Describe the problem

Permutation tests may crash when datasets are large (50 time points, oct6, 13 subjects in my case). mne.stats.permutation_cluster_1samp_test with out_type='mask' caused memory overflow and the process was killed on a machine with 64GB RAM and 16GB swap. With out_type='indices', the process only used about 4GB RAM.

Since out_type is default to 'indices' for mne.stats.spatio_temporal_cluster_1samp_test and mne.stats.spatio_temporal_cluster_test, it would be nice that out_type in mne.stats.permutation_cluster_1samp_test, mne.stats.permutation_cluster_test also defaults to 'indices' to prevent the memory error.

Describe your solution

Current:
mne.stats.permutation_cluster_1samp_test(..., out_type='mask', ...)

Proposed:
mne.stats.permutation_cluster_1samp_test(..., out_type='indices', ...)