dMRI and tractography reveal subtle hypoplasia of the corpus callosum. A, Representative sagittal view of color-coded dMRI and enlargement of a representative CC showing the regions of interest (ROIs) used for tractography: orange, splenium; yellow, rostrum; remaining green CC, body. The key to tissue color bidirectional orientations in the color-coded dMRI is as follows: blue, dorsal-ventral; green, medial-lateral; red, anterior-posterior. B, Representative dorsal three-dimensional views of CC tractography in WT, Slc12a6^Δ18/Δ18, and nsSlc12a6^Δ18/Δ18 brains. Each region of tractography has been color coded according to its seed ROI and overlaid on a two-dimensional color-coded dMRI horizontal plane at the level of the anterior commissure. A bilateral streamline deficit appears in the external capsule (ec) of the caudal cortex of the Slc12a6^Δ18/Δ18 brain. These streamlines are adjacent to the auditory cortices. C, The number of voxels for each of the ROIs was quantified, giving a measure of volume, while slight, but insignificant, decreases in CC volume were observed in the rostrum and body. When calculated for the entire CC, volume was decreased by ∼12% and 11% of WTs, which were statistically significant, *p = 0.0019 and *p = 0.044, compared with WTs, respectively. D, Streamline number was unchanged in each of the ROIs. E, When streamlines were filtered to isolate those passing through the external capsule adjacent to the auditory cortex, significantly fewer streamlines were found in the Slc12a6^Δ18/Δ18 brain (*p = 0.0325, n = 3, compared with WTs, t test). Values represent means ± SE. Scale bar, 1 mm in all images. post., Posterior; ant., anterior.