[Eeglablist] stange posterior waveforms

Cedric Cannard ccannard at protonmail.com
Wed Jul 16 10:02:54 PDT 2025


Dear Lily,

Attachments are not supported by EEGLABlist. Can you share an image via a drive or figshare? 

What is your reference and how many electrodes do you have? Is this visible in just on person or several? 

Cedric



On Tuesday, 15 July 2025 at 07:03, Lily via eeglablist <eeglablist at sccn.ucsd.edu> wrote:

> Hello everyone,
> 
> 
> I’m currently conducting an ERP experiment on semantic judgment, and I've noticed some unusual patterns in a few participants’ waveforms that I hope to get your input on:
> 
> 
> 1. Large parietal and occipital amplitude
> Several subjects show unusually large amplitudes at occipital, parietal, and parieto-occipital electrodes, although the frequency content doesn’t seem to change. These epochs exceed the amplitude observed at other electrodes but still remain within the preset simple voltage threshold (±75 µV) and peak-to-peak threshold (100 µV). Thus, they aren't being automatically rejected.
> 
> 
> - Is it normal to see such elevated posterior amplitudes (perhaps alpha-related), or could it indicate participants are dozing off or otherwise inattentive?
> - If it is the latter, what’s the recommended way to handle them? Should I simply remove these trials, or is it acceptable to apply ICA to remove the related components? I’ve tried removing one or two brain-source components, which seems to "repair" the epochs, but I’m concerned whether this approach might introduce other artifacts or unintended distortions.
> - I’ve included example waveforms in Attachment 1('larger amplitude').
> 
> 
> 2. Posterior inverted V-shaped artifacts
> I’m also seeing another type of artifact in the posterior sites, resembling an inverted V shape. It looks somewhat like a blink artifact, but whereas blink artifacts are usually frontal, these are clearly posterior. I’ve attached a representative example in Attachment 2('v_shape_artifacts').
> 
> 
> 
> 
> Thanks in advance for any guidance or suggestions!
> Best regards
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