[FieldTrip] rereference to repaired channel

"Jörn M. Horschig" jm.horschig at donders.ru.nl
Mon Dec 12 16:18:34 CET 2011


Hi Ingrid,

ft_channelrepair is simply taking the average of the neighbouring 
channels. So, you would need to specify your neighbours and based on 
this, and it just gets the average of the neighbours of your mastoid. 
However, imho, you cannot really define neighbours for a mastoid 
channel. There is a method to interpolate not using nearest neighbours 
but spherical splines, however this is not (yet) part of FieldTrip, see 
also here:
http://bugzilla.fcdonders.nl/show_bug.cgi?id=634
But, I am not sure if this would help in this case either. If you really 
want to use linked mastoid as a reference, I would probably reject that 
subject due to bad signal quality. I don't know if anyone on the list 
has a better idea (or more experience).

Second, if you first rereference, all noise from this one channel will 
leak into all other channels, as you said. As long as your mastoid is 
not a bad channel, there should be no difference whether you first 
rereference or then interpolate bad channels or the other way around 
(assuming that I got no mathematical blackout right now).
Should be easy to verify, e.g. if you have channels A, B, and C (all 
neighbours) and want to reference to R, then your referenced channels 
would be A-R, B-R and C-R.
If B is a bad channel, and you interpolate first, you get B=(A+C)/2 
(i.e. just the average of the neighbouring sensors). If you then 
rereference you will obtain (A+C)/2 - R, which is the same as (A+C) /2 - 
(R+R)/2 = (A-R + C-R) /2.
If you first rereference and then interpolate you obtain B-R = (A-R + 
C-R) /2 immediately (because A-R and C-R are the only neighbours of B-R).

Hope it helps! Best,
Jörn

On 12/11/2011 9:06 PM, Ingrid Nieuwenhuis wrote:
> Hi all,
>
> I've measured EEG data with Cz as the reference. I now want to 
> rereference to linked mastoids. However, in one participant one of the 
> mastoid electrodes is really noisy. Is it valid to throw out the noisy 
> channel, then fix it (with ft_channelrepair), and then rereference to 
> this fixed mastoid channel? In other words, is channelrepair a linear 
> operation? Or would I be mixing (a little) Cz into all channels 
> now...? Any other options?
>
> Or another way of asking: do I get the same result if I first 
> rereference and then fix bad channels, compared to first fixing bad 
> channels and then rereference? In the participant with the bad mastoid 
> chan, I don't have a choice, I have to first fix the channel first, or 
> I'd be mixing noise into all channels. But, in other participants, 
> where I also have bad channels (but not the mastoid ones), does the 
> order matter, which order is better? I'd think first rereference, then 
> fix... Correct?
>
> Thanks!!
> Ingrid
>


-- 
Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group

P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Contact:
E-Mail: jm.horschig at donders.ru.nl
Tel:    +31-(0)24-36-68493
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