Brain scans on Australian kids with ADHD have shed light on why they have trouble remembering and don't recognise distractions.

Researchers at Flinders University in Adelaide tracked the brain activity of 150 children and teenagers with attention deficit hyperactivity disorder (ADHD) to build a neuro-cognitive profile of their behaviour.

Using the card game Snap, the scientists tested how well sufferers off their medication were able to remember numbers in the short term as part of their "working memory".

"In Snap, you have to recognise that the same two cards have appeared in a row, but we found that children diagnosed with ADHD had incredible difficulty detecting doubles at all," said the study's lead researcher, PhD student Hannah Keage.

"They just weren't able to select that information about the cards properly and hold it on-line in their head."

And tests of brain activity revealed why, with charts showing "weaker" and slower brain activity than the extremes shown in a "normal" brain.

So their brain recruited less neurons than was normal to get involved in that operation, Ms Keage said.

"Clearly, they're distracted because they're not getting the right information in the brain."

The researcher, who presented her findings at the World Congress of Neuroscience in Melbourne on Monday, also tested the ability of ADHD kids to cope with distractions.

Using the same computer card game, the patients were asked to play while distracting images also flashed up on screen.

These children didn't recognise these distractions as a deviation from the task or as a distraction at all, and they displayed the same "weak" brain activity.

"That means they essentially have difficulty recognising new usual things as new, which makes learning new things very difficult," Ms Keage said.

Repeating the tests once the children had resumed their stimulant medication showed vastly improved uptake of both tasks, proving the drugs many children take for the condition work.

The team hopes that better understanding the behavioural deficits in ADHD kids will help in the development of new therapies that directly target these problem areas.

"There's scope there now to try to train these children to recognise new information or remember things differently to help overcome these learning disabilities," Ms Keage said.