The language network
Highlights
► There are two dorsal and two ventral fiber tracts relevant for language. ► The connection between temporal and premotor cortex supports speech repetition. ► The connection between temporal cortex and Broca's area supports complex syntax. ► The ventral fiber tracts subserve semantic and basic syntactic processes.
Introduction
Macrocircuits in the brain are realized by white matter fiber bundles which are composed of millions of axons. The myelin that surrounds the axons is essential for the fast transmission of electrical impulses and, thus, for the transmission of information during sensory, motor and cognitive functions [1]. With the advent of diffusion weighted magnetic resonance imaging (dMRI), white matter fiber bundles can now be tracked non-invasively, in vivo in the human brain (for a review see [2]) by using, for example, functional magnetic resonance imaging (fMRI)-based regions of interest (ROIs) to map the anatomy to the function. Different approaches have been taken to linking language function and brain structure using tractography (for a review see [3]).
The neural circuit supporting language functions must connect the language-relevant brain regions located in the inferior frontal cortex, including Broca's area, and the superior temporal cortex, including Wernicke's area (for reviews see [4•, 5•]). For the production of language, motor and premotor regions are relevant, and for language perception, sensory input systems such as the auditory systems for hearing and the visual system for reading must be recruited.
Recent research investigating the structural network underlying language has mainly focused on the core functions of language, that is, semantic and syntactic processes, leaving the sensory–motor interface undiscussed or in the background. Therefore, we will only briefly touch on the input–output aspect of language processing when discussing speech repetition and will mainly focus on those fiber bundles whose functional relevance to semantic and syntactic processing has been demonstrated, either through fMRI-based defined ROIs, through phylogenetic and ontogenetic data or through lesion studies.
Section snippets
The dorsal pathways
The two dorsal long-range fiber bundles discussed in the literature as being most important for language are the arcuate fascile (AF) and parts of the superior longitudinal fascile (SLF), connecting the frontal cortex (FC) and the TC [6, 7•, 8]. Since the AF and the SLF are not easily distinguishable in their horizontal parts, given the available methods, some researchers use the nomenclature AF/SLF. The AF is the fiber tract which directly connects Broca's area and Wernicke's area. It has been
Ventral pathways
Ventrally, the FC and the TC are connected via at least two pathways, the uncinate fascile (UF) and another fiber tract, which is either named the extreme capsule fiber system (ECFS) or the inferior-fronto-occipital fascile (IFOF) in the literature, running through the extreme capsule (see also [25]). The UF connects the anterior inferior FC with the anterior TC [9, 15•, 26, 27]. The IFOF connects the FC with posterior parts of the brain, that is, the posterior TC [28, 29], the occipital cortex
Conclusion
The reviewed studies point towards a neural language network with at least two dorsal and two ventral pathways, each serving a different language function. The two dorsal pathways support auditory-to-motor mapping and the processing of syntactically complex sentences, respectively. The two ventral pathways subserve, on the one hand, semantic and, on the other, basic syntactic processes. However, further studies may provide a more fine-grained picture of the dorsal and ventral pathways of the
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
A.F. was supported by a grant by the European Research Council (ERC-2010-AdG_20100407 for project 269505 NEUROSYNTAX). S.G. was partly supported by the German Research Foundation as a member of the Berlin School of Mind and Brain, Humboldt University, Berlin, Germany, as well as by the German National Merit Foundation.
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