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

[Katarzyna Dudzikowska]

Dear all,

I am working on preprocessing data with EEGLAB with the end goal of running
a frontal asymmetry analysis. As I am trying to settle on proper
preprocessing steps, I have a few issues/doubts and I would love to get
some input from the more knowledgeable crowd (and I am certain you are as
this is my first time working with EEG data :) )

As I will be looking at alpha frequency, I judged that filtering between
0.5 and 45Hz should work for me, but this begets more questions
(obviously).
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?
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.
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?

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.