Sanne works in the finance department of a mid-sized company. Her colleagues know two versions of her. One version builds, on a Friday afternoon, a report the department has been waiting on for months, forgets to eat lunch and sends an email at ten past six saying 'done, have a look'. The other version sits on a four-line expense form for three days. She knows it has to be done. She wants to do it. It just does not happen.
Seen from the outside, that is hard to square. If you can do something that difficult, why not something simple? To Sanne it is just as puzzling, and after years of it, shame usually settles on top. Yet there is an explanation that fits what researchers see in the brain.
What runs differently in the brain
In 2009 a team led by Nora Volkow published a study in JAMA comparing the brains of 53 adults with ADHD to those of 44 adults without it. Using PET scans, they measured how many dopamine receptors and dopamine transporters were available in specific brain regions. Dopamine is a substance brain cells use to talk to each other. One of the networks in which it plays a leading role is the reward system: the regions that light up when something seems worth doing.
In the participants with ADHD, two parts of that reward system, the nucleus accumbens and the midbrain, had on average fewer of those receptors and transporters available. And the fewer there were, the more attention problems the participants reported. The researchers themselves wrote carefully that this fits the idea that the reward system responds less well in ADHD. They had not measured sensitivity to reward separately, so that step remains an interpretation. These are also group averages. You cannot tell from a single scan whether someone has ADHD.
What such a difference does in practice has been worked out by researchers such as Gail Tripp and Jeff Wickens. Normally the brain learns to release a dopamine signal as soon as something announces a reward. You see the coffee cart turn into the corridor and you are already pleased, before you have any coffee. According to Tripp and Wickens, that forward shift of the signal works less well in ADHD. The result: a reward that arrives in three weeks does not feel like a reward now. There is nothing pulling. A review by Luman, Oosterlaan and Sergeant showed as early as 2005 that children with ADHD, in experiments, more often choose a smaller reward now over a larger one later, more strongly than other children.
Interest as the engine
Put those two things side by side and Sanne's week makes more sense. The expense form has a reward that is far away and small: money that will come anyway, a tick in a system. The report was new, complicated, and there was a deadline behind it with people waiting. Those are exactly the things that do get the reward system moving: something interesting or new, or something that is urgent right now.
The American psychiatrist William Dodson calls this an 'interest-based nervous system'. Good to know: this is an observation from his consulting room, not a theory that has been tested in research. But it describes something many people with ADHD recognise immediately, and it lines up with what the dopamine studies show. Finding something important is often not enough to get moving with ADHD. Finding it interesting works better. Urgency almost always works, which explains why so many people with ADHD only get going the night before a deadline.
Hyperfocus: the other side of the same coin
When something does pull, the brain sometimes can hardly let go of it. Hours disappear. Messages stay unread, food is forgotten, so is the three o'clock meeting. This is called hyperfocus.
For a long time hyperfocus was mostly something people talked about and researchers rarely studied. In 2019 Kathleen Hupfeld and colleagues looked at it in two groups of adults, more than six hundred people in total. Participants with more ADHD traits reported more frequent and more intense hyperfocus, at school, in hobbies and in front of screens. In a 2021 review, Brandon Ashinoff and Ahmad Abu-Akel concluded that the phenomenon is still poorly defined and that good studies are scarce. They propose a definition: attention that becomes so intense through engagement with a task that you barely notice your surroundings, while the task itself usually goes better.
Hyperfocus and distractibility look like opposites, but they probably come from the same system. The brain regulates attention less on command and more on what the task stirs up. With a dull task, attention slides away. With a task that grips, everything else slides away.
When it stalls
Stalling on a simple task goes by popular names such as 'ADHD paralysis' or 'task paralysis'. These are not scientific terms. In research it is called procrastination and difficulty initiating tasks. A small 2014 study by Niermann and Scheres among students found that it is mainly the attention-related traits of ADHD that go together with procrastination, more than hyperactivity or impulsivity. It is a first finding with 54 participants, so not evidence you would build a house on. But it fits the picture that procrastination in ADHD has little to do with wanting. The system that is supposed to say 'worth it' simply does not say it.