Cortical axonal tracts are disrupted in brains of Itsn1 mutants. A, FA analysis of Itsn1^Gt/Gt brain. Differences in FA between WT and Itsn1^Gt/Gt are localized to midline crossing structures. These differences are highlighted when overlapped onto the WT FA map. A higher t-statistic (closer to red) means a greater difference between WT and Itsn1^Gt/Gt FA values for a given voxel. Shown are representative transverse sections from dorsal to ventral brain (left to right). The highest statistical differences are observed along tracts that approach the longitudinal medial fissure. B, Representative white matter tracts from each genotype show loss of tract structure around the midline in crossing fibers from mutants of Itsn1. Significant loss of order can be seen in Itsn1^Gt/Gt and Itsn1^Gt/GtItsn2^−/− corpus callosum, ventral hippocampal commissure (white arrowhead) in the fornix/fimbria, and anterior commissures. Cerebral peduncles are indistinguishable between genotypes. Since tractography maps are calculated within established constants (see Materials and Methods), in some cases adjacent neuroanatomical tracts are visualized. For example, a piece of the corpus callosum is visible in the WT image of the fornix/fimbria as are tracts extending to the brain stem ventral and caudal to the fornix.