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). This 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.

The command mri_volcluster identifies clusters of activations that meet specified criteria. It then creates images, summary files and even numbered labels for the output clusters. 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).

Relevant literature: Smoothing and cluster thresholding for cortical surface-based group analysis of fMRI data. Hagler DJ Jr, Saygin AP, Sereno MI. NeuroImage (2006).

Surface

The same idea applies to data on a surface (vertices). The command mri_surfcluster identifies clusters of vertices that meet specified criteria.

Relevant literature: Hayasaka and Nichols (2003) (pdf)