Dear David,
this "conjunction map" is a descriptive approach, illustrating the overlap between the different thresholded T maps. Thus you would probably go with the usual thresholds when looking at the different contrasts (e.g. .001 uncorrected on voxel level plus cluster correction, or .05 FWE on voxel level) and use no further threshold when generating the "conjunction map". Of course you could also pre-specify a certain extent threshold in voxel units (an overlap in activation has to contain at least x voxels to be considered as relevant).
Best,
Helmut
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