Inappropriate usage of the Brunner-Munzel test in recent voxel-based lesion-symptom mapping studies

Voxel-based lesion-symptom mapping (VLSM) techniques have been important in elucidating structure-function relationships in the human brain. Rorden, Karnath, and Bonilha (2007) introduced the non-parametric Brunner-Munzel rank order test as an alternative to parametric tests often used in VLSM analyses. However, the Brunner-Munzel statistic produces inflated z scores when used at any voxel where there are less than 10 subjects in either the lesion or no lesion groups. Unfortunately, a number of recently published VLSM studies using this statistic include relatively small patient populations, such that most (if not all) examined voxels do not meet the necessary criteria. We demonstrate the effects of inappropriate usage of the Brunner-Munzel test using a dataset included with MRIcron, and find large Type I errors. To correct for this we suggest that researchers use a permutation derived correction as implemented in current versions of MRIcron when using the Brunner-Munzel test.