after working through the tutorials, I succeed in the analysis of my EEG-data so far, but one problem remains: I aquired my data using BrainVision Recorder and received the 3 files - .eeg, .vhdr and .vmrk - for each session. When I import them to Brainstorm of review the rawfiles, the amplitude scale seems to be distorted by a factor of about 100000x:
I have to turn scale to 100000 to receive aproximatel the same view I have in BrainVision Recorder at a scale of 100 microvolt.
The files themselves don’t seem to be harmed, as they show correctly in other software. The waveforms seem right, too - it’s just the ampltude scale which doesn’t fit.
I’d be glad to receive any hint on what this problem might be about…
This could be because one of the channels in the file is not actually an EEG channel and/or because there is a bad channel in the file.
I know that for displaying the time series, some other programs normalize the value of the electrodes electrode by electrode. Brainstorm normalizes them all at once and scales them all based on the highest values found in all the channels. If one channel has value in the range of 1 while all the others are in microVolts (1e-6), all the other channels would appear completely flat.
If this is what is happening, you can either change the type of the channel responsible for this behavior (right-click on the channel file > Edit), or mark it as bad.
Another option could be that you have some sort of artifact at the beginning of the recordings that causes some very high values. Then when you scroll through the file, this high maximum is kept and you can’t see anything.
To re-calculate the best scaling each time you change the time displayed in the figure, selection the option “Display > Auto-scale amplitude” in the Event tab.
If you can’t find any reason for which the display looks like this: you can send me one example, I will have a look at it.
Upload your 3 files eeg/vhdr/vmrk somewhere (ex: dropbox) and send me the link in a separate email.
Depending on the time window you are looking at, the amplitudes might be displayed in Volts or in microVolts (I just fixed a little bug - it was showing “No units” instead of “Amplitude (V)”).
They both show that the maximum amplitude for your brain activity is around 300 microVolts (=0.0003 Volts). During the artifacts, the amplitude goes up to 0.01 Volts (100000 microVolts).
Is there anything wrong with that?
If you enable the “auto-scale amplitude” option, it would alternate between the two units.
You can also apply a 0.5Hz high-pass filter (or lower) for the visualization, from the Filter tab. It would remove the slow components of the recordings, and make your signals look more like “small oscillations around zero”. Many EEG viewers include a high-pass filter by default for the visualization.
Can you describe more precisely what do you think is not appropriate in the behavior the Brainstorm viewer, how it differs from what you see in other programs?
that’s it: I hadn’t considered that the scale’s unit might have changed. That makes perfect sense now: I just changed the scale unit in my other software to Volts, which led to the same image as in brainstorm (with high-pass filter at 0.1Hz). So, with the information that the amplitude in the “no unit”-sections was shown in Volts, everything works perfectly.
Many thanks again for this great software you are giving to us and especially for your great support and efforts!
I found no other package, neither commercial nor free, fitting my needs and wishes so well. Again: Many thanks!
Note for BrainVision / BrainAmp users:
A bug was found in the application of the channel gain, the amplitudes read where sometimes incorrect.
Please update Brainstorm for reading correct amplitudes from the BrainVision files.