A study published today in JAMA Pediatrics warns that kids’ literacy and language skills suffer with screen use, and MRI scans of their brains appear to back up the findings.

The study: Forty-seven 3- to 5-year-olds took a test to measure their cognitive abilities, and their parents were asked to answer a detailed survey about screen time habits. Questions included: How frequently do they use that screen? What type of content are they viewing? And is there an adult sitting with the child talking about what they’re watching? The answers were scored against a set of screen time guidelines put out by the American Academy of Pediatrics. The kids also had their brains scanned in an MRI machine.

Brain changes: The scans revealed that kids who spent more time in front of screens had what the authors call lower “white matter integrity.” White matter can be roughly thought of as the brain’s internal communications network—its long nerve fibers are sheathed in fatty insulation that allows electrical signals to move from one area of the brain to another without interruption. The integrity of that structure—how well organized the nerve fibers are, and how well developed the myelin sheath is—is associated with cognitive function, and it develops as kids learn language.

Lead author John Hutton of Cincinnati Children’s Hospital told MIT Technology Review there’s a clear link between higher screen use and lower white matter integrity in the children his team studied. That structural change appears to be reflected in the results of the cognitive test the kids took as well, which showed high screen time associated with lower levels of language and literacy skills. “The effect size is substantial, as these findings also rigorously controlled for multiple comparisons across the brain,” Hutton says.

The big caveats: It's a small and preliminary study. "It's absolutely not clear that screen time causes differences in brain development and there are many factors that could explain the association found here," Signe Lauren Bray, a researcher at the University of Calgary who was not involved in the study, said via email. Bray has done fMRI studies on kids brains, and pointed to other work that suggested kids who spent more time in front of screen tended to display more symptoms of ADHD. But that study also suggested that the symptoms could be the very reason why kids were spending time in front of screens in the first place. Socioeconomics could also play a role, she said, and screen time effects could disappear if those factors were taken into account.

Just tell me how much screen time is okay for my kid already. Unfortunately, that’s not so easy. “It’s hard to say what the ‘safe’ age or amount of screen time is,” Hutton says. “My motto is ‘Screen-free until three’—this at least gets kids to preschool with a solid anchor in the real world, where their basic sense of connection with caregivers and early language skills have solidified.”

It’s a small study, but also big. “While relatively small for a behavioral study, this is actually a fairly large MRI study, especially involving young children, [and] the first to explore associations between screen time and brain structure,” Hutton says. Next steps include more tests on kids and efforts to figure out how parents’ screen use might influence their children.

The take-home message: “Caution is warranted,” Hutton says. “Children are not small grown-ups, and their needs change with development.”

This story was updated on November 5 to include comment from Signe Lauren Bray.