Topographical relationships between arcuate fasciculus connectivity and cortical thickness.

The arcuate fasciculus (AF) connects cortical regions important in language processing, but how fiber coherence and organization relates to gray matter macrostructure remains uncharacterized. We used high-resolution structural and 30-direction diffusion imaging data from 36 healthy adults (24 male/12 female; mean age, 30.5 ± 9.8 years) to establish the relationships between AF microstructure and regional variations in cortical gray matter within language networks. Cortical pattern-matching algorithms were used to measure gray matter thickness at high-spatial density, and a validated diffusion tractography method was used to reconstruct the AF in the left and right hemisphere of each subject. Relationships between imaging measures and neuropsychological scores of verbal fluency were additionally assessed. Results revealed positive and highly topographical associations between arcuate fractional anisotropy (FA) and cortical thickness within anterior and posterior language regions and surrounding cortices, more prominently in the left hemisphere. These regional cortical thickness/FA relationships were primarily attributable to variations in radial diffusivity. Associations between cortical thickness and verbal fluency were observed in perisylvian language-related regions. Language scores were associated with left-hemisphere AF axial diffusivity, but not with AF FA or radial diffusivity. These findings thus suggest that particular components of white matter microstructure and regional increases in cortical thickness benefit aspects of language processing. Furthermore, the topographical relationships between independent measures of white matter and gray matter integrity suggest that rich developmental or environmental interactions influence brain structure and function where the presence and strength of such associations may elucidate pathophysiological processes influencing language systems.