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I'm not sure which subrick to select for the overlay, and which subrick to select for the threshold.
Usually you want to put a statistical sub-brick (t value -- sub-brick 1 in this case) for thresholding. As for overlay it is your choice: if you want to colorize the average percent signal change, stick in sub-brick 0 in thise case; if you want to show the t value distribution, overlay your brain with sub-brick 1 then.
Gang