[Eeglablist] Preprocessing - Doubts regarding filtering and removing line noise

[Delorme, Arnaud]

Dear Katia,

> a) If my low pass filter is set to 45 Hz, do I still need to try and remove
> the line noise (clean line)?  As far as I understand line noise is usually
> at a higher frequency, so does it mean that my filter will have taken care
> of it already, or am I confusing things?

No you do not need to remove line noise if you low pass filter at 45Hz.

> b) Is it correct that it is good practice to apply a low and high filter in
> two separate steps? I am sure that I read that somewhere, but I cannot find
> it anymore and, assuming I haven't dreamt it, I cannot remember what the
> reasoning was.

It is usually equivalent, although there is sometimes some small numerical differences. I also like to do it separately, but this goes back to a problem I encountered 15 years ago when I was using a sub-optimal filtering function that could not do both properly at the same time.

> c) Also, more curious than anything: the first thing I did in the
> preprocessing (after assigning channel locations) was to downsample to
> 250Hz. If I then filter out everything above 45Hz anyway, is downsampling
> even necessary?

Downsampling will make data processing faster. If you do not mind waiting, it is not necessary. Given your cutoff frequency is 45 Hz, I would say it is recommended in your case.

Best wishes,

Arno


> 
> I will be extremely grateful for your input.
> 
> All the best,
> Katia Dudzikowska
> 
> P.S. I have some other questions, but as they regard other preprocessing
> steps I figured I would ask them separately, for the sake of future
> searches, hope it's ok.
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