We would like to thank Dr. J. Michael Tyszka, Dr. Miriam Scadeng and Dr. David Dubowitz for helping us to identify the 3-D structures extracted from the DT dataset. Dr. Jason Wood developed the Iris Explorer modules used to produce part of the results in the paper. This work was supported by National Science Foundation grants #ACI-9982273 and #ASC-89-20219, the National Institute on Drug Abuse, the National Institute of Mental Health and the NSF, as part of the Human Brain Project, Office of Naval Research Volume Visualization grant #N000140110033, and the National Library of Medicine ``Insight'' Project #N01-LM-0-3503.The first DT-MRI dataset is courtesy of the University of Utah SCI Institute, the second dataset is courtesy of Dr. Mark Bastin, University of Edinburgh, UK. Finally, we would like to thank our reviewers for a very detailed review and multiple valuable suggestions.