[Eeglablist] rejecting/interpolating bad channels?

[Tarik S Bel-Bahar]

Greetings, I am wondering why as well.
In particular, does anyone have an EEGLAB-based algorithm
that catches these extremely bad channels, or very noisy ones without big
amplitude changes, in either continuous or epoched data ?

Andrew, the bad channel rejection probably works differently if you are
using continuous or epoched data.
I bet that the epoched (or windowed) data bad channel detection would catch
the big bad channel.


On Mon, May 9, 2011 at 4:34 PM, Andrew Hill <andrewhill at ucla.edu> wrote:

> Thanks James - Exactly what I needed.
>
> Another question for the list about automagic channel rejection.
> This picture shows some EEG and the channels that are marked as bad with
> kurtosis / probability thresholds of 5:
> http://salamander.net/stage/pastebin/autorejects.jpg
>
> You will notice it doesn't remove channels that are wildly out of range -
> even dropping the thresholds on the methods to 2 or under, those extremely
> bad channels aren't "caught"...
>
> Anyone have an idea why?
>
> Best,
> Andrew
>
> On May 9, 2011, at 6:10 AM, James Desjardins wrote:
>
> Hi Andrew,
>
> The outputs listed in the pop_rejchan help text can be added following
> the EEG output at the command line as follows:
>
> [EEG,indelec] = pop_rejchan(EEG,'elec',1:64], ...
> 'threshold',5,'norm','on','measure','kurt');
>
> Then used in eeg_interp as follows:
> EEG = eeg_interp(EEG,indelec)
>
>
> Also, it may be good practice to store that vector in the EEG
> structure so you can keep the information when you subsequently save
> the dataset. Other rejection variables are stored in the EEG.reject
> field.
>
> The following should work nicely and the marked channels will be
> stored when you save the dataset:
>
> [EEG,EEG.reject.indelec] = pop_rejchan(EEG,'elec',1:64], ...
> 'threshold',5,'norm','on','measure','kurt');
>
> EEG = eeg_interp(EEG,EEG.reject.indelec)
>
>
> I have been using a procedure similar to this in my data processing
> stream where I interpolate bad channels then include them in my
> average reference (the interpolation allows for a consistent and
> spatially balanced average reference channel across participants
> regardless of what channels are marked bad). I keep track of which
> channels have been interpolated (in the case above using the
> EEG.reject.indelect field) and then I do not include them in
> procedures where interpolation is inappropriate (eg. ICA).
>
> I hope that this is helpful.
>
> ps. I am going to use the term "automagically" regularly from now on.
>
> James Desjardins
> Technician, MA Student
> Department of Psychology, Behavioural Neuroscience
> Cognitive and Affective Neuroscience Lab
> Brock University
> 500 Glenridge Ave.
> St. Catharines, ON, Canada
> L2S 3A1
> 905-688-5550 x4676
>
>
> Quoting Andrew Hill <andrewhill at ucla.edu>:
>
> Hi folks,
>
>
> I'm trying to figure out how to first automagically detect bad
>
> channels and then interpolate them on the fly, before saving my EEG
>
> data set.
>
> Using pop_rejchan seems to do what it should:
>
> ...
>
> EEG = pop_rejchan(EEG ,'threshold',5,'norm','on','measure','kurt');
>
> Computing kurtosis for channels...
>
> 4 electrodes labeled for rejection
>
> Removing 4 channel(s)...
>
> ...
>
>
> But these are of course not removed - just marked as red in the plot
>
> and I'm assuming labeled in the EEG object somewhere.
>
>
> I see an output parameter "indelec" in the pop_rejchan help, but
>
> after running pop_rejchan I don't see an "indelec" variable created,
>
> nor a EEG.indelec one.
>
>
> Ideally I'd like to take the indices of any marked-bad channels and
>
> then immediately interpolate, maybe something like this?
>
>
> EEG = pop_rejchan(EEG, 'elec',[1:64]
>
> ,'threshold',5,'norm','on','measure','kurt');
>
> EEG = eeg_interp(EEG, EEG.indelec)
>
>
> But that of course isn't syntatically correct - how/where is this
>
> information stored?
>
> Is anyone else doing something like this?
>
>
> Also, does the pop_reref function know enough to exclude channels
>
> that have been marked bad?
>
> I couldn't see anything relevant in the function help.
>
>
> Thanks,
>
> Andrew
>
>
>
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