[Eeglablist] PSD artifact in merged EEG data

[João Paulo Viegas Amaro]

Hello all,

I’m encountering an issue when merging EEG data from two different runs.
The power spectral density (PSD) plot of the merged data consistently shows
an unusual artifact — a sort of “bouncy” pattern — which does not appear
when plotting each run individually. I have observed this in most of my 20
subjects, and the artifact is almost always limited to the merged data.
However, in one case, a similar pattern did appear in a single run on its
own.

I’ve attached an example from one subject (shared in a Google Drive link),
which includes the code, PSD plots for each individual run, as well as the
merged data, for comparison.

The data were recorded using EGI’s 256-channel EEG system at a sampling
rate of 500 Hz. The plots were generated immediately after loading the
data, with no preprocessing or other modifications applied beforehand.

Has anyone come across this type of artifact or have any idea what might be
causing it?

Link:
https://urldefense.com/v3/__https://drive.google.com/drive/folders/1NWPBNA03aeL0EAheAWbmay41elEqnelq?usp=sharing__;!!Mih3wA!HzTxfZi2NwXU5fzmy2gQR2Lwk8rEyvzE6vm6VZX0Gl4YMmhgA6H-ZPjgMGGQPdN33T4tCVU0b0gg5uRbLQNev2dO1QGnrQ0$ 

Thank you in advance for your help!

Best regards,

João Amaro