More is not always better: increased fractional anisotropy of superior longitudinal fasciculus associated with poor visuospatial abilities in Williams syndrome

Abstract

We used diffusion tensor imaging to examine white matter integrity in the dorsal and ventral streams among individuals with Williams syndrome (WS) compared with two control groups (typically developing and developmentally delayed) and using three separate analysis methods (whole brain, region of interest, and fiber tractography). All analysis methods consistently showed that fractional anisotropy (FA; a measure of microstructural integrity) was higher in the right superior longitudinal fasciculus (SLF) in WS compared with both control groups. There was a significant association with deficits in visuospatial construction and higher FA in WS individuals. Comparable increases in FA across analytic methods were not observed in the left SLF or the bilateral inferior longitudinal fasciculus in WS subjects. Together, these findings suggest a specific role of right SLF abnormality in visuospatial construction deficits in WS.

Figure 1.

Results overlaid on whole brain. A, TBSS voxel-based analysis results overlaid on FA template in coronal (top row) and axial views (second and third rows). Regions that show significant difference in FA values between WS and TD and between WS and DD groups are shown. Top row shows PLIC/CPT/STR/SCT regions (Talairach coordinate, y = −8), middle row shows SLF region (z = 29), and bottom row shows UF/IFO regions (z = −12). p = 0.05 corrected. B, Probabilistic maps of SLF and ILF for each group overlaid on a three-dimensional whole-brain T1 image. The SLF is projected onto the top of the brain, and the ILF is projected onto the bottom of the brain. A value of 100% indicates that all subjects in the group show overlapping tracts in that particular voxel. Right hemisphere is shown on right (Rt).

Figure 2.

FA and ADC results. Mean FA and 1/ADC values from ROQS (A) and mean FA values from fiber tracking (B) are plotted for each fiber tract: left superior longitudinal fasciculus (L SLF), right SLF (R SLF), L inferior longitudinal fasciculus (L ILF), and R ILF. Error bar represent SEMs. * indicates fibers that showed a main effect of group using one-way ANOVA, and significant differences between WS and TD and between WS and DD (Table 2).