In addition, the remaining connections are then set to unit strength, resulting in a greatly sparsified binary network which is then subjected to standard graph analysis. Since the appropriate value of the threshold is a free and completely undetermined parameter, most practitioners vary the threshold across a broad range and then compute and compare graph metrics for the resulting networks.

The practice of thresholding functional networks has two immediate consequences, a much sparser topology which then tends to result in more and more separate clusters or modules, and a topology that Inhibitors,research,lifescience,medical discards all (even strong) negative correlations. While the status of negative Inhibitors,research,lifescience,medical correlations in resting fMRI remains controversial,35-38 it could be argued that the presence of an anticorrelation between two nodes does contribute information about their community membership. Building on this idea, variants of the Q-metric and other related measures that take into account Inhibitors,research,lifescience,medical the full weight distribution of a network have been proposed.39 These new metrics can also be applied

to functional networks regardless of their density (including fully connected networks), thus eliminating the need for thresholding entirely. Figure 3. Methodological issues in the analysis of functional connectivity. Panels (B) and (C) illustrate the effect of thresholding and binarizing. Panels (D) to (G) illustrate the issue of degenerate solutions in modularity. (A) A whole-brain Inhibitors,research,lifescience,medical functional connectivity … The second issue relates to the optimization of the module partition given a cost or quality metric like Newman’s Q. Studies of various real-world networks have shown that identifying the single optimal partition can not only be computationally difficult,

but that many real networks Inhibitors,research,lifescience,medical can be partitioned at near-optimal levels in a number of different or “degenerate” ways;40 Aggregating these degenerate solutions can provide additional information about the robustness with which a given node pair is affiliated with the same or a Carnitine palmitoyltransferase II different module. This idea has been developed further into a quantitative approach called “consensus clustering.” 41 Consensus E7080 chemical structure clustering has not yet been widely applied to brain networks,39,42 but it may soon become a useful tool since it provides information about the strength with which individual neural elements affiliate with their “home community.” An attractive hypothesis is that elements with generally weak affiliation are good candidates to assume functional roles as hub nodes that crosslink diverse communities.