Objectives: To relate anatomic connectivity alterations in specific tracts to cognitive performance drawing on brain areas connected by those tracts.
Methods: The methods include combining high-angular diffusion weighted imaging (HARDI) acquisition methods (128-directions, high b-values, 32-channel head coil) with advanced computational methods for reconstructing the diffusion orientation density function (generalized q-space imaging and fiber tractography) to map the detailed white matter connectivity in individual participants.
Results: We have successfully collected preliminary HDFT data from a sample of 8 individuals with autism and 9 controls aged 40-65 years, and found important group differences, even in this small sample. The fractional anisotropy of the left superior longitudinal fasciculus is reduced in autism (F(1 ,14) = 6.02, p < .05) and it reliably predicts a lower-level language ability (word reading speed) (r = .69, p < .05) (measured with the WRAT). In addition, the number of fibers in the right uncinate fasciculus was reduced (F(1 ,14) = 9.58, p < .01). Importantly, the number of fibers in this tract reliably predicts higher level language abilities in autism, namely inference-making ( r = .71, p < .05) and oral expression (r = .77, p < .05) (measured by the Test of Language Competence). In both cases, the tract properties show a strong correlation with the type of language performance that relies on the specific interregional connectivity provided by the tract. This extremely encouraging preliminary finding exemplifies the ability to relate cognitive performance to connectivity on an individual basis. Moreover, these tract measures do not predict performance in controls, where connectivity is apparently not a limiting factor.
Conclusions: These findings constitute some of the first evidence that white matter properties are disrupted in older adults with autism and that these measures are related to cognitive performance. These preliminary results demonstrate the feasibility of the approach, its applicability to older participants, and its potential fruitfulness.