Many children and teenagers with ADHD may find it difficult to turn in homework — they get too distracted to complete it, they lose the pens and pencils needed to do it. Their daydreams constantly interrupt their thoughts. They re-read the same sentences several times because they can’t seem to stay focused as well as their peers. They fidget. They touch everything. Some talk — nonstop.

Attention deficit hyperactivity disorder is a condition that not long ago some claimed was made up. Some still do. It remains difficult to diagnose and treat. Doctors and psychologists use batteries of verbal and written tests as well as checklists to evaluate behavior. But in recent years, scientists have been able to see things associated with ADHD by looking at the brains of people who have it.

Most recently, scientists at the University of Michigan looked into the minds of more than 750 youth and discovered that those with ADHD lag behind their peers when measuring how their brains form connections between key networks. For instance, those with ADHD have less-developed connections between a part of the brain that controls internal-directed thought, such as daydreaming, and a part that controls external-directed tasks, such as homework, according to a study published Monday in the Proceedings of the National Academy of Sciences. That lag in connection development may help explain why youth with ADHD struggle to stay focused, according to the news release.

University of Michigan psychiatry professor Chandra Sripada, lead author of the study, told The Washington Post that the transition between “internally focused modes of attention” and “externally focused modes of attention” work somewhat like a seesaw.  In an “immature” brain, the transitions are poorly executed. As the brain matures, the transitions become more graceful. “Those with ADHD, their minds swing back and forth too abruptly,” he said.

“The default network is maturing very rapidly between youth and adulthood,” Sripada told NPR. “It’s neither a hero nor villain — you need to be able to turn it on appropriately and turn it off appropriately.”

For the study, researchers examined brain scans of 275 young people ages 7 to 21 with ADHD and 481 others without it to see how each group’s brain networks communicate. Sripada said the idea that there is a lag in the maturation of the brain of people with ADHD has been shown through structural studies. His study looked at the function — what the brain is actually doing.

The research may help scientists study the course of ADHD from childhood to adulthood. In some cases, children seem to out-grow the disorder; in other cases, the disorder follows them. Similar studies on brain network maturation may help explain the difference, according to study. Eventually, Sripada said, these types of studies may help provide a marker that will make it possible for doctors to use objective tools such as brain scans to aid in ADHD diagnosis.

“The results of this study set the stage for the next phase of this research, which is to examine individual components of the networks that have the maturational lag,” Sripada said. “This study provides a coarse-grained understanding, and now we want to examine this phenomenon in a more fine-grained way that might lead us to a true biological marker, or neuromarker, for ADHD.”

To help develop a neuromarker, Sripada is conducting two follow-up studies — one for youth and one for adults.