[Mne_analysis] Question about permutation testing

[Dirk van Moorselaar]

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Dear MNE users,

I have a question about using permutation tests to compare decoding accuracy across time between two conditions. Specifically, I am interested to see whether these conditions not only differ during stimulus presentation, but also already in anticipation of a new (search) display. I have a within subject design so I am running the following code  

t_obs, clusters, cluster_pv, H0 =  mne.stats.permutation_cluster_1samp_test(X_1 - X_2)

If I then inspect the results, I get significant clusters during stimulus presentation (which is nice. However, there are  no significant anticipatory clusters, even though numerically the conditions clearly differ.
Interestingly, I am able to make these anticipatory clusters significant if instead of analysing the whole time course, I limit the permutation test to the anticipatory window. I assume that this is the case because the anticipatory cluster is smaller than the reactive cluster????

My question is whether this is a valid approach, and if not whether there is another alternative way to still show that the conditions already differ before stimulus processing.

Thanks in advance,

Dirk van Moorselaar