If you had trouble concentrating, were feeling forgetful or restless, realized you've misplaced things, and kept getting distracted, chances are you'd want to know why. For children and adults with attention deficit hyperactivity disorder, or ADHD, these are just some of the symptoms they can face.

Now scientists are diving deeper into ADHD, looking for neurobiological markers in the brain that could explain how the developmental disorder, which is frequently misunderstood, affects people's behavior, emotions, and focus.

In recent years, imaging studies have revealed size differences in certain parts of the brain associated with ADHD. Researchers think that disrupted connectivity in the brain might be another neurological manifestation of the disorder.

"Growing evidence from [brain imaging] studies may help better reconceptualize ADHD by linking brain-based features with improved clinical care models and treatment outcomes," writes psychologist Jacqueline Saad and colleagues in their new paper, published in PLOS One.

This latest study, which looked at white matter parts of the brain, didn't observe any structural differences associated with ADHD. This is not a setback as the findings represent a sustained effort from researchers to recognize and understand brain features underpinning ADHD.

ADHD is a common but complex neurobehavioral disorder that emerges as patterns of inattentive, impulsive, and sometimes hyperactivity behavior. It affects millions of children and adults worldwide.

It is currently diagnosed solely based on behavioral signs, which must be present for six months or more. The behaviors are commonly dismissed as poor self-control or a lack of discipline.

Even though the exact cause of ADHD remains unclear, neuroscientists have demonstrated in some studies that ADHD stems from underlying neurological differences.

Without understanding these differences, we risk condemning ADHD as bad behavior instead of acknowledging it for what it is: another form of neurodiversity where people's brains seem to be wired to work another way.

Previous research from 2017 found brain differences related to ADHD that were small but clearly evident among some 1,700 children and adults with ADHD and 1,500 people without – the largest dataset at the time.

In 2018, another brain imaging study found evidence that structural changes associated with ADHD symptoms were recognizable in children as young as 4 years old who had less gray matter in areas associated with activity and attention than their peers.

But it seems distinguishing between different subtypes of ADHD on brain scans – the objective of this latest study – is more difficult, which could have something to do with the way that ADHD presents from one person to the next, and over time.

"We need more sophisticated but clinically relevant models that recognize ADHD results from a combination of deficits that interact to produce varying symptoms for every person who experiences ADHD," Alison Poulton, an Australian pediatrician specializing in treating children with the disorder, wrote in 2017. (Poulton was not involved in this new study.)

People presenting with inattentive ADHD, one of three subtypes of the disorder, often struggle to concentrate on tasks, make careless mistakes, and are easily distracted, while those with hyperactive-impulsive ADHD fidget, seem impatient, and can make rash decisions.

There's also combined ADHD, where people tend to display both inattentive and hyperactive-impulsive symptoms.

In this new study, the researchers looked at brain scans from 35 children and teenagers with ADHD (inattentive or combined ADHD) and compared them to 28 so-called neurotypical controls.

Using state-of-the-art spatial analyses and a 3D modeling technique called tractography, the researchers examined the microstructure and network connectivity of white matter in the brain, looking for differences in what, admittedly, was a small group of children.

On a structural level, the researchers could not observe any underlying differences in white matter that would differentiate the inattentive and combined subtypes from each other or separate the kids with ADHD from unaffected controls.

These results are in line with some previous research, the researchers write, though there have been only a handful of studies looking for structural differences in white matter between ADHD subtypes, and more research is warranted.

"While the present study focused on whether hard-wired networks may shed insight that distinguishes the subtypes, further exploration of the functional dynamics of brain network organization may potentially provide greater insight toward the underlying neurobiology of the ADHD subtypes," the team writes.

A person's behavioral expression of ADHD might also change from time to time, through their teen years and into adulthood, making ADHD subtypes a somewhat transient classification.

"Perhaps this is another potential reason for the null findings as ADHD presentation types are not stable across time, and thus they may be better linked with functional measures," of brain activity, Saad and colleagues conclude.

To add to an already complex puzzle, some ADHD researchers also posit there could be as many as seven different types of ADHD.

So it seems we have a long way to go yet before we truly understand how ADHD is wired in the brain, but we should appreciate this form of neurodiversity all the same.

The study was published in PLOS One.