[FieldTrip] calculating the ITC or PLV from animal EEG data

[jan-mathijs schoffelen]

Dear Russell,

There's an important detail that you may have glossed over, and relates indeed to the difference between PLV and ITC.

ITC can be defined within a single electrode (i.e. it is a univariate measure), and it quantifies the phase  consistency across observations, of oscillations relative to an event of interest.
PLC can only be defined across two electrodes (i.e. it is a bivariate measure), and it quantifies the phase difference consistency across observations, of oscillations (i.e. relative to one another).

The reference electrode does not count as a second electrode in this case. 

In other words, it looks like you can only compute ITC. In order to visualize it, you can do something like this:

freqnew = freq;
freqnew = rmfield(freq, 'fourierspctrm');
freqnew.powspctrm = itc;

cfg = ... (build the cfg according to your needs)
figure;ft_singleplotER(cfg, freqnew);



Best wishes,

Jan-Mathijs  

On Jan 3, 2013, at 11:02 PM, Russell G Port wrote:

> Hello All,
> 
> I was hoping someone could lend me a hand with an issue I have. I have animal EEG sets, which I have processed with fieldtrip. Each animal had 1 hippocampal
> electrode and one reference electrode, and were run in a CED system. From the way it was setup in the past; each animal was listed as a channel. I.E. channel 1 
> corresponded to animal 1, etc etc. When I read it into fieldtrip,as would be correct normally each channel is again an animal and each run it a data set (which 
> would normally be a subject). This is fine, since I have found ways to work around this, and can also separate each animal's data into seperate data sets
> and rename their channel "electrode 1". What interests me though is computing the phase-locking value over trials, and/or making grand averages of the phase-locking
> value over certain conditions (a certain subset of mice), I think another name for this phase-locking value would be intertrial coherence..? I am found useful things on 
> this board, i.e. to use ft_connectivityanlaysis, or using
> 
> tmpdat = freq.fourierspctrm;
> tmpdat = tmpdat./abs(tmpdat); ; % this will normalise each trial for its amplitude;
> itc = abs(mean(tmpdat)); % this will give the itc
> From reading other messages I know that everyone on this board is knowledgeable. How would other people compute this PLV for the kind of data I have? Is it the same as
> ITC? I assume theres critical details that I am glossing over. Lastly is there a way using ft_singleplotERS, to plot this PLV measure (or atleast measure of 
> coherence), since when plotting cross-spectra you would need a reference channel?
> 
> Thanks
> Russell Port
> Upenn Neuroscience Graduate Group
> 
> 
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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