NEW YORK, Dec. 10 (Xinhua) -- Children who spend more than seven hours a day on screens like smartphones and video games experience "premature thinning of the cortex," according to a new and ongoing study.

Funded by U.S. National Institute of Health (NIH), the study plans to follow more than 11,000 adolescents for a decade to see how childhood experiences impact the brain.

The first findings from the study is already available. By scanning brains of 4,500 participants, researchers found that children who use smartphones, tablets and video games more than seven hours a day showed premature thinning of cortex.

"That's typically thought to be a maturational process. So what we could expect to see later is happening a little bit earlier," said Dr. Gaya Dowling, an NIH doctor working on the project when interviewed by CBS News.

Dowling said it is still early to tell whether the change was caused by the screen time, and whether or not it is a bad thing. "It won't be until we follow them over time that we will see if there are outcomes that are associated with the differences that we're seeing in this single snapshot," she said.

Early results from the study also showed that as little as two hours of screen time daily could negatively affect children: Those who have more than two hours of screen time a day got lower scores on tests focused on thinking and language skills.

As Dowling noted, a full picture of the screen-time effects won't be possible until years down the line, when the study is complete.