Correction for Multiple Comparisons/Cluster Analysis (Volume)
With so many voxels in fMRI maps, it is very likely that many voxels will appear to be active purely by random chance (ie, a false positive). The is known as the "Problem of Multiple Comparisons". One way around this is to do a cluster analysis in which active voxels are eliminated unless they appear in a cluster, the idea being that false positives will not appear next to each other.
This program (mri_volcluster) will find clusters in a volume. A cluster is a set of contiguous voxels which meet a threshold criteria. The set may also have to reach a certain minimum number of voxels to be considered a cluster. The results can be saved in four ways: (1) a text summary file, (2) a new volume which is same as the input volume but with all the voxels that do not belong to a cluster set to zero, (3) a volume with each voxel's value equal to the cluster number in the summary file to which the voxel belongs, and (4) one cluster can be saved as a label file. The search space within the volume can be restricted to be within a mask. Two voxels are considered contiguous if they share a common row, column, or slice (except for --allowdiag).