[Eeglablist] stange posterior waveforms

[Lily]

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