[FieldTrip] quick fieldtrip question

[Marshall, T.R. (Tom)]

Thanks Eelke. This seems like what I want, but I can't seem to actually implement it.

To provide a little more detail, what I currently have are two data sets on which I have performed frequency analysis. One of them has a bunch of channels missing that I had to throw out. Now I want to have a look at the difference between the two. Of course it only makes sense to do this for the channels that are common to both datasets.

In order to do my subtraction, I have this:

***
%find channels that exist in both datasets
common_channels=tdcs_low{1}.label(ismember(tdcs_low{1}.label,notdcs_low{1}.label));

%throw away channels that are unique to the notdcs dataset
cfg=[];
cfg.channels=common_channels;

notdcs_fewer_channels{1}=ft_preprocessing(cfg,notdcs_low{1});
***

...which gives the following error

***
??? Error using ==> ft_checkdata>freq2raw at 1583
this only works for dimord='rpt_chan_freq_time'

Error in ==> ft_checkdata at 257
        data = freq2raw(data);

Error in ==> ft_preprocessing at 284
  data = ft_checkdata(data, 'datatype', 'raw', 'hassampleinfo', 'yes');
***

In the above datasets, the 'dimord', looks like channels x frequencies x time. Not sure what 'rpt' stands for in the error message, nor why there should be a 4th dimension.

Best,
Tom

----- Original Message -----
From: "Eelke Spaak" <eelke.spaak at donders.ru.nl>
To: "T.R. Marshall (Tom)" <t.marshall at fcdonders.ru.nl>, "Email discussion list for the FieldTrip project" <fieldtrip at donders.ru.nl>
Sent: Monday, 26 September, 2011 2:22:56 PM
Subject: Re: quick fieldtrip question

Hi Tom,

I'm forwarding my response, and your question, to the FieldTrip
mailing list, so that anyone who in the future might wonder the same
thing will benefit. BTW, if you're not subscribed to the list yet, you
really should (see http://fieldtrip.fcdonders.nl/discussion_list for
instructions).

The easiest way to select channels to remove or retain is by using the
cfg.channel option which is understood by ft_preprocessing:

cfg = [];
cfg.channel = {'chan1', 'chan2'}; % this will *keep* only those channels
data = ft_preprocessing(cfg, data);

or:

cfg = [];
cfg.channel = {'all', '-chan1', '-chan2'}; % this will *remove* those channels
data = ft_preprocessing(cfg, data);

For more information, type 'doc ft_channelselection' into Matlab, or
see http://fieldtrip.fcdonders.nl/reference/ft_channelselection .

Hope this helps!

Best,
Eelke


2011/9/26 Marshall, T.R. (Tom) <t.marshall at fcdonders.ru.nl>:
> Hi Eelke,
>
> So I have a very quick question about channel selection in fieldtrip.
>
> I want to plot a contrast of my tDCS and no-tDCS data. My problem is that I had to throw away a lot more channels in the tDCS condition than in the no-tDCS condition, so datasets comprise different channels. I spent a few minutes trying to write fiddly code to compare the entries in the 'labels' field of the data structure, but surely fieldtrip has some general purpose function just to keep some channels and throw away others? I can't find exactly this on the wiki...
>
> Hope you can help. Thanks in advance.
> Tom
>
> --
> Tom Marshall, MSc.
> Neuronal Oscillations Group, Donders Centre for Cognitive Neuroimaging
> tel: +31(0)243668487
> email: t.marshall at fcdonders.ru.nl
> postal: PO Box 9101, 6500HB, Nijmegen, The Netherlands
> visiting: Kapittelweg 29, 6525EN, Nijmegen, The Netherlands
>

-- 
Tom Marshall, MSc.
Neuronal Oscillations Group, Donders Centre for Cognitive Neuroimaging
tel: +31(0)243668487
email: t.marshall at fcdonders.ru.nl
postal: PO Box 9101, 6500HB, Nijmegen, The Netherlands
visiting: Kapittelweg 29, 6525EN, Nijmegen, The Netherlands