Dynamic processing in the human language system: synergy between the arcuate fascicle and extreme capsule

The production and comprehension of human language is thought to involve a network of frontal, parietal, and temporal cortical loci interconnected by two dominant white matter pathways. These two white matter bundles, often referred to as the dorsal and ventral processing tracts, are hypothesized to have markedly different language functions. The dorsal tract is thought to process phonological processing, while the ventral tract is considered to abet semantics. This proposed functional differentiation of tracts is similar to the ventral and dorsal dichotomy proposed for the visual and auditory systems. The present study evaluated this characterization of the language system in the context of various components involved in its function. Twenty-four chronic stroke patients completed a battery of 10 language tests designed to measure performance on the comprehension and production of phonology, morphology, semantics, and syntax. The patients also completed diffusion MRI scanning. Lesions were confined to the left hemisphere, but the size and location of the insult varied so that patients had damage to a single tract, both tracts, or neither tract. Individual FA maps were generated, and focal areas of hypointensity served as markers of white matter damage. Whole-brain voxel-by-voxel correlations revealed that only phonological and semantic tasks fit into the dual-stream model, while syntax and morphology involved both pathways. ROI analyses of the arcuate fascicle and extreme capsule supported this finding. These data suggest that natural language function is more likely to reflect a synergistic system rather than a segregated dual-stream system.