In speech, meaning is conveyed by single words and their combination into sentences. While processing words and sentences share some linguistic mechanisms and anatomical substrates, their dissociation is not uncommon in aphasia patients. How these clinical observations fit into current dual stream models of language is poorly understood.

98 patients with PPA were recruited at the Northwestern University, Chicago.

            Diffusion images (60 diffusion-weighted 2.3mm isotropic, b-value=1,000 s/mm2) were acquired on a 3T Siemens Trio MRI system and processed for Diffusion Tensor Imaging and Spherical Deconvolution. 

            Tractography was used to dissect tracts of the dorsal (three segments of the arcuate fasciculus) and ventral networks (uncinate fasciculus, inferior fronto-occipital fasciculus, and the inferior longitudinal fasciculus). In addition, the temporal longitudinal tract, a recently described connection between posterior and anterior temporal regions was analysed. Tract volume, fractional anisotropy, and mean diffusivity were extracted for each tract and correlated with language performances (Peabody Picture Vocabulary Test, Boston Naming Test, and both the Sentence Production Priming Test and the Sentence Comprehension Test from the Northwestern Assessment of Verbs and Sentences).

In the ventral network correlations between performance in single word processing tests and diffusion measurements were significant only for the temporal longitudinal tract and the uncinate fasciculus. None of the ventral network tracts showed correlation with tests for sentence processing.

Correlations with sentence processing tests were significant for the anterior and posterior segments of the arcuate. None of the dorsal tracts showed statistically significant correlations with tests for single word processing.

These results suggest a parallel processing for single words along the ventral network and for sentences along the dorsal network. Our findings reconcile dual stream language models with classical aphasiology classifications.