The brain changes after ADHD medication. That does not make the signal a biomarker.

Methylphenidate has been used to treat ADHD for decades. We know considerably more about whether it improves ADHD symptoms than we do about exactly how those improvements are represented in the electrical activity of the brain.

EEG seems like an obvious place to look.

If medication changes attention, inhibition or cognitive processing, perhaps those changes should also appear in neural responses. If they appear consistently enough, researchers might eventually be able to use EEG to understand treatment mechanisms or even monitor whether an individual is responding.

A new systematic review in the Journal of Attention Disorders examines how close we are to doing either. Researchers reviewed EEG studies investigating the acute effects of methylphenidate in people with ADHD. The literature included event-related potentials as well as quantitative and spectral EEG measures, covering different stages of information processing.

One result appeared more consistently than most. Across studies examining the P3, methylphenidate was frequently associated with increased P3 amplitude. The P3 is an event-related potential occurring several hundred milliseconds after a relevant stimulus and is commonly associated with processes including attention allocation and updating of information in working memory.

That makes the finding interesting. If methylphenidate improves aspects of attentional processing, a change in P3 is biologically plausible.

But consistency was much harder to find elsewhere.

Studies examining earlier sensory responses, motor preparation, response monitoring and spectral EEG measures produced a much more mixed picture. Differences in tasks, participant characteristics, medication protocols, EEG processing and study design made it difficult to build a single account of what methylphenidate does to brain activity.

For me, this is where the paper becomes particularly relevant.

My own doctoral work used auditory electrophysiology to investigate processing in adults with ADHD. Electrophysiological measures are extraordinarily sensitive. They allow us to detect changes in neural activity that may never be visible from behaviour alone. But sensitivity and clinical usefulness are not the same thing.

A neural measure can change after medication without being able to tell us whether the medication is helping a particular person. It can correlate with a cognitive process without being specific to ADHD. A group-level difference can be statistically convincing while still being far too variable to guide an individual clinical decision.

This is the hurdle biomarker research repeatedly encounters.

To become clinically useful, a marker has to do considerably more than produce an interesting p-value. It needs to be reliable, reproducible and sufficiently sensitive and specific to answer a clinical question better than the tools already available.

According to this review, EEG is not there yet for monitoring methylphenidate treatment response. That conclusion does not make the EEG findings unimportant. Understanding how medication alters neural processing can tell us more about ADHD neurobiology and may eventually help researchers identify subgroups or mechanisms that are obscured when everyone with ADHD is analysed together.

But there is a large distance between detecting what medication does to a brain signal and using that signal to decide what medication will do for a person.

The new review is a useful reminder not to collapse those two questions. EEG may be showing us something real. We are still working out exactly what it means.

Reference

Quinn, D. M., Earley, C., Scarlat, I.-M., Whelan, R., & McGrath, J. (2026). Electroencephalographic signatures of methylphenidate in ADHD: A systematic review and 10 recommendations for future research. Journal of Attention Disorders.

https://doi.org/10.1177/10870547261483457