Dynamic Processing in the Human Language System: Synergy between the Arcuate Fascicle and Extreme Capsule

Abstract

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.

The impact of phonological versus semantic repetition training on generalisation in chronic stroke aphasia reflects differences in dorsal pathway connectivity

“…Importantly, whole-brain correlations showed that only performance on tasks loading strongly on either phonology or semantics fit into this dual-stream model, whereas complex linguistic functions of syntax and morphology required integrity of both pathways (Rolheiser, et al, 2011).…”

“…The dorsal stream, on the other hand, is proposed to be strongly lefthemisphere dominant (Berthier, Lambon Ralph, Pujol, & Green, 2012;Catani & Mesulam, 2008) Although independent, the ventral and dorsal streams are highly interactive. Rolheiser, Stamatakis and Tyler (2011) considered 24 chronic stroke patients' performance across 10 tests involving key aspects of language production and comprehension and how this related to the results of diffusion weighted imaging. Phonological processing was found to load most heavily on the arcuate fascicle, implicated in the dorsal stream (Hickok & Poeppel, 2007).…”

Functional Dynamics of Dorsal and Ventral Reading Networks in Bilinguals

“…Whereas the dorsal pathway, encompassing parietal lobe, superior temporal gyrus (STG) and inferior frontal gyrus (IFG) pars opercularis, is thought to subserve phonological processing, the ventral pathway, including vOT and anterior IFG regions (i.e., pars triangularis and pars obitalis), supports mapping of orthographic-lexical stimuli onto semantic representations (Sandak et al 2004). These findings have also been bolstered by studies looking at white-matter pathways (Saur et al 2008;Rolheiser et al 2011;Friederici, 2012). Although results in studies with monolingual samples show a differential engagement of dorsal versus ventral reading regions, there is limited evidence as to what extent similar functional involvement of these networks is present in bilingual reading, and which aspects of reading (e.g., direct phonology versus assembled phonology) modulate their recruitment.…”

Development of white matter and reading skills

“…1C) is a fiber tract that connects the posterior inferior frontal cortex, including Broca's area, and the lateral temporal cortex, including Wernicke's area. This pathway is important for phonological awareness, an essential skill in reading development (6,24,25). The ILF (orange in Fig.…”