A new study looked at 3-to-5-year olds who used TVs, tablets, and smartphones for more than an hour a day.
Researchers said those children had lower levels of development in the brain's white matter.
That's an area key to the development of language, literacy, and cognitive skills.
The new study used a special type of MRI.
It is the first study to document screen time and brain structure development in preschool-aged kids.
The study was published in the journal JAMA Pediatrics.