How many clusters in the insular cortex?

The insular cortex has been divided into functional clusters whose number differs in studies, and is strongly dependent on the criteria employed in its parcellation. Recently, using resting state functional MRI, we reported two clusters, one anterior ventral and one posterior dorsal, in the insula, and confirmed our result in a meta-analytic study. Other authors, using different parcellation strategies, have obtained different divisions in insular clusters. The hierarchical clustering that we performed on the ROI-wise analysis moderately supports a tripartite subdivision of the insula, as has also been suggested by others. Given the dataset, the determination of the optimal number of parcels is not a trivial task. Several methods for calculating the optimal number of clusters have been proposed, but none of them are problem free. The coexistence of different parcellation schemes can be explained by hypothesizing an intrinsically hierarchical nature of the resting state brain networks: indeed, in this view, different levels of complexity could be nested in a hierarchical structure that starts from a bipartite insular subdivision and move to more complex parcellation schemes where each of the two previous clusters are further subdivided.