filt vs filtfilt (MATLAB implementation)

[sappelhoff]

@sappelhoff So I'm currently taking a grad course on signal processing, and I think I finally understand the key difference between MNE's highpass filter and EEGLAB's: MNE defaults to applying the filter using a zero-phase method that doesn't result in any phase shift in the signal (the equivalent of filtfilt in MATLAB/Scipy), whereas the EEGLAB filter function I re-implemented here only does a single forward pass (using lfilter), so you end up with phase shift in the signal.

As far as I can tell, other than theoretically being faster (which it isn't in the current implementation) there's absolutely no reason to use a forwards-only filter like EEGLAB does instead of the backwards-forwards zero-phase approach used by MNE. I'm guessing this is a weird legacy carry-over for EEGLAB from the days when people were running EEG analyses on Pentium 3's and PowerPC G4's so CPU cycles were at a major premium. I'll update the "differences" docs to reflect this when I get a chance!

Originally posted by @a-hurst in #71 (comment)