Review Article • DOI: 10.2478/s13380-014-0232-8 • Translational Neuroscience • 5(4) • 2014 • 239-252 Translational Neuroscience WHITE MATTER ARCHITECTURE OF THE LANGUAGE NETWORK Vanja Kljajevic* Abstract University of the Basque Country (UPV/EHU), The relevance of anatomical connectivity for understanding of the neural basis of language was recognized in Vitoria-Gasteiz, Spain the 19th century, and yet this topic has only recently become the subject of wider research interest. In this paper, I review recent findings on white matter tracts implicated in language: the arcuate fasciculus, superior longitudinal fasciculus, extreme capsule, uncinate fasciculus, middle longitudinal fasciculus, inferior longitudinal fasciculus, and inferior fronto-occipital fasciculus. The reviewed findings on these tracts were reported in studies that used a variety of methods, from post-mortem dissection and diffusion imaging to intraoperative electrostimulation with awake surgery patients. The emerging picture suggests that there is currently no consensus with regard to the exact number and identity of the tracts supporting language, their origins, trajectories, and terminations, as well as their functional interpretation. Keywords • White matter • Anatomical connectivity • Language • Dorsal stream • Ventral stream • Aphasia Received 15 August 2014 © Versita Sp. z o.o. accepted 24 August 2014 List of abbreviations of cognitive processes. The left hemisphere WM. Diffusion-Weighted Magnetic Resonance temporo-perisylvian language network is one Imaging (DW-MRI) is a recently-developed AF – arcuate fasciculus among several large-scale neurocognitive imaging method that allows noninvasive in vivo BA – Brodmann area networks that have been identied so far in studying of the brain’s structural connectivity DTI – diffusion tensor imaging the human brain, along with the prefrontal [6], complementing traditional research EC – external capsule executive function network, the fronto-parietal methods, such as post-mortem fiber dissection, EmC – extreme capsule spatial network, the limbic/paralimbic network histochemical tract-tracing, intraoperative IFG – inferior frontal gyrus supporting explicit memory and motivation, electrostimulation, and conventional MRI [7]. IFOF – inferior fronto-occipital fasciculus the inferior temporal network supporting Diffusion Tensor Imaging (DTI) provides an ILF – inferior longitudinal fasciculus face and object recognition [1], and possibly opportunity to study microstructural properties MdLF – middle longitudinal fasciculus the default mode network [2]. Large-scale of WM by measuring the diffusion of water MFG – middle frontal gyrus networks consist of distant but interconnected molecules within the tissue [8,9]. Typically MRI – magnetic resonance imaging local networks, which in turn are restricted to studied DTI measures that are thought to SFG – superior frontal gyrus single cytoarchitectonic elds or adjacent areas indicate WM integrity are fractional anisotropy SLF – superior longitudinal fasciculus [3]. (FA) and mean diffusivity (MD). Changes in their STG – superior temporal gyrus Insufficient insight into the anatomical values may indicate pathological processes; STS – superior temporal sulcus connectivity of the human brain, relative to for instance, decreased FA and increased MD UF – uncinate fasciculus more rapidly growing knowledge on brain’s values are often found in the WM of Alzheimer’s WM – white matter functional connectivity, has been repeatedly patients [10]. Other absolute diffusivities (radial pointed out in literature as a major limitation and axial) as well as mode of anisotropy are less 1. Introduction in the current understanding of neurocognitive often reported in literature. networks [4,5]. One reason for the still partial Unlike conventional T1-weighted MR images By connecting gray matter brain areas, white understanding of the brain’s structural on which WM appears homogenous and does matter (WM) fiber tracts contribute to the connectivity is the lack of powerful tools that not allow differentiation among various WM formation of networks that afford emergence would enable insights into the ne structure of structures, diffusion-based methods allow * E-mail: [email protected] 239 Translational Neuroscience virtual in vivo dissection of WM, visualization of basis of language, according to which aphasia model. Regardless, the dorsal connection even smaller tracts [7,11], as well as estimates results not only from damage to the cortical between Broca’s and Wernicke’s areas via the of the direction of axon bers, which enables grey matter areas supporting language AF postulated in the early model has been tractography [12]. This makes diffusion-based function (IFG, STG), but also from a disruption incorporated in current models of the neural techniques an indispensable tool for studying of the connections between these areas (AF). basis of language, although the classical view the structural connectivity of the living brain The Broca-Wernicke-Lichtheim’s model had that a single WM tract connects anterior and [13]. Building on insights acquired by methods predominated over other language models for posterior language areas had to be revised. such as post-mortem blunt dissection and over a century. Its principles on the functional Other WM tracts have been revealed to support intraoperative electrostimulation, diffusion specialization of brain areas and importance of various language functions, from naming and imaging techniques are currently demarcating areas’ connectivity for functionality represent semantics to syntax, phonology, reading and new borders of knowledge on the complex the foundation of current models of the neural writing. architecture of WM, despite their often basis of language. Further development of discussed limitations, such as spatial resolution, the model was enabled among others by 2. The dual-stream model of difficulty in interpreting orientation of mixing contributions such as Dejerine’s work on language processing tracts within a voxel, and the risk of delineating anatomical connectivity towards the end structural connectivity maps that in actual fact of the 19th century and Geschwind’s work, Language is a higher cognitive function that may not correspond to anatomical connectivity which in particular recognized the role of the brings together various processes, requiring [14-16]. inferior parietal region in language [21,22]. In executive resources and involvement of a range The relevance of anatomical connectivity for recognition of Geschwind’s contribution to of brain areas. Language comprehension, understanding the neural basis of language the neuroanatomy of language, researchers for example, involves auditory/ visual word was also recognized in the 19th century; working in this eld sometimes refer to recognition, lexical and morphological for instance, to explain how brain damage the inferior parietal region as “Geschwind’s processes, syntactic analysis or parsing, caused speech disturbances in a 23-year old territory” [23]. conceptual interpretation, referential processes, aphasic woman, Meynert [17] considered both With subsequent insights from studies and so on [35]. The main processes involved in affected grey matter areas and WM tracts. In involving brain-damaged patients and the language production are conceptual processes, the 1860s, Pierre Paul Broca established that advancement of neuroimaging, it has become word selection and retrieval, sequencing at injury in the inferior frontal gyrus (IFG) of the clear that language is more distributed in the the sentence and word levels, articulation, left cerebral hemisphere was associated with a brain than previously thought [24-26]. Even and monitoring of speech output [25]. Being profound speech loss [18]. Carl Wernicke found Broca’s historic patients Leborgne and Lelong, so complex, language is resource-demanding; out that lesion in the left superior temporal whose dening lesions were typically described for instance, a change in word order may gyrus (STG) was associated with impairment as affecting the posterior third of the left IFG, require selective attention, whereas processes in speech comprehension [19]. In addition to have been discussed in light of new evidence that heavily rely on temporary storage and distinguishing between two types of language obtained by CT and MR imaging of their brains manipulation of stored information, such as disorders, presumably caused by lesions to that revealed more extended lesions [27,28]. syntactic movement, may require additional these two respective brain areas — motor (or In general, aphasia has turned out to be more working memory. Thus, researchers now focus expressive) aphasia and sensory (or receptive) complex than originally assumed. Returning on interactivity of anatomically distant brain aphasia, Wernicke hypothesized that so-called to conduction aphasia for illustration, we nd areas that support various aspects of language conduction aphasia (“Leitungsaphasie”) would that lesions associated with this syndrome functioning, rather than on functional result from injury to the WM bers connecting rarely affect only the AF [29], damage to the specializations of isolated “language” areas [36]. the IFG and the STG. Namely, disruption of AF does not necessarily cause the syndrome Building on Wernicke’s speech processing the anatomical connection between the [30], and that the syndrome may occur due model, which involved two processing pathways IFG and STG, i.e.