Hi everyone,
We have been seeing a strange and persistent pattern in our EEG recordings and would like to hear if anyone has experienced something similar.
Setup:
Brain Products actiCAP + BrainAmp amplifier
Multiple cap sizes, matched to head size & sex
Participants: asian adults (Both Healthy and disease model)
resting EEG (eye-closed)
did not show this issue Problem Across all participants and experiments, we observe occipital-dominant activity in the topography, regardless of frequency bands.
It appears consistently even with different caps, operators, and recording days.
(an example of a patient, but healthy participants also showed same patterns)
While similar occipital activity can be seen in the high-frequency range with our previous Grass system, it is far less pronounced compared to the Brain Products recordings.
What we’ve checked Environmental artifacts → unlikely (appears in all subjects)
Alpha–beta misclassification → no change in alpha topography when redefined
Population-specific head shape → possible, but Waveguard data from same population was normal
Multiple preprocessing pipelines → same result
I’d like to know: Has anyone observed a similar occipital-dominant pattern with Brain Products systems? Any insights on possible causes (hardware, grounding, reference) or troubleshooting steps would be greatly appreciated.
I used Brainproducts for my EEG studies in the past, haven’t observed that. What electrode did you choose? Does it still show up after average referencing?
Thank you for your reply. We are using active electrodes (actiCAP, BrainProducts), and the occipital-dominant pattern still persists even after average referencing.
were all the participants tested in the same environment? having artifact present in all participants doesn’t necessarily rule out environmental artifacts. Say if you’re collecting data in an infirmary, bedside equipment, or sockets, can potentially interfere with signals
Even in lab settings we would sometimes ask participants to turn off their phones during the experiment
Since this happens consistently across participants, caps and operators, it’d be reasonable for us to first rule out something that’s present in all sessions you’re running.
Say for example are you running EEG concurrently with any other device? Presenting auditory stimuli? anything that may interact with the electromagnetic environment in the lab?
Were the participants given instructions on what to expect during the close-eye recording session? Is it possible that participants interpreted the instructions in a way that require them to perform mental imagery, leading to occipital activations?
Were the electrodes properly cleaned before each session? what’s the result of saline water testing result? Can we rule out hardware issue with electrodes or amplifiers?
Sorry for throwing all these questions but using a method of elimination with educated guesses may be an efficient start
Thank you for the thoughtful suggestions. To address each point:
Concurrent devices / stimuli: Sessions are strictly resting-state EEG only. No auditory or other stimuli are presented. A monitor is present in front of participants, but it is powered off during recordings, and we do not believe it introduces meaningful artifacts.
Lab environment: The lab is quiet and spacious; only the BrainProducts system for acquisition and a laptop for monitoring are active.
Participant instructions: For eyes-closed recordings, participants are instructed to relax comfortably without specific tasks or imagery and to keep their eyes closed throughout.
Electrodes & cleaning: Electrodes are thoroughly cleaned and dried before each session.
Operators: Three different operators have collected data to date, and the same occipital-dominant pattern appears across all operators.
Saline test: Saline tests have not revealed electrode differences, and we have not identified issues.
We are currently discussing this issue with the BrainProducts support team as well, but the exact cause has not yet been identified.
As far as ruling things out: another idea is to check your analysis script very carefully. Something I’ve seen a lot is filtering the data in one band (say “alpha”), and then filtering in another band on the already alpha-filtered data. I don’t think that’s what’s going on here, but double-check; the solution is to copy the original unfiltered data before each band filter gets applied. Also check for index variables within a for-loop; maybe it’s supposed to be something like filtered_data[ix] but instead says filtered_data[0].
I will add to the chorus of voices suggesting that “it happens for all subjects” does not rule out an environmental artifact. Or, possibly, the fit of the caps is different enough so that a certain part is always snug and another part is always loose? Or the participants lean the back of their head on a headrest, and actiCap is sensitive to that in a way that the Grass system wasn’t?
If you still have the Grass system, I’d probably do a recording with both systems on the same day, same person (a lab member), and use the same analysis script to process both sets of data. And of course: view the raw data traces and the spectra (before and after filtering) to see if anything odd jumps out at you.
