Children reach developmental milestones when the relevant tracts in their brain are myelinated, he said; the language “explosion” at 18 months, for example. When the connection is fully myelinated between the part of the brain known as Wernicke’s area (comprehension of words) and Broca’s (production of speech), children go from understanding words to being able to say them. “The adage from neuroscience is the neurons that fire together wire together,” Dr. Hutton said. “Practice doing anything reinforces connections.”

The average ScreenQ score in the study was about 9, with a range of 1 to 19. About 41 percent of the children had a screen in the bedroom; about 60 percent had their own portable devices. The median screen time per day was an hour and a half, but the range was from none to 12 hours.

After controlling for age, gender and income, the children with higher ScreenQ scores had lower measures of structural integrity and myelination, especially in tracts involved with language and literacy skills.