[Eeglablist] Effect of anti-aliasing low-pass filter on connectivity analysis

Mon Aug 24 16:27:47 PDT 2015

Dear Andreas,

I have been wondering about this but keep forgetting to ask you about it:

> no stopband (below Nyquist)

Why do you need to avoid having a stopband below Nyquist?
I appreciate your kind help.

Makoto

On Wed, Jun 24, 2015 at 4:02 AM, Andreas Widmann <widmann at uni-leipzig.de>
wrote:

> Dear Makoto,
>
> to my understanding filter causality and Granger causality are not
> directly related. The output of a causal linear filter is identical to the
> output of a non-causal linear filter but shifted on the time axis (delayed;
> but equally across channels and bands!). Non-linear (here min phase)
> filters distort the phase spectrum and should, to my understanding, not be
> used for GC analysis.
>
> Barnett and Seth (2011, J Neurosci Meth) show that GC is in theory (but
> not in practice) invariant under filtering. They do recommend filtering to
> achieve stationarity (e.g., drift, line noise; also in recent 2015 J
> Neurosci paper). The main problem with filtering and GC is the increase in
> required model order ("We have shown that a primary cause is the large
> increase in empirical model induced by filtering; high model orders become
> necessary in order to properly fit the modified aspects of the power
> spectrum (low power in stop band, steep roll-off, etc.).“).
>
> That is, to my understanding for a carefully designed anti-aliasing filter
> (linear, zero-phase) the impact should be limited. The anti-aliasing filter
> as it is implemented in the repaired pop_resample function (in develop but
> not yet in eeglab13 branch) will have no stopband (below Nyquist) and a
> rather shallow roll-off (and low order) with default cutoff (fc = 0.9 *
> Nyq) and transition band width (df = 0.2 * Nyq). The cutoff and transition
> band width can be manually defined by the user, so you can try to apply a
> more shallow roll-off, e.g. with fc = 0.8 and df = 0.4. This conclusion
> should, however, be actually tested with a simulation. From a practical
> perspective any M/EEG signal has been filtered with an anti-aliasing filter.
>
> > As the ERP handbook by Luck (or his other book) recommends,
> anti-aliasing should better have the margin of 4-5 times of the new
> sampling rate e.g. if you downsample signlas to 250 Hz, anti-aliasing
> low-pass at 125 Hz is the standard, but recommendation is 75 Hz or even 50
> Hz. Well, I haven't tested it myself so I am not sure what bad it would do
> if I use 125 Hz (any comment on this, anyone?) but in this case, I guess
> the anti-aliasing low-pass filter does affect the subsequest connectivity
> analysis--am I correct (assuming that I analyze EEG up to 50 Hz)?
> To my understanding this conservative oversampling ratio is intended to
> improve signal fidelity (resolution, noise) rather than anti-aliasing
> alone. Given the result demonstrated by Barnett and Seth I would not
> recommend applying a lowpass filter with a stopband below Nyquist.
>
> Best,
> Andreas
>
> > Am 24.06.2015 um 02:59 schrieb Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:
> >
> > Dear Iman,
> >
> > > Using causal filter may adversely effect the direction of information
> >
> > flow in the GC analysis. It is recommended that one use a
> >
> > non-causal filter (for example, finite impulse response filters) with
> >
> > zero phase lag
> >
> >
> > Really? The impulse response of the non-causal FIR filter spreads in
> both ways in the time domain, which means info of future events leak to
> past... I thought using causal filter with minimum phase makes more sense.
> >
> > Makoto
> >
> > On Tue, Jun 23, 2015 at 4:29 PM, Iman Mohammad-Rezazadeh <
> irezazadeh at ucdavis.edu> wrote:
> > http://journal.frontiersin.org/article/10.3389/fnhum.2015.00194/abstract
> >
> >
> >
> > Using causal filter may adversely effect the direction of information
> >
> > flow in the GC analysis. It is recommended that one use a
> >
> > non-causal filter (for example, finite impulse response filters) with
> >
> > zero phase lag (Mullen et al., 2012, Coben and Rezazadeh, 2015)
> >
> >
> >
> >
> >
> > From: eeglablist-bounces at sccn.ucsd.edu [mailto:
> eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Makoto Miyakoshi
> > Sent: Tuesday, June 23, 2015 2:07 PM
> > To: Vito De Feo
> > Cc: EEGLAB List
> > Subject: Re: [Eeglablist] Effect of anti-aliasing low-pass filter on
> connectivity analysis
> >
> >
> >
> > Thank you Vito for your response. Forgive me to ask you one more
> question.
> >
> >
> >
> > As the ERP handbook by Luck (or his other book) recommends,
> anti-aliasing should better have the margin of 4-5 times of the new
> sampling rate e.g. if you downsample signlas to 250 Hz, anti-aliasing
> low-pass at 125 Hz is the standard, but recommendation is 75 Hz or even 50
> Hz. Well, I haven't tested it myself so I am not sure what bad it would do
> if I use 125 Hz (any comment on this, anyone?) but in this case, I guess
> the anti-aliasing low-pass filter does affect the subsequest connectivity
> analysis--am I correct (assuming that I analyze EEG up to 50 Hz)?
> >
> >
> >
> > Makoto
> >
> >
> >
> > On Mon, Jun 22, 2015 at 9:31 AM, Vito De Feo <
> vito.defeo at zmnh.uni-hamburg.de> wrote:
> >
> > Dear Makoto,
> > this will not affect the connectivity analysis if the frequency of
> interest are far from the Nyquist frequency. For example if you downsample
> to 500 Hz (Nyquist freq = 250 Hz) you will have no problem in the band
> 0-100 Hz.
> > Best
> > Vito
> >
> >
> > Il giorno 20/giu/2015, alle ore 00:28, Makoto Miyakoshi ha scritto:
> >
> > > Dear List,
> > >
> > > If I use zero-phase low-pass filter for anti-aliasing, does it affect
> the subsequent connectivity analysis? I ask this because EEGLAB
> pop_resample() automatically applies it. If it does, is there a workaround?
> Should I use minimum phase causal filter for anti-aliasing?
> > >
> > > --
> > > Makoto Miyakoshi
> > > Swartz Center for Computational Neuroscience
> > > Institute for Neural Computation, University of California San Diego
> >
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> >
> >
> >
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> > --
> >
> > Makoto Miyakoshi
> > Swartz Center for Computational Neuroscience
> > Institute for Neural Computation, University of California San Diego
> >
> >
> >
> >
> > --
> > Makoto Miyakoshi
> > Swartz Center for Computational Neuroscience
> > Institute for Neural Computation, University of California San Diego
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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