Kids spend an increasing fraction of their formative years online, and it is a habit they dutifully carry into adulthood. Under the right circumstances, however, a love affair with the Internet may spiral out of control and even become an addiction.

Whereas descriptions of online addiction are controversial at best among researchers, a new study cuts through much of the debate and hints that excessive time online can physically rewire a brain.

The work, published June 3 in PLoS ONE, suggests self-assessed Internet addiction, primarily through online multiplayer games, rewires structures deep in the brain. What's more, surface-level brain matter appears to shrink in step with the duration of online addiction.

"I'd be surprised if playing online games for 10 to 12 hours a day didn't change the brain," says neuroscientist Nora Volkow of the National Institute on Drug Abuse, who wasn't involved in the study. "The reason why Internet addiction isn't a widely recognized disorder is a lack of scientific evidence. Studies like this are exactly what is needed to recognize and sette on its diagnostic criteria," if it is a disorder at all, she says.*

Defining an addiction

Loosely defined, addiction is a disease of the brain that compels someone to obsess over, obtain and abuse something, despite unpleasant health or social effects. And "internet addiction" definitions run the gamut, but most researchers similarly describe it as excessive (even obsessive) Internet use that interferes with the rhythm of daily life.

Yet unlike addictions to substances such as narcotics or nicotine, behavioral addictions to the Internet, food, shopping and even sex are touchy among medical and brain researchers. Only gambling seems destined to make it into the next iteration of the Diagnostic and Statistical Manual of Mental Disorders, or DSM, the internationally recognized bible of things that can go awry with the brain.

Nevertheless, Asian nations are not waiting around for a universal definition of Internet addiction disorder, or IAD.

China is considered by many to be both an epicenter of Internet addiction and a leader in research of the problem. As much as 14 percent of urban youth there—some 24 million kids—fit the bill as Internet addicts, according to the China Youth Internet Association. By comparison, the U.S. may see online addiction rates in urban youth around 5 to 10 percent, say neuroscientists and study co-authors Kai Yuan and Wei Qin of Xidian University in China.

The scope of China's problem may at first seem extraordinary, but not in the context of Chinese culture, says neuroscientist Karen M. von Deneen, also of Xidian University and a study co-author.

Parents and kids face extreme pressure to perform at work and in school, but cheap Internet cafes lurk around the corner on most blocks. Inside, immersive online game realities like World of Warcraft await and allow just about anyone to check out of reality.

"Americans don't have a lot of personal time, but Chinese seem to have even less. They work 12 hours a day, six days a week. They work very, very hard. Sometimes the Internet is their greatest and only escape," according to von Deneen. "In online games you can become a hero, build empires, and submerge yourself in a fantasy. That kind of escapism is what draws young people."

Out of sight of parents, some college kids further cave to online escapism or use gaming to acquire resources in-game and sell them in the real world. In a recent case Chinese prison wardens allegedly forced inmates into the latter practice to convert digital gold into cold-hard cash.

Several studies have linked voluntary and excessive online use to depression, poor school performance, increased irritability and more impulsiveness to go online (confounding addicts' efforts, if they want to at all, to stop pouring excessive time into online games). To study the effects of possible Internet addiction on the brain, researchers began with the Young Diagnostic Questionnaire for Internet addiction.

This self-assessment test, created in 1998 by psychiatrist Kimberly Young of Saint Bonaventure University in New York State, is an unofficial standard among Internet addiction researchers, and it consists of eight yes-or-no questions designed to separate online addicts from those who can manage their Internet use. (Questions range from, "Do you use the Internet as a way of escaping from problems or of relieving an anxious mood?" to "Have you taken the risk of losing a significant relationship, job, educational or career opportunity because of the Internet?".)

The China-based research team picked 18 college-age students who satisfied addict criteria, and these subjects said they spent about 10 hours a day, six days a week playing online games. The researchers also selected 18 healthy controls who spent less than two hours a day online (an unusually low number, says von Deneen). All of the subjects were then plopped into an MRI machine to undergo two types of brain scans.

Brain drain

One set of images focused on gray matter at the brain's wrinkled surface, or cortex, where processing of speech, memory, motor control, emotion, sensory and other information occurs. The research team simplified this data using voxel-based morphometry, or VBM—a technique that breaks the brain into 3-D pixels and permits rigorous statistical comparison of brain tissue density among people.

The researchers discovered several small regions in online addicts' brains shrunk, in some cases as much as a 10 to 20 percent. The affected regions included the dorsolateral prefrontal cortex, rostral anterior cingulate cortex, supplementary motor area and parts of the cerebellum.

What's more, the longer the addiction's duration, the more pronounced the tissue reduction. The study's authors suggest this shrinkage could lead to negative effects, such as reduced inhibition of inappropriate behavior and diminished goal orientation.

But imaging neuroscientist Karl Friston of University College London, who helped pioneer the VBM technique, says gray matter shrinkage is not necessarily a bad thing. "The effect is quite extreme, but it's not surprising when you think of the brain as a muscle," says Friston, who was not involved in the study. "Our brains grow wildly until our early teens, then we start pruning and toning areas to work more efficiently. So these areas may just be relevant to being a good online gamer, and were optimized for that."

(Friston says London taxi drivers provide a telling comparative example of the brain's ability to reshape itself with experience. In the 2006 study, researchers compared taxi drivers' brains with those of bus drivers. The former showed increased gray matter density in their posterior hippocampi—a region linked to maplike spatial navigation and memory. That probably comes as no surprise to London cabbies, who spend years memorizing a labyrinthine system of 25,000 streets, whereas bus drivers have set routes.)

As another crucial part of the new study on Internet addiction, the research team zeroed in on tissue deep in the brain called white matter, which links together its various regions. The scans showed increased white matter density in the right parahippocampal gyrus, a spot also tied to memory formation and retrieval. In another spot called the left posterior limb of the internal capsule, which is linked to cognitive and executive functions, white matter density dropped relative to the rest of the brain.

Disorder under construction

What the changes in both white and gray matter indicate are murky, but the research team has some ideas.

The abnormality in white matter in the right parahippocampal gyrus may make it harder for Internet addicts to temporarily store and retrieve information, if a recent study is correct. Meanwhile, the white matter reduction in the left posterior limb could impair decision-making abilities—including those to trump the desire to stay online and return to the real world. The long-term impacts of these physical brain changes are even less certain. Rebecca Goldin, a mathematician at George Mason University and director of research for STATS, says the recent study is a big improvement over similar work published in 2009. In this older study a different research group found changes in gray matter in brain regions of Internet addicts. According to Goldin, however, the study lacked reliable controls.

The sample sizes of both studies were small—fewer than 20 experimental subjects each. Yet Friston says the techniques used to analyze brain tissue density in the new study are extremely strict. "It goes against intuition, but you don't need a large sample size. That the results show anything significant at all is very telling," Friston notes.

In the end all of the researchers interviewed by Scientific American emphasized significance only goes so far in making a case for IAD as a true disorder with discrete effects on the brain. "It's very important that results are confirmed, rather than simply mining data for whatever can be found," Goldin says.

Correction (06/17/11): This story was updated throughout to correct the spelling of Karen von Deneen's last name.