Four regions in the right hemisphere showed significant group differences in two-year change in cortical white matter extent (see Table 3 and Figure 2). Two-year white matter changes were greater in non-users versus initiators such that non-users showed increasing white matter over time relative to initiators. On the lateral surface, one white matter cluster was located toward the far lateral aspect of the precentral gyrus, but also extended into lateral borders of the precentral and supramarginal regions. The other lateral cluster was located in white matter along the central portion of the middle temporal gyrus. On the medial surface, an anterior cluster extended along the posterior two-thirds of the white matter associated with the superior frontal gyrus, while a posterior cluster was located along the caudal portion of the lingual gyrus. No significant differences in white matter volume were observed between groups at baseline.
