Significant differences between non-users and initiators in two-year FA change were observed in two voxel clusters (Table 3 and Figure 3). For both clusters, the non-users had greater FA increases than initiators over the two year follow-up interval. One cluster was located in the left hemisphere along the dorsal portion of the caudate and extending toward the thalamus. The other cluster was located in the right hemisphere along the mid-temporal to polar-temporal regions of the inferior fronto-occipital fasciulus (IFOF), extending anteriorly to the junction of the IFOF and the uncinate fasciculus. No clusters were found that indicated a greater FA increases in the initiator group. Non-users did not differ in FA from initiators at baseline. To clarify the nature of the FA changes in the two significant clusters, mean radial and axial diffusivity values were extracted within each cluster. In the non-user group, both clusters demonstrated a greater decrease in radial versus axial diffusivity over the two year follow-up, consistent with increased fiber myelination (68), a pattern that was not evident for the initiator group. However, these are only descriptive
