The brain regions that were detected in the multivariate analysis are partially consistent with the results from previous univariate ENIGMA-OCD meta-analyses and mega-analyses6,7. Medicated adult OCD patients showed thinner frontal, temporal and parietal cortices, and smaller hippocampal and larger pallidum volumes compared to HC, whereas no differences were found for unmedicated adult patients. Although we were unable to detect significant features for medicated patients and unmedicated patients vs. HC classifications, these earlier results fit with the finding that classification performance for medicated patients vs. HC was better than for unmedicated patients vs. HC, as differences in brain anatomy of unmedicated patients appear to be minimal. Interestingly, the classification between medicated vs. unmedicated patients was even better, which suggests that this results from the minimization of heterogeneity in stratified patient groups as opposed to the larger heterogeneity seen in case-control comparisons. Finally, as the medication used for treating OCD is also used for treatment of many other psychiatric disorders, we anticipate that these results are not specific to OCD.
