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(CBS NEWS) – Early results from the most ambitious study of the developing adolescent brain ever undertaken show differences in some of the brains of heavy screen users.

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The data, supported by the National Institutes of Health, also found children who spent more than two hours a day on screens scored lower on thought and language tests than those who spent less time on phones and other electronic devices. 60 Minutes will report on the earliest data from the study Sunday, December 9 at 7:00 p.m., ET/PT on CBS.

Researchers at 21 sites have begun interviewing thousands of nine and 10-year-olds and scanning their developing brains.

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They will follow these and other participants for 10 years to determine how their brains change as they mature. The scans of 4,500 kids in the study show a thinning of the cortex in some of the brains of those spending more than seven hours a day playing video games or engaging with smartphones and tablets.

The thinning is normal but is usually expected to take place later in a child’s development.

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For more on this story, visit CBS News.