first I want to express my sincere gratitude of what people have created here for sharing sleepdata, it is really astounding. And I will try to contribute to this in the near future with own data, further software and more annotation data for sleep EEG.
I am currently trying to analyze (detecting slow oscillations, sleep spindles etc.) the SHHS EEG data.
However, I realized that the data specifications (also in the manual) and the edf files indicate that the data range for the EEG, EOG channels is only +-125 µV (250 µV scale range), i.e. the data has a cutoff at +125 µV and -125 µV; the EMG is even cut at +-31.25 µV.
Given that the signal was high-pass filtered at 0.15 Hz, and by looking at the data, a lot of this cutoff seems to make the data very limited for further EEG analyses.
For example many slow waves are just "cut off" at the top and bottom (since many typically slow waves reach beyond this scale, even in older subjects).
I wonder how the data was adequately scored giving this limitation, and this could have also perturbing influences on the power analysis, right?
Was this handled already in some way?
And, was this limitation truly at recording, or does there exist raw data without this limitation, or did I miss anything regarding this data?
Or is there some SHHS data without this limitation?
I do not know if this is a known issue, but I could not find anyone to bring this up in the forum before, and it seemed rather critical to me regarding the usability of the EEG data.
Thanks for the kind words - glad you're enjoying the resource!
I don't know the answer to your question, but I pinged a couple members of the NSRR team who should be more familiar with this area.
yours is a very good question, thank you for pointing out one of the "flaws" of the SHHS data. I do not know of other versions of the SHHS with different parameters, although I have noticed a certain heterogeneity in the signals quality, so you might find that some are more usable than others. As pointed out by collaborators who are familiar with this dataset, the SHHS was not originally collected with very advanced EEG analysis in mind, and the high-pass filter is applied at recording, causing a loss of the slow oscillations in the power spectrum. This, however, should not have affected the visual scoring, as the scoring rules are thresholded at 75 microvolts and the scoring does not care if the signals overwhelms the channel width.
I hope this is of help to you, and that you can still make use of other features of this database, e.g. the spindle power or other bands.
Dear Michael and Sara,
thanks for the fast replies. I know one can not get any detail perfect when collecting such large datasets and the people already did a very good job. There are plenty of other datasets here and EEG is also not my only focus.
I was just very surprised by this, since it seems very untypical to have such a limit in EEG range for sleep data.
The fact that there is so many EEGs data indeed can compensates for many limitations in some analysis.
However I am just worried that also some (especially interesting) spindle activity which is typically occuring during the up states (or down-states) of slow oscillations (i.e. slow waves and K-complexes) of high amplitude is not adequately captured.
I will have to screen the data a little more then, maybe there are some datasets that are of better use than others, but this will inevitably introduce an selection bias of the data.
Wish you nice festivities already!