# [FieldTrip] Question about coherence statistics

laetitia.lalla at inserm.fr laetitia.lalla at inserm.fr
Thu Mar 31 13:11:31 CEST 2016

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Dear FielTrip community,

Sorry to bother you again...

I have a question about the statistical testing to assess coherence
differences and the related paper by E Maris, JM Schoffelen and P Fries
(Journal of Neuroscience Methods, 2007)

I believe I am exactly in the framework described in this paper, but I
guess there is still some crucial information that I don't understand...

This is my story :

- It's a single-subject study

- I want to compare two sets of trials observed in different conditions.

- The 2 sets of trials are exactly the same size, so my coherence
estimate won't be biased by the "unequal sample size problem".

- I just want to compare for 1 channel pair, for 1 frequency bin [6Hz
10Hz], so I guess the Multiple Comparison problem won't affect my
analysis too much (either in the spatial or the spectral dimension).

Based on the paper, I could refer myself to the point "2.7.1. A non
parametric satistical test for a single signal pair and a single
frequency bin".

- Since my interest is in the coherence difference for a single signal
pair, I can choose [|C1(f)|-|C2(f)|] as a test statistic.

- And I can perform the montecarlo simulation.

My question is the following : How can I implement the nonparametric
test of the coherence difference between my 2 conditions with the
fieldtrip functions ?

This is what I did :

1) Extract and preprocess the signal from my condition 1.

2) freq1= ft_freqanalysis(cfg, signal1) (with cfg.method='mtmfft',
cfg.output='powandcsd') →

3) coh1= ft_connectivityanalysis(cfg, freq1)

I did the same thing for the other condition → coh2.

And then for the statistics :

cfg = [];

cfg.statistic = 'diff'; (% because the statistic test than I chose is
the difference [|C1(f)|-|C2(f)|])

cfg.parameter='cohspctrm';

cfg.method = 'montecarlo';

cfg.numrandomization = 1000;

cfg.ivar = 1;

cfg.alpha=0.05;

cfg.tail=0;

cfg.correcttail='alpha';

stat=ft_freqstatistics(cfg, coh1, coh2);

And it gives me the following error :

Error using ft_checkconfig (line 153)

The field cfg.design is required.

Here, I really don't understand why I'm asked for the design matrix...
When doing the statistics for the Power, I understood that the design
matrix was telling which trial belonged to the condition 1 and which
trial belonged to the condition 2. But here, it doesn't make any sense
because my coherence was calculated by averaging on the trials...

Does the design matrix mean something differently here ? Or maybe I
forgot one parameter to put in the cfg ?

Any help will be appreciated !

Thanks a lot for your time,

Laetitia Lalla

PhD student in Neurosciences

INMED, Marseille, France

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