[FieldTrip] DPSS and montage

[Robert Oostenveld]

Dear Casper

Let me give an example montage, which assumes that you start with a monopolar dataset with 4 channels that you want to convert to a 3-channel bipolar dataset.

bipolar.labelorg  = {'1', '2', '3', '4'}
bipolar.labelnew  = {'1-2', '2-3', '3-4'}
bipolar.tra       = [
  +1 -1  0  0 
   0 +1 -1  0 
   0  0 +1 -1
];

The FT_APPLY_MONTAGE function can be used on a variety of FT data structures, such as prepocessed raw data, electrode and gradiometer definitions, freqanalyzed data (in fourier representation). If you imagine having a plain NxM data matrix with the N=4 original channels and M is the number of timepoints, you'll see that applying the montage is equivalent to multiplying the data with "bipolar.tra"

The reason for having the FT_APPLY_MONTAGE function is to deal with the bookkeeping, e.g. reordering channels and checking that all channels are present.

best
Robert

PS I'll add this example to the reference docs.

 


On 12 Oct 2011, at 11:40, Casper van Heck wrote:

> Dear fieldtrippers,
> 
> I've been using Fieldtrip as an Intern for two months now, and have repeatedly run into a problem when using DPSS. When using the below settings, the computer locks up within seconds (although it does seem to do 'something'), stays that way for up to ten minutes (which is my personal cutoff, where I force a reboot), and then reports an 'out of memory'.
> 
> cfg.method = 'mtmfft'; 
> cfg.output = 'pow';
> cfg.taper = 'dpss';
> cfg.foilim = [20 200];
> cfg.tapsmofrq = 1;
> 
> The data fed to ft_freqanalysis is the result of roughly 30 seconds continuous data of a single (originally bipolar) channel processed by ft_preprocessing.
> When using 'hanning' it completes without a hitch, but DPSS doesn't. I've tried the same thing using a different, more powerful computer, which succesfully completed 30 seconds using the above settings, but when setting the tapsmofrq to 3 the RAM was filled up in a period of a few seconds, and the computer choked. The 'normal' computer has 2GB of ram, and a dual-core intel running at 2GHz, and the more powerful computer has 8GB ram with an quad-core AMD at 3.4GHz. What am I doing wrong? 
> 
> Secondly, I was wondering about the 'montage' funcionality, since I'll be making use of bipolar arrangements, but the parameters of the montage-structure as defined on http://fieldtrip.fcdonders.nl/reference/ft_apply_montage don't help me much in making one of my own. Would it be possible to take a look at an existing montage structure, such as the one referred to on http://fieldtrip.fcdonders.nl/getting_started/neuralynx_fcdc?
> 
> Sincerely,
> 
> Casper van Heck 
>  
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