[FieldTrip] Coherence of single trials

Monika Mellem monikamellem at gmail.com
Tue Nov 27 00:22:51 CET 2012

Dear Fieldtrippers,

I am trying to calculate coherence estimates on individual trials of our
EEG data and then want to perform a within-subjects statistical test, but
so far I have not been able to figure out how to do this.  I am able to
calculate coherence for our 22 subjects and then perform statistics on the
grand average, so I understand the basic usage of these functions
(ft_freqanalysis, ft_connectivityanalysis, ft_freqgrandaverage,
ft_freqstatistics).  But now we want to do statistical testing on
individual subjects.  Is it possible to keep coherence for individual
trials in Fieldtrip as they seem necessary for this statistical analysis?

I am setting cfg.keeptrials = 'yes' as an input to ft_freqanalysis, and get
the cross-spectra of individual trials (I also tried computing the fourier
spectra but it made no difference in the following step).  However, when
computing the coherence in ft_connectivityanalysis, Fieldtrip averages over
the trials.  Is there a way to output coherence for individual trials
instead?  Please see the full cfg structure settings below.

Also, I did look into the option of cfg.statistic = 'indepsamplesZcoh' as
an input to ft_freqstatistics since previous posts suggested this was for
single subject coherence statistics, but our data is paired as the same
stimulus appears in the 2 conditions we are comparing.  Is there a paired
test for single subject coherence testing, or should we use cfg.statistic =

Many thanks for any suggestions you may have!
Monika Mellem
PhD Candidate
Interdisciplinary Program in Neuroscience
Georgetown University
Washington, DC
msm79 at georgetown.edu

cfg settings for ft_freqanalysis and ft_connectivityanalysis
cfg = [];
cfg.prestim = -0.5;
cfg.poststim = 1.5;
cfg.foilim = [2 30];
cfg.freqrange = 'low';
% cfg.output     = 'fourier';
cfg.output     = 'powandcsd';
cfg.method = 'mtmconvol';
cfg.keeptrials = 'yes';
cfg.channel    = {'all'};
cfg.channelcmb = {refchan 'all'};

cfgcoh.channelcmb = cfg.channelcmb;
cfgcoh.method = 'coh';

v = genvarname(['CRSP_' condition '_low']);
eval([v '= ft_multitaper_powcoh_EGI(cfg, data);']);  %Wrapper around

w = genvarname(['COH_' condition '_low']);
eval([w '= ft_connectivityanalysis(cfgcoh, ' v ');']);
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