shows the statistical map of the significantly asymmetric regions in fractional anisotropy of healthy subjects thresholded by P < 0.005, P < 0.001, P < 0.0005, P < 0.00005, and P < 0.00001 with a cluster size of >60 voxels at P < 0.005. Colored dots in the left hemisphere (neurological convention, left hemisphere on the left side of the picture) indicate voxels with higher fractional anisotropy in the left hemisphere than right, and colored dots in the right hemisphere indicate voxels with higher fractional anisotropy in the right hemisphere than the left. The white thin lines in the images, which were extracted from the template, show boundary voxels where a pairwise t test was conducted.
Fig. 3 The statistical map of the statistically significant asymmetric regions in fractional anisotropy of healthy subjects. Colored dots in the left hemisphere (neurological convention) indicate voxels with higher fractional anisotropy in the left hemisphere (more ...)
summarizes hemispheric asymmetry from fractional anisotropy in healthy subjects. Left-greater-than-right asymmetry was found in genu, rostrum, and splenium of the corpus callosum, cingulum bundle, optic radiation, and superior cerebellar peduncles, while right-greater-than-left asymmetry was found at the prefrontal regions of anterior limb of the internal capsule fibers, the uncinate fasciculus, internal capsule, and the superior and inferior aspect of the arcuate fasciculus (superior longitudinal fasciculus and inferior longitudinal fasciculus).
Hemispherical asymmetry of fractional anisotropy in healthy subjects
Right-greater-than-left asymmetry of the uncinate fasciculus appears in the stem and the inferior aspect of the fiber bundle. However, we did not find significant left-greater-than-right asymmetry at the uncinate fasciculus. Having in mind the previous report (Kubicki et al., 2002
), we explored the effect of smoothing kernel size on the results. demonstrates three consecutive coronal slices of the statistical asymmetry maps in healthy subjects derived from three different Gaussian kernel sizes, that is, FWHM of 3 × 3 × 3 mm, 6 × 6 × 6 mm, and 9 × 9 × 9 mm. For this particular structure, statistical maps with smaller smoothing filter with FWHM 3 × 3 × 3 mm () demonstrate the left-greater-than-right asymmetry in controls in superior portion of uncinate fasciculus, which is consistent with the previous manual result (Kubicki et al., 2002
) measured at the similar location. Since the uncinate fasciculus is coherent only for a very short distance, the smoothing kernel of 6 × 6 × 6 mm FWHM might enhance the partial volume effect by including low anisotropy voxels located outside the stem of this fiber bundle, reducing the statistical power in this region. In the map of larger smoothing kernel size with FWHM 9 × 9 × 9 mm, the anisotropy of the cingulum bundle was smeared into neighboring low anisotropy structures, and the asymmetry was not detected significantly in the cingulum bundle ().
Fig. 4 Statistical asymmetry map of fractional anisotropy in healthy subjects: similarity and difference between Gaussian kernel size of 3 × 3 × 3 mm, 6 × 6 × 6 mm, and 9 × 9 × 9 mm in FWHM. All voxels having lower (more ...)
Note that significant asymmetry was detected at fiber crossing areas, such as left-higher-than-right in the medial regions where the superior longitudinal fasciculus and fibers from posterior limb of the internal capsule are crossing, and right-higher-than-left in the posterior regions where corpus callosum fibers (splenium) cross the superior longitudinal fasciculus.
The statistical asymmetry map of fractional anisotropy in schizophrenics is shown in and is summarized at . Here, it is clear that patients with schizophrenia show reduced hemispheric asymmetry in fractional anisotropy compared with healthy subjects.
Fig. 5 The statistical map of the statistically significantly asymmetric regions in fractional anisotropy in schizophrenics. Colored dots in the left hemisphere (neurological convention) indicate voxels with higher fractional anisotropy in the left hemisphere (more ...)
Hemispherical asymmetry of fractional anisotropy in schizophrenic subjects
We found anisotropy asymmetry in patients with schizophrenia at cingulum bundle, callosal fibers, optic radiation, and middle and posterior portions of superior longitudinal fibers. Normal asymmetry in controls was not found at anterior limb of the internal capsule and at the uncinate fasciculus.
shows the results of our study superimposed on the three-dimensional model of white matter fiber tracts, obtained with the method described elsewhere (Park et al., 2003b
). The colors in the fiber bundles are derived from the voxel’s z
-values and show left–right asymmetry. The portion of the figure above the yellow line illustrates left-greater-than-right FA asymmetry similar to and , whereas the portion of the figure below the line shows FA asymmetry in the opposite direction (i.e., right-greater-than-left). These z
-values for the cingulum bundle and the anterior portion of the CC clearly demonstrate a left-greater-than-right asymmetry in controls that is present, albeit, reduced in schizophrenia.
Fig. 6 Asymmetry of fractional anisotropy (FA) in normal controls (NC) and patients with schizophrenia (SZ) superimposed on the three-dimensional tractography. The colors in the fiber bundles are derived from the left– right asymmetry z-values displayed (more ...)