[NBThelp] artifacts in spectrogram and DFA

[Matt E. <mat...@ps...>]

Dear NBT list,

First, thanks for the toolbox. It looks like it is going to be a great resource for the field!

I'm hoping to use NBT to conduct DFA on some .bdf and ANT EEprobe .cnt files, but I have a question related to artifacts I'm noticing in my EEGlab spectral plots. The files are continuous data from a resting period, sampled at 1024 Hz, and about 200,000 samples in length. The spectra always have a regular, saw-tooth or spiking appearance in all the channels for both file types (even after converting the .bdf files to .cnt in ASA-lab software).

I've noticed that downsampling the data and removing the channel mean appears to improve the look of the spectra, but I'm having trouble identifying the actual underlying issue. I'm especially concerned that the saw-tooth artifacts might be due to large voltage changes in my time domain data, potentially due to skin potentials or some other artifact, but these are only evident when I plot the data in matlab rather than using the EEGlab channel data scroll function or the NBT visualization tools.

I know that similar artifacts have been discussed in regards to biosemi data on the EEGlab list before: http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html, and http://darb.ketyov.com/professional/raw-spectrumZoomed.jpg, but the proposed fix to cast the raw data first to double and subtract the channel mean hasn't solved the issue in my raw .bdfs, and it's unclear to me whether that accounts for the same problem in the native .cnt files. Also, since the ultimate goal is to conduct DFA, I'm very reluctant to simply use a low high-pass filter or mean detrend every channel, without guidance regarding the implications of that for DFA.

Any advice about how to correct the issue in prep for DFA would be greatly appreciated!

Thanks,
Matt

Matthew J. Euler, Ph.D.
Department of Psychology
University of Utah