[FieldTrip] Cluster-based permutation tests on time-frequency and size of conditions

Joram van Driel joramvandriel at gmail.com
Wed Mar 18 14:51:33 CET 2015

Hi Martina,

In general, I'd advice to do some kind of trial-selection procedure when
comparing error versus correct trials, in order to trial-count-match the
two conditions. Otherwise you run into problems considering: SNR (higher
for the correct condition), and RT (errors are usually faster, resulting in
a time-on-task confound). What I always do is pick from the correct
condition a similar number of trials that are close to the RT distribution
of the error trials (i.e. the faster correct trials). That way you solve
both problems at once (and probably the cluster-based permutation test in
field trip will work as well, as a bonus ;)).


On Wed, Mar 18, 2015 at 2:31 PM, Martina Rossi <martina.rossi76 at yahoo.it>

> Dear All,
> I would like to get some feedback from the community about a statistical
> analysis problem I need to tackle with my study.
> I want to apply the cluster-based permutation tests on time-frequency data
> considering two conditions (correct vs error).
> Unfortunately, these two conditions have different sizes (correct >>
> error).
> Right now, I am only considering subjects having a ratio "error/correct"
> bigger than 1/5, yet this is only an arbitrary threshold I set.
> The question is the following:
> is there a formal way to identify a threshold by which two conditions can
> be realiably compared with the cluster-based permutation tests?
> If the cluster-based approach is not suitable in this scenario, is there
> any other approach you would suggest?
> I shall perhaps point out that I am working on EEG data recorded with a 32
> channel system (impedance levels < 10 kΩ).
> Looking forward to hear your feedback :)
> Kind Regards,
> Martina Rossi
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Joram van Driel
Postdoc @ Vrije Universiteit Amsterdam
Cognitive Psychology
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