Research on EEG in ADHD has consistently shown differences in brain rhythms, in the alpha band. Alpha oscillations are thought to be linked to attentional processes and are sometimes reduced in people with attention-deficit hyperactivity disorder.
Building on this work, Kannan and colleagues asked whether these rhythms could be influenced by stimulating the brain with transcranial alternating current stimulation (tACS).
Continuous performance task (CPT) in a virtual room
To measure attention, the study used a CP task. In a CPT, participants watch a stream of stimuli and respond only to specific targets.
In this study, the task was to press a space bar when the letter “A” was followed by the letter “K”.
The task is repetitive and requires sustained focus, so it’s commonly used to assess attention lapses. This is especially useful in ADHD research.
Instead of a standard computer task, the CPT is done in a virtual seminar room. This makes the task feel more like everyday situations, but still allows precise measurement of performance and behavior.
(A) Equipment used, including EEG in ADHD study during the experiment. (B) CP task in the virtual seminar room, from a participant’s perspective.In the CP task, there are two different phases: non-distractor (NDP) and distractor phases (DP). During DP, virtual classmates performed more complex actions to distract the participants.
Now, the researchers can analyze behavioral measures:
- Omission error rate (missed responses to target stimuli).
- Commission error rate (invalid responses to non-targets).
- Reaction time variability – RTV.
Setting up brain stimulation and EEG in ADHD participants
Authors wanted to investigate whether tACS could increase alpha activity in adults with ADHD. And whether this could improve attention and related behavioral symptoms. Previous studies showed alpha-band differences in posterior brain regions in ADHD and healthy controls. Therefore, the stimulation electrodes were placed over Cz and Oz to target these areas.
Participants received tACS at their individual alpha frequency (IAF), after a resting EEG was used to determine IAF. Active stimulation lasted 18 minutes, while sham consisted of a brief, ramped current to mimic sensations.
Measuring EEG in ADHD study
Using mbt’s mobile EEG device, authors were able to collect EEG data in all phases of the study.
For the analysis, they used wavelet transformation to see how the activity changes over time and frequencies. This approach is especially useful for tasks like the CPT, where attention can vary rapidly. By applying wavelet analysis, authors could examine how alpha power evolved before and after stimulation.
Similar to stimulation sites, they also focused on the posterior regions for EEG: Pz, POz, CPz, P3 and P4 locations.
Adding eye-tracking measurements
Researchers tracked eye movements and head motion during the CP task as well! They used the Tobii eye-tracking system to additionally track how distracted the participants were during CPT. This is possible by measuring time looking at the canvas, distractors, or gaze wandering.
Check out our post here to see how eye-tracking and EEG can be synchronized.
Behavior and subjective evaluation during CPT
(A) Omission error rate, (B) commission error rate and (C) RTV before and after the sham and active stimulation. Sham measurements are in blue, and active tACS stimulation is in red.Participants’ performance on CPT didn’t seem to change significantly between the tACS and sham conditions. But they also filled out a questionnaire on core ADHD symptoms during the experiment. So let’s see if there are any subjective differences they felt after stimulation:
(A) Hyperactivity, (B) inattention and (C) impulsivity scores. These ranged from -3 (strongly disagree) to 3 (strongly agree). We can see scores pre and post sessions – sham are in blue, and tACS sessions are in red.When participants rated how they felt after the session, there was no difference between tACS stimulation and sham. Even though some ratings shifted slightly over time, people didn’t report feeling less affected by their ADHD symptoms.
So, the alpha-tACS didn’t seem to change how participants experienced their attention either.
Eye-tracking and movement data
Measures of gaze stability and head movement also showed no significant changes between active and sham. This suggests that alpha-tACS did not meaningfully affect motor restlessness or physical engagement in the virtual environment.
Were there any changes in EEG in ADHD patients during tACS stimulation?
Results of the wavelet analysis during CPT performance: pre- and post-intervention. The intervention was either a sham or active stimulation session. Labeled area in the spectrogram is called ERO and it stands for event related potential.
(A) Boxplots show mean alpha power pre and post for sham and active stimulation (blue and red). We can also see (B) alpha power before and after the sham stimulation vs. (C) in case of active stimulation.Alpha power increased from pre- to post-stimulation, which is clearly visible in the plots above. However, this increase is there in both the tACS and sham conditions. When the authors directly compared tACS to sham, no significant effects appeared. While alpha activity does change over the course of the experiment, they can’t be attributed to tACS.
The authors suggest that stronger effects might require longer or repeated stimulation. Or maybe different electrode placements and targeting other brain frequencies.
This study highlights both the promise and current limits of using tACS to modulate attention in real-world settings. You can find the original linked here, for more details!
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