For the study, published this week in JAMA Pediatrics, researchers at the Cincinnati Children's Hospital Medical Center compared brain scans of 47 otherwise healthy children aged 3 to 5 with parental reports of their screen use.

They found that that children who exceed the one-hour-per-day guideline for screen time set by the American Academy of Pediatrics (AAP) had lower structural integrity of white matter in parts of the brain that are associated with literacy skills such as imagery, mental control and self-regulation.

The children who exceeded the AAP guidelines also performed worse on cognitive tests, according to a press release. For instance, they had lower expressive language and were slower to rapidly name objects in tests of processing speed.

"This is important because the brain is developing the most rapidly in the first five years," lead author Dr. John Hutton, a pediatrician at Cincinnati Children's Hospital, told CNN. "That's when brains are very plastic and soaking up everything, forming these strong connections that last for life."

The researchers noted that in just one generation, portable electronics have rapidly changed childhood, as screen time has become a regular part of learning and playing that only increases from infancy onward.

The children in the study averaged around two hours of screen time per day, though the highest amount reported was 12 hours per day. Around 60 percent had their own portable electronic device and 41 percent had a screen in their bedroom, The New York Times reported.