes, and girls were selected because they tend to have less experience with video games than boys. Fifteen of the girls were given the task of playing the video game for an average of 90 minutes a week over the course of three months. The others were told to avoid playing video games.

Both groups were monitored for changes in brain function as well as brain structure. Earlier research conducted in Germany had shown that juggling practice led to a thickening in areas of the cerebral cortex, so Haier and his colleagues were pretty sure they'd find a link between what they saw in the functional MRI (about more efficient brain function) and in the structural MRI (about cortex thickening).

And that's where the brain puzzle threw them for a new loop.

"In science, everyone makes a very big deal about having a hypothesis before you go on a fishing expedition," Haier said. "Never once in 20 years has my hypothesis worked out the way I thought it would. The brain is always a surprise."

The researchers analyzed the brain changes in the game-playing group compared with the control group, and they found that the Tetris players' brain function became more efficient in areas linked to critical thinking, reasoning, language and information processing - just as Haier found in 1992. They also discovered that the cortex became thicker - just as the German researchers had discovered. The only problem was ... they weren't the same areas.

"We all were surprised when we put the images together and saw that there was no overlap," Haier said. The cortex became thicker in areas of the brain linked to the planning of complex movements as well as the coordination of sensory information.

Haier had hoped that he and his colleagues would come up with a mechanism to explain in physiological terms how the brain became more efficient through game-playing. "The obvious thing would be if you get more brain tissue, you have more neurons to work on a problem, so therefore that area of the brain doesn't have to work as hard," he said.

Now he realizes the problem isn't as simple as he thought. "What this study does, really, is lay the groundwork for a whole series of studies to untangle all this," he said.

In a news release, the University of New Mexico's Jung said he'd like to see what happens to game-playing brains over time.

"We hope to continue this work with larger, more diverse samples to investigate whether the brain changes we measured revert back when the subjects stop playing Tetris," Jung said. "Similarly, we are interested if the skills learned in Tetris, and the associated brain changes, transfer to other cognitive areas such as working memory, processing speed, or spatial reasoning."

Haier would love to figure out how the different areas of the brain interact during mental training, on a time scale of milliseconds. But that job may be beyond the capability of functional MRI scans, which can monitor changes only on the scale of seconds. "If we're interested in information flow in the millisecond range, by the time fMRI can see it, it's too late," Haier said.

So Haier is setting his sights on yet another new technology, and it's a real mouthful.Magnetoencephalography, or MEG, monitors the faint magnetic fields produced by the brain's electrical activity. Haier thinks MEG scans could reveal how the parts of the brain that become more efficient interact with the parts that develop thicker tissue.

"The time resolution of this technology is a millisecond, so you can see changes in the brain millisecond by millisecond," he said.

As Haier talked about how he'd design those future experiments in game-playing, which would have to be conducted within a magnetically shielded environment, I could tell he was already trying to fit the puzzle pieces together in his mind.

"I want to know what the heck is going on in those brains," he said.