I have a question about how Z-score values for specific voxels within a cluster appear to change depending on the Z threshold used in FEAT analyses.
I ran an fMRI analysis using a simple blocked design and initially used cluster-based thresholding with a Z threshold of 2.0. I then reran the analysis using a threshold of 2.3, without making any other changes to the design file. When looking at the overlaid images, the clusters for Z>2.3 seem to make sense in terms of being in the same location and of a slightly smaller extent relative to Z>2.0. However, when looking at the thresh_zstat maps in FSLView, the Z values for voxels in the significant clusters sometimes differ between the 2.3 and 2.0 maps. I then compared the unthresholded zstat.nii images of the 2.3 and 2.0 results in FSLView, and all the voxels across the brain have the same Z values between the two versions except for the voxels within the clusters that were significant, which had either higher, lower, or the same Z values for the 2.3 results. The histograms for the thresh_zstat images also were different.
I am confused about this because I thought that the Z values assigned to each voxel should be the same regardless of what Z-threshold is used for the cluster-based thresholding performed in FSL. I’m sure there must be something that I don’t understand, perhaps about the cluster-based thresholding, so if anyone has an explanation I would greatly appreciate it.