[FieldTrip] How to identify clusters in space?

[Stefan Arnau]

Dear fieldtrip-list,

I am currently trying to figure out a how to solve a specific problem 
related to cluster permutation statistics. I am quite often in a 
situation where I want to test a specific effect against zero.

In case of within subject effects, I can create null-hypothesis data for 
each participant by performing the same kind of analysis with the factor 
of interest being permuted across trials. For each subject, I can then 
pass both datasets, the dataset created on the actual labels as well as 
the dataset created on permuted labels, to the ‘ft_timelockstatistics’ 
or the ‘ft_freqstatistics’ function in order to identify significant 
effects on the group level.

For correlation analysis, there is the possibility to test 
between-subject correlations against zero using the 
’ft_statfun_correlationT’ option in cfg.statistics. The 
‘ft_timelockstatistics’ for example then takes care of correlating the 
data with the measure of interest and also performs the cluster-based 
permutation test.

My problem now is that I don’t know how to perform such a test against 
zero on a group level when not working with correlations but, for 
example, with regression coefficients.

In theory, I know how to create a H0-distribution of sum-t or 
number-of-points statistics obtained from clusters identified in 
permuted data and I also know how to correct the clusters in the real 
data accordingly. With “in theory” in mean that I know how to do it in 
time or in time-frequency space. The part that I don’t understand is how 
clustering is done additionally in space, that is across channels.

It would be very helpful if someone could provide a minimum example of 
matlab code or refer me to the respective file or code-sequence in the 
fieldtrip code that specifically deals with identifying clusters in 
channel-by-time and/or channel-by-frequency-by-time data. Apart from the 
specific problem of testing a given group-level statistic against zero, 
this would of course also help me a lot to understand better how 
clustering in space behaves.

Thanks in advance and best regards

Stefan

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