[FieldTrip] time windows and number of channels in cluster based permutation tests

[Michele Scaltritti]

Dear FieldTrip users,

I have a rather general question regarding cluster-based permutation test.

In the tutorial for Cluster-based permutation tests on event related
fields, I read that sensitivity of the analysis depends on the length of
the time interval that is analyzed. Elsewhere, I found a similar reasoning
regarding channels, in the sense that it the exclusion of the sensors where
no effect is likely to be present should yield a more sensitive analysis.

In my experiment, I analyzed a limited set of central electrodes (where
previous evidence suggests that the difference should be located) and found
a significant positive cluster, and a (smaller) significant negative one.
When doing the same analysis considering all the electrodes, no significant
negative cluster is found, just a positive one. Among other things, this
may reflect a different sensitivity as a function of the number of channels
considered in the analysis. I explored a bit the different parameters (such
as neighbors channels definition), but I did not identify the source of the
difference between the analyses.

Leaving aside my specific case, in more general terms I cannot understand
how sensitivity is increased by focusing on a smaller time-windows, or on a
specific set of electrodes. After all, the number of comparisons we make
should not affect the number of samples surpassing the threshold, nor the
clustering as a function of the spatial and temporal adjacency of these
supra-theshold samples. I understand that the same cluster may vary when
other samples are considered in the analyses (for example, samples from
other electrodes may get included), but again, I am failing to see how this
may be related to the sensitivity of the analysis. The cluster-level
statistics, for example, should not be weakened by this.

I apologize if this message underlies any gross misunderstanding.

Michele Scaltritti
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