Manual filter or digital filter first for EEG?

I am getting conflicting advise regarding how to clean my EEG data:

1) Manually remove artefacts first and then apply digital filters

OR

2) Apply digital filters first and then manually remove artefacts

The reason given for 1) is because artefacts are more visible and avoids accidentally accepting artefacts as EEG data.

The reason given for 2) is because it avoids accidentally rejecting EEG data.

Both reasoning makes sense to me and I am a little confused as to what I should do: 1) or 2). Anyone can advise? Thank you very much.

• It depends on the nature of the artefacts. E.g. eye movements, jaw movements, clenching etc , produce so strong movements that entire trials including them can be excluded. So if you address these kind of artifices procedure 1 is the one to go. On the other hand, artefacts due to other neurophysiological sources like heart beat can be eliminated by other approaches (like ICA or beamforming), here you would apply filtering in advance and then try to remove these artefacts. – Irreducible Sep 20 '18 at 5:29
• Yes, I'm starting to see it depends on the data type as well. I'm just starting out in EEG signal processing and starting to see the nuances that can make it complicated. Thank you for your comment, it was insightful. – Krithika Sep 20 '18 at 8:12

When it comes to preprocessing of EEG data, the first step is to filter the signal. Filtering is done such that it preserves information across different bands of the EEG data (alpha, beta, gamma, delta). Typically, many researchers use 0.5 - 50 Hz band in the first step (removes DC components). Note that the filter should be zero-phase such that no delay or phase changes are introduced in the EEG data on using the filter. One simple way is to use filtfilt command in MATLAB.