Variability and reproducibility of directed and undirected functional connectivity estimates in the human brain.

We analyzed and compared inter- and intra-subject variability of connectivity matrices as well as graph-theoretical measures in a large ($n=1003$) database of healthy subjects which underwent four consecutive rsfMRI sessions. We analyzed both directed (Granger Causality and Transfer Entropy) and undirected (Pearson Correlation and Partial Correlation) time-series association measures and related graph-theoretical measures. While matrix weights exhibit a higher reproducibility in undirected as opposed to directed methods, this difference disappears when looking at global graph metrics and, in turn, exhibits strong regional dependence in local graphs metrics. Our results warrant caution in the interpretation of connectivity studies.