Cortico-subcortical organization of language networks in the right hemisphere: An electrostimulation study in left-handers

Intraoperative mapping of the right hemisphere: a systematic review of protocols that evaluate cognitive and social cognitive functions

The right hemisphere of the brain is often referred to as the non-dominant hemisphere. Though this is meant to highlight the specialized role of the left hemisphere in language, the use of this term runs the risk of oversimplifying or minimizing the essential functions of the right hemisphere. There is accumulating evidence from functional MRI, clinical lesion studies, and intraoperative mapping data that implicate the right hemisphere in a diverse array of cognitive functions, including visuospatial functions, attentional processes, and social cognitive functions. Neuropsychological deficits following right hemisphere resections are well-documented, but there is a general paucity of literature focusing on how to best map these functions during awake brain surgery to minimize such deficits. To address this gap in the literature, a systematic review was conducted to examine the cognitive and emotional processes associated with the right hemisphere and the neuropsychological tasks frequently used for mapping the right hemisphere during awake brain tumor surgery. It was found that the most employed tests to assess language and speech functions in patients with lesions in the right cerebral hemisphere were the naming task and the Pyramids and Palm Trees Test (PPTT). Spatial cognition was typically evaluated using the line bisection task, while social cognition was assessed through the Reading the Mind in the Eyes (RME) test. Dual-tasking and the movement of the upper and lower limbs were the most frequently used methods to evaluate motor/sensory functions. Executive functions were typically assessed using the N-back test and Stroop test. To the best of our knowledge, this is the first comprehensive review to help provide guidance on the cognitive functions most at risk and methods to map such functions during right awake brain surgery.

Systematic Review Registration

PROSPERO database [CRD42023483324].

Functional and oncological outcomes after right hemisphere glioma resection in awake versus asleep patients: a systematic review and meta-analysis

The right hemisphere has been underestimated by being considered as the non-dominant hemisphere. However, it is involved in many functions, including movement, language, cognition, and emotion. Therefore, because lesions on this side are usually not resected under awake mapping, there is a risk of unfavorable neurological outcomes. The goal of this study is to compare the functional and oncological outcomes of awake surgery (AwS) versus surgery under general anesthesia (GA) in supratentorial right-sided gliomas. A systematic review of the literature according to PRISMA guidelines was performed up to March 2023. Four databases were screened. Primary outcome to assess was return to work (RTW). Secondary outcomes included the rate of postoperative neurological deficit, postoperative Karnofsky Performance Status (KPS) score and the extent of resection (EOR). A total of 32 articles were included with 543 patients who underwent right hemisphere tumor resection under awake surgery and 294 under general anesthesia. There were no significant differences between groups regarding age, gender, handedness, perioperative KPS, tumor location or preoperative seizures. Preoperative and long-term postoperative neurological deficits were statistically lower after AwS (p = 0.03 and p < 0.01, respectively), even though no difference was found regarding early postoperative course (p = 0.32). A subsequent analysis regarding type of postoperative impairment was performed. Severe postoperative language deficits were not different (p = 0.74), but there were fewer long-term mild motor and high-order cognitive deficits (p < 0.05) in AwS group. A higher rate of RTW (p < 0.05) was documented after AwS. The EOR was similar in both groups. Glioma resection of the right hemisphere under awake mapping is a safer procedure with a better preservation of high-order cognitive functions and a higher rate of RTW than resection under general anesthesia, despite similar EOR.

Beyond Broca's and Wernicke's: Functional Mapping of Ancillary Language Centers Prior to Brain Tumor Surgery

Functional MRI is a well-established tool used for pre-surgical planning to help the neurosurgeon have a roadmap of critical functional areas that should be avoided, if possible, during surgery to minimize morbidity for patients with brain tumors (though this also has applications for surgical resection of epileptogenic tissue and vascular lesions). This article reviews the locations of secondary language centers within the brain along with imaging findings to help improve our confidence in our knowledge on language lateralization. Brief overviews of these language centers and their contributions to the language networks will be discussed. These language centers include primary language centers of "Broca's Area" and "Wernicke's Area". However, there are multiple secondary language centers such as the dorsal lateral prefrontal cortex (DLPFC), frontal eye fields, pre- supplemental motor area (pre-SMA), Basal Temporal Language Area (BTLA), along with other areas of activation. Knowing these foci helps to increase self-assurance when discussing the nature of laterality with the neurosurgeon. By knowing secondary language centers for language lateralization, via fMRI, one can feel confident on providing neurosurgeon colleagues with appropriate information on the laterality of language in preparation for surgery.

Speech and Language Errors during Awake Brain Surgery and Postoperative Language Outcome in Glioma Patients: A Systematic Review

Awake craniotomy with direct electrical stimulation (DES) is the standard treatment for patients with gliomas in eloquent areas. Even though language is monitored carefully during surgery, many patients suffer from postoperative aphasia, with negative effects on their quality of life. Some perioperative factors are reported to influence postoperative language outcome. However, the influence of different intraoperative speech and language errors on language outcome is not clear. Therefore, we investigate this relation. A systematic search was performed in which 81 studies were included, reporting speech and language errors during awake craniotomy with DES and postoperative language outcomes in adult glioma patients up until 6 July 2020. The frequencies of intraoperative errors and language status were calculated. Binary logistic regressions were performed. Preoperative language deficits were a significant predictor for postoperative acute (OR = 3.42, p < 0.001) and short-term (OR = 1.95, p = 0.007) language deficits. Intraoperative anomia (OR = 2.09, p = 0.015) and intraoperative production errors (e.g., dysarthria or stuttering; OR = 2.06, p = 0.016) were significant predictors for postoperative acute language deficits. Postoperatively, the language deficits that occurred most often were production deficits and spontaneous speech deficits. To conclude, during surgery, intraoperative anomia and production errors should carry particular weight during decision-making concerning the optimal onco-functional balance for a given patient, and spontaneous speech should be monitored. Further prognostic research could facilitate intraoperative decision-making, leading to fewer or less severe postoperative language deficits and improvement of quality of life.

Large-scale differences in functional organization of left- and right-handed individuals using whole-brain, data-driven analysis of connectivity

Handedness influences differences in lateralization of language areas as well as dominance of motor and somatosensory cortices. However, differences in whole-brain functional connectivity (i.e., functional connectomes) due to handedness have been relatively understudied beyond pre-specified networks of interest. Here, we compared functional connectomes of left- and right-handed individuals at the whole brain level. We explored differences in functional connectivity of previously established regions of interest, and showed differences between primarily left- and primarily right-handed individuals in the motor, somatosensory, and language areas using functional connectivity. We then proceeded to investigate these differences in the whole brain and found that the functional connectivity of left- and right-handed individuals are not specific to networks of interest, but extend across every region of the brain. In particular, we found that connections between and within the cerebellum show distinct patterns of connectivity. To put these effects into context, we show that the effect sizes associated with handedness differences account for a similar amount of individual differences in the connectome as sex differences. Together these results shed light on regions of the brain beyond those traditionally explored that contribute to differences in the functional organization of left- and right-handed individuals and underscore that handedness effects are neurobiologically meaningful in addition to being statistically significant.

Benefit of Action Naming Over Object Naming for Visualization of Subcortical Language Pathways in Navigated Transcranial Magnetic Stimulation-Based Diffusion Tensor Imaging-Fiber Tracking

Visualization of functionally significant subcortical white matter fibers is needed in neurosurgical procedures in order to avoid damage to the language network during resection. In an effort to achieve this, positive cortical points revealed during preoperative language mapping with navigated transcranial magnetic stimulation (nTMS) can be employed as regions of interest (ROIs) for diffusion tensor imaging (DTI) fiber tracking. However, the effect that the use of different language tasks has on nTMS mapping and subsequent DTI-fiber tracking remains unexplored. The visualization of ventral stream tracts with an assumed lexico-semantic role may especially benefit from ROIs delivered by the lexico-semantically demanding verb task, Action Naming. In a first step, bihemispheric nTMS language mapping was administered in 18 healthy participants using the standard task Object Naming and the novel task Action Naming to trigger verbs in a small sentence context. Cortical areas in which nTMS induced language errors were identified as language-positive cortical sites. In a second step, nTMS-based DTI-fiber tracking was conducted using solely these language-positive points as ROIs. The ability of the two tasks’ ROIs to visualize the dorsal tracts Arcuate Fascicle and Superior Longitudinal Fascicle, the ventral tracts Inferior Longitudinal Fascicle, Uncinate Fascicle, and Inferior Fronto-Occipital Fascicle, the speech-articulatory Cortico-Nuclear Tract, and interhemispheric commissural fibers was compared in both hemispheres. In the left hemisphere, ROIs of Action Naming led to a significantly higher fraction of overall visualized tracts, specifically in the ventral stream’s Inferior Fronto-Occipital and Inferior Longitudinal Fascicle. No difference was found between tracking with Action Naming vs. Object Naming seeds for dorsal stream tracts, neither for the speech-articulatory tract nor the inter-hemispheric connections. While the two tasks appeared equally demanding for phonological-articulatory processes, ROI seeding through the task Action Naming seemed to better visualize lexico-semantic tracts in the ventral stream. This distinction was not evident in the right hemisphere. However, the distribution of tracts exposed was, overall, mirrored relative to those in the left hemisphere network. In presurgical practice, mapping and tracking of language pathways may profit from these findings and should consider inclusion of the Action Naming task, particularly for lesions in ventral subcortical regions.

Bihemispheric Navigated Transcranial Magnetic Stimulation Mapping for Action Naming Compared to Object Naming in Sentence Context

Preoperative language mapping with navigated transcranial magnetic stimulation (nTMS) is currently based on the disruption of performance during object naming. The resulting cortical language maps, however, lack accuracy when compared to intraoperative mapping. The question arises whether nTMS results can be improved, when another language task is considered, involving verb retrieval in sentence context. Twenty healthy German speakers were tested with object naming and a novel action naming task during nTMS language mapping. Error rates and categories in both hemispheres were compared. Action naming showed a significantly higher error rate than object naming in both hemispheres. Error category comparison revealed that this discrepancy stems from more lexico-semantic errors during action naming, indicating lexico-semantic retrieval of the verb being more affected than noun retrieval. In an area-wise comparison, higher error rates surfaced in multiple right-hemisphere areas, but only trends in the left ventral postcentral gyrus and middle superior temporal gyrus. Hesitation errors contributed significantly to the error count, but did not dull the mapping results. Inclusion of action naming coupled with a detailed error analysis may be favorable for nTMS mapping and ultimately improve accuracy in preoperative planning. Moreover, the results stress the recruitment of both left- and right-hemispheric areas during naming.

Organizing Variables Affecting fMRI Estimates of Language Dominance in Patients with Brain Tumors

Numerous variables can affect the assessment of language dominance using presurgical functional magnetic resonance (fMRI) in patients with brain tumors. This work organizes the variables into confounding and modulating factors. Confounding factors give the appearance of changed language dominance. Most confounding factors are fMRI-specific and they can substantially disrupt the evaluation of language dominance. Confounding factors can be divided into two categories: tumor-related and fMRI analysis. The tumor-related confounds further subdivide into tumor characteristics (e.g., tumor grade) and tumor-induced conditions (aphasia). The fMRI analysis confounds represent technical aspects of fMRI methods (e.g., a fixed versus an individual threshold). Modulating factors can modify language dominance without confounding it. They are not fMRI-specific, and they can impact language dominance both in healthy individuals and neurosurgical patients. The effect of most modulating factors on fMRI language dominance is smaller than that of confounding factors. Modulating factors include demographics (e.g., age) and linguistic variables (e.g., early bilingualism). Three cases of brain tumors in the left hemisphere are presented to illustrate how modulating confounding and modulating factors can impact fMRI estimates of language dominance. Distinguishing between confounding and modulating factors can help interpret the results of presurgical language mapping with fMRI.

Prior Neurosurgery Decreases fMRI Estimates of Language Laterality in Patients with Gliomas within Anterior Language Sites

The impact of previous surgery on the assessment of language dominance with preoperative fMRI remains inconclusive in patients with recurrent brain tumors. Samples in this retrospective study included 17 patients with prior brain surgery and 21 patients without prior surgery (38 patients total; mean age 43.2, SD = 11.9; 18 females; seven left-handed). All the patients were left language dominant, as determined clinically. The two samples were matched on 10 known confounds, including, for example, tumor laterality and location (all tumors affected Brodmann areas 44/45/47). We calculated fMRI language dominance with laterality indices using a whole-brain and region of interest approach (ROI; Broca’s and Wernicke’s area). Patients with prior surgery had decreased fMRI language dominance (p = 0.03) with more activity in the right hemisphere (p = 0.03) than patients without surgery. Patients with prior brain surgery did not display less language activity in the left hemisphere than patients without surgery. These results were replicated using an ROI approach in the affected Broca’s area. Further, we observed no differences between our samples in the unaffected Wernicke’s area. In sum, prior brain surgery affecting Broca’s area could be a confounding factor that needs to be considered when evaluating fMRI language dominance.

Unexpected absence of aphasia following left temporal hemorrhage: a case study with functional neuroimaging to characterize the nature of atypical language localization

Unexpected absence of aphasia after left-hemisphere perisylvian damage is often assumed to reflect right-hemisphere language lateralization, but other potential explanations include bilateral language representation, or sparing of critical left-hemisphere regions due to individual variability. We describe the case of a left-handed gentleman who presented with no aphasia after a left temporal hemorrhage. We used functional neuroimaging to determine how his language network had been spared. In this case, we observed unequivocal right-hemisphere lateralization of language function, explaining his lack of aphasia. We discuss the variability of language organization and highlight outstanding questions about the implications of damage in different scenarios.

Contralesional functional network reorganization of the insular cortex in diffuse low-grade glioma patients

Diffuse low-grade gliomas (DLGGs) growing on the insular lobe induce contralesional hemispheric insular lobe compensation of damaged functioning by increasing cortical volumes. However, it remains unclear how functional networks are altered in patients with insular lobe DLGGs during functional compensation. Thirty-five patients with insular DLGGs were classified into the left (insL, n = 16) and right groups (insR, n = 19), and 33 healthy subjects were included in the control group. Resting state functional magnetic resonance imaging was used to generate functional connectivity (FC), and network topological properties were evaluated using graph theoretical analysis based on FC matrices. Network-based statistics were applied to compare differences in the FC matrices. A false discovery rate was applied to correct the topological properties. There was no difference in the FC of edges between the control and insL groups; however, the nodal shortest path length of the right insular lobe was significantly increased in the insL group compared to the control group. Additionally, FC was increased in the functional edges originating from the left insular lobe in the insR group compared to the control group. Moreover, there were no differences in topological properties between the insR and control groups. The contralesional insular lobe is crucial for network alterations. The detailed patterns of network alterations were different depending on the affected hemisphere. The observed network alterations might be associated with functional network reorganization and functional compensation.

New insights into the anatomo-functional architecture of the right sagittal stratum and its surrounding pathways: an axonal electrostimulation mapping study

The sagittal stratum (SS) is a large sheet-like structure where major axonal fiber tracts cross, though its anatomical delineations are still debated. Here we investigated the poorly studied anatomo-functional organization of the right SS using direct electrical stimulation (DES) in patients undergoing wide-awake surgery for a cerebral glioma. Seventeen patients were included. There were six males, the mean age was 38 years old. One patient underwent surgery twice. Fourteen patients were right-handed and one was ambidextrous. Behavior tasks were used to monitor online the patients' functions during DES, including visual and somesthetic processes, semantics, language, spatial and social cognition. Beyond the cortical DES, the mapping of axonal pathways evoked various functional responses. At the level of the core of the right SS, there were visual disturbances, visual hemi-agnosia, semantic paraphasia, left spatial neglect, confusion and comprehension difficulties, anomia, and mentalizing disturbances. At the level of the surrounding axonal pathways, there were left spatial neglect, anomia, vertigo, dysesthesia, and hearing disturbances. Our functionally defined three-dimensional map indicates that this complex region has a multilayered functional architecture, and supports an organization founded on two anatomical systems: a core system formed by the optic radiations, inferior longitudinal fasciculus, and inferior fronto-occipital fasciculus, and a peripheral one composed of surrounding or intersecting white matter tracts, including the superior longitudinal fasciculus/arcuate fasciculus, thalamocortical radiations, auditory radiations, and parieto-insular vestibular system. These results should prompt neurosurgeons to achieve awake DES mapping within the right SS because of the likelihood of causing multiple and irreversible structural disconnections.

Oscillatory and structural signatures of language plasticity in brain tumor patients: A longitudinal study

Recent evidence suggests that damage to the language network triggers its functional reorganization. Yet, the spectro‐temporal fingerprints of this plastic rearrangement and its relation to anatomical changes is less well understood. Here, we combined magnetoencephalographic recordings with a proxy measure of white matter to investigate oscillatory activity supporting language plasticity and its relation to structural reshaping. First, cortical dynamics were acquired in a group of healthy controls during object and action naming. Results showed segregated beta (13–28 Hz) power decreases in left ventral and dorsal pathways, in a time‐window associated to lexico‐semantic processing (~250–500 ms). Six patients with left tumors invading either ventral or dorsal regions performed the same naming task before and 3 months after surgery for tumor resection. When longitudinally comparing patients' responses we found beta compensation mimicking the category‐based segregation showed by controls, with ventral and dorsal damage leading to selective compensation for object and action naming, respectively. At the structural level, all patients showed preoperative changes in white matter tracts possibly linked to plasticity triggered by tumor growth. Furthermore, in some patients, structural changes were also evident after surgery and showed associations with longitudinal changes in beta power lateralization toward the contralesional hemisphere. Overall, our findings support the existence of anatomo‐functional dependencies in language reorganization and highlight the potential role of oscillatory markers in tracking longitudinal plasticity in brain tumor patients. By doing so, they provide valuable information for mapping preoperative and postoperative neural reshaping and plan surgical strategies to preserve language function and patient's quality of life.

Microsurgical anatomy of language

The localizationist model, which focused on classical cortical areas such as Broca's and Wernicke's, can no longer explain how language processing works. Over recent years, several studies have revealed new language-related cortical and subcortical areas, resulting in a transition from localizationist concepts to a hodotopical model. These studies have described language processing as an extensive and complex network of multiple interconnected cortical areas and subcortical pathways, differing from the classical circuit described by the localizationist perspective. The hodotopical model was made possible by a paradigm shift in the treatment of cerebral tumors, especially low-grade gliomas: total or subtotal tumor resections with cortical and subcortical mapping on awake patients have become the gold standard treatment for lesions located in the dominant hemisphere. In this article, we review current understating of the microsurgical anatomy of language.

Handedness and White Matter Networks

The development and persistence of laterality is a key feature of human motor behavior, with the asymmetry of hand use being the most prominent. The idea that asymmetrical functions of the hands reflect asymmetries in terms of structural and functional brain organization has been tested many times. However, despite advances in laterality research and increased understanding of this population-level bias, the neural basis of handedness remains elusive. Recent developments in diffusion magnetic resonance imaging enabled the exploration of lateralized motor behavior also in terms of white matter and connectional neuroanatomy. Despite incomplete and partly inconsistent evidence, structural connectivity of both intrahemispheric and interhemispheric white matter seems to differ between left and right-handers. Handedness was related to asymmetry of intrahemispheric pathways important for visuomotor and visuospatial processing (superior longitudinal fasciculus), but not to projection tracts supporting motor execution (corticospinal tract). Moreover, the interindividual variability of the main commissural pathway corpus callosum seems to be associated with handedness. The review highlights the importance of exploring new avenues for the study of handedness and presents the latest state of knowledge that can be used to guide future neuroscientific and genetic research.

Mapping critical cortical hubs and white matter pathways by direct electrical stimulation: an original functional atlas of the human brain

OBJECTIVE

The structural and functional organization of brain networks subserving basic daily activities (i.e. language, visuo-spatial cognition, movement, semantics, etc.) are not completely understood to date. Here, we report the first probabilistic cortical and subcortical atlas of critical structures mediating human brain functions based on direct electrical stimulation (DES), a well-validated tool for the exploration of cerebral processing and for performing safe surgical interventions in eloquent areas.

METHODS

We collected 1162 cortical and 659 subcortical DES responses during testing of 16 functional domains in 256 patients undergoing awake surgery. Spatial coordinates for each functional response were calculated, and probability distributions for the entire patient cohort were mapped onto a standardized three-dimensional brain template using a multinomial statistical analysis. In addition, matching analyses were performed against prior established anatomy-based cortical and white matter (WM) atlases.

RESULTS

The probabilistic maps for each functional domain were provided. The topographical analysis demonstrated a wide spatial distribution of cortical functional responses, while subcortical responses were more restricted, localizing to known WM pathways. These DES-derived data showed reliable matching with existing cortical and WM atlases as well as recent neuroimaging and neurophysiological data.

CONCLUSIONS

We present the first integrated and comprehensive cortical-subcortical atlas of structures essential for humans' neural functions based on highly-specific DES mapping during real-time neuropsychological testing. This novel atlas can serve as a complementary tool for neuroscientists, along with data obtained from other modalities, to improve and refine our understanding of the functional anatomy of critical brain networks.

Inferior Longitudinal Fasciculus' Role in Visual Processing and Language Comprehension: A Combined MEG-DTI Study

The inferior longitudinal fasciculus (ILF) is a white matter tract that connects the occipital and the temporal lobes. ILF abnormalities have been associated with deficits in visual processing and language comprehension in dementia patients, thus suggesting that its integrity is important for semantic processing. However, it remains elusive whether ILF microstructural organization per se impacts the visual semantic processing efficiency in the healthy brain. The present study aims to investigate whether there is an association between ILF’s microstructural organization and visual semantic processing at the individual level. We hypothesized that the efficiency of visual semantic processing positively correlates with the degree of anisotropy of the ILF. We studied 10 healthy right-handed subjects. We determined fractional anisotropy (FA) of the ILF using diffusion tensor imaging (DTI). We extracted N400m latency and amplitude from magnetoencephalography (MEG) signals during a visual semantic decision task. N400m and mean FA of the ILF were left lateralized with the higher FA value in the left hemisphere. Inter-individual analysis showed that FA of the ILF negatively correlated with the N400m latency and amplitude, which suggests that high ILF anisotropy is associated with more efficient semantic processing. In summary, our findings provide supporting evidence for a role of the ILF in language comprehension.

Individual Differences and Hemispheric Asymmetries for Language and Spatial Attention

Language and spatial processing are cognitive functions that are asymmetrically distributed across both cerebral hemispheres. In the present study, we compare left- and right-handers on word comprehension using a divided visual field paradigm and spatial attention using a landmark task. We investigate hemispheric asymmetries by assessing the participants' behavioral metrics; response accuracy, reaction time and their laterality index. The data showed that right-handers benefitted more from left-hemispheric lateralization for language comprehension and right-hemispheric lateralization for spatial attention than left-handers. Furthermore, left-handers demonstrated a more variable distribution across both hemispheres, supporting a less focal profile of functional brain organization. Taken together, the results underline that handedness distinctively modulates hemispheric processing and behavioral performance during verbal and nonverbal tasks. In particular, typical lateralization is most prevalent for right-handers whereas atypical lateralization is more evident for left-handers. These insights contribute to the understanding of individual variation of brain asymmetries and the mechanisms related to changes in cerebral dominance.

White matter pathways for prosodic structure building: A case study

The relevance of left dorsal and ventral fiber pathways for syntactic and semantic comprehension is well established, while pathways for prosody are little explored. The present study examined linguistic prosodic structure building in a patient whose right arcuate/superior longitudinal fascicles and posterior corpus callosum were transiently compromised by a vasogenic peritumoral edema. Compared to ten matched healthy controls, the patient's ability to detect irregular prosodic structure significantly improved between pre- and post-surgical assessment. This recovery was accompanied by an increase in average fractional anisotropy (FA) in right dorsal and posterior transcallosal fiber tracts. Neither general cognitive abilities nor (non-prosodic) syntactic comprehension nor FA in right ventral and left dorsal fiber tracts showed a similar pre-post increase. Together, these findings suggest a contribution of right dorsal and inter-hemispheric pathways to prosody perception, including the right-dorsal tracking and structuring of prosodic pitch contours that is transcallosally informed by concurrent syntactic information.

Semantic representation in the white matter pathway

Object conceptual processing has been localized to distributed cortical regions that represent specific attributes. A challenging question is how object semantic space is formed. We tested a novel framework of representing semantic space in the pattern of white matter (WM) connections by extending the representational similarity analysis (RSA) to structural lesion pattern and behavioral data in 80 brain-damaged patients. For each WM connection, a neural representational dissimilarity matrix (RDM) was computed by first building machine-learning models with the voxel-wise WM lesion patterns as features to predict naming performance of a particular item and then computing the correlation between the predicted naming score and the actual naming score of another item in the testing patients. This correlation was used to build the neural RDM based on the assumption that if the connection pattern contains certain aspects of information shared by the naming processes of these two items, models trained with one item should also predict naming accuracy of the other. Correlating the neural RDM with various cognitive RDMs revealed that neural patterns in several WM connections that connect left occipital/middle temporal regions and anterior temporal regions associated with the object semantic space. Such associations were not attributable to modality-specific attributes (shape, manipulation, color, and motion), to peripheral picture-naming processes (picture visual similarity, phonological similarity), to broad semantic categories, or to the properties of the cortical regions that they connected, which tended to represent multiple modality-specific attributes. That is, the semantic space could be represented through WM connection patterns across cortical regions representing modality-specific attributes.

One of the most challenging questions in cognitive neuroscience is how semantic knowledge, for example, that “scissors” and “knives” are related in meaning, can emerge from primary sensory dimensions such as visual forms. It is often assumed that in the human brain, semantics are stored in regions of the brain cortex, where distinct types of modality-specific information are transferred to and bind together. We tested an alternative hypothesis—“representation by connection”—in which higher-order semantic information could be coded by means of connection patterns between cortical regions. Combining data from behavior and brain imaging of 80 patients with brain lesions, we applied machine learning to construct the mapping models between the lesion patterns on axonal tracts (white matter) and item-specific object-naming performances. We found that specific white matter lesions produced deficits in object naming associated with the object’s semantic space, but not relevant to its primary dimension. The naming performances of semantically related objects were better predicted from white matter lesion-pattern models. That is, the higher-order semantic space could be coded in patterns of brain connections by linking cortical areas that do not necessarily contain such information.

A Novel Technique for Region and Linguistic Specific nTMS-based DTI Fiber Tracking of Language Pathways in Brain Tumor Patients

Navigated transcranial magnetic stimulation (nTMS) has recently been introduced as a non-invasive tool for functional mapping of cortical language areas prior to surgery. It correlates well with intraoperative neurophysiological monitoring (IONM) findings, allowing defining the best surgical strategy to preserve cortical language areas during surgery for language-eloquent tumors. Nevertheless, nTMS allows only for cortical mapping and postoperative language deficits are often caused by injury to subcortical language pathways. Nowadays, the only way to preoperatively visualize language subcortical white matter tracts consists in DTI fiber tracking (DTI-FT). However, standard DTI-FT is based on anatomical landmarks that vary interindividually and can be obscured by the presence of the tumor itself. It has been demonstrated that combining nTMS with DTI-FT allows for a more reliable visualization of the motor pathway in brain tumor patients. Nevertheless, no description about such a combination has been reported for the language network. The aim of the present study is to describe and assess the feasibility and reliability of using cortical seeding areas defined by error type-specific nTMS language mapping (nTMS-positive spots) to perform DTI-FT in patients affected by language-eloquent brain tumors. We describe a novel technique for a nTMS-based DTI-FT to visualize the complex cortico-subcortical connections of the language network. We analyzed quantitative findings, such as fractional anisotropy values and ratios, and the number of visualized connections of nTMS-positive spots with subcortical pathways, and we compared them with results obtained by using the standard DTI-FT technique. We also analyzed the functional concordance between connected cortical nTMS-positive spots and subcortical pathways, and the likelihood of connection for nTMS-positive vs. nTMS-negative cortical spots. We demonstrated, that the nTMS-based approach, especially what we call the “single-spot” strategy, is able to provide a reliable and more detailed reconstruction of the complex cortico-subcortical language network as compared to the standard DTI-FT. We believe this technique represents a beneficial new strategy for customized preoperative planning in patients affected by tumors in presumed language eloquent location, providing anatomo-functional information to plan language-preserving surgery.

Awake surgery for hemispheric low-grade gliomas: oncological, functional and methodological differences between pediatric and adult populations

INTRODUCTION

Brain mapping through a direct cortical and subcortical electrical stimulation during an awake craniotomy has gained an increasing popularity as a powerful tool to prevent neurological deficit while increasing extent of resection of hemispheric diffuse low-grade gliomas in adults. However, few case reports or very limited series of awake surgery in children are currently available in the literature.

METHODS

In this paper, we review the oncological and functional differences between pediatric and adult populations, and the methodological specificities that may limit the use of awake mapping in pediatric low-grade glioma surgery.

RESULTS

This could be explained by the fact that pediatric low-grade gliomas have a different epidemiology and biologic behavior in comparison to adults, with pilocytic astrocytomas (WHO grade I glioma) as the most frequent histotype, and with WHO grade II gliomas less prone to anaplastic transformation than their adult counterparts. In addition, aside from the issue of poor collaboration of younger children under 10 years of age, some anatomical and functional peculiarities of children developing brain (cortical and subcortical myelination, maturation of neural networks and of specialized cortical areas) can influence direct electrical stimulation methodology and sensitivity, limiting its use in children.

CONCLUSIONS

Therefore, even though awake procedure with cortical and axonal stimulation mapping can be adapted in a specific subgroup of children with a diffuse glioma from the age of 10 years, only few pediatric patients are nonetheless candidates for awake brain surgery.

Direct evidence for the contributive role of the right inferior fronto-occipital fasciculus in non-verbal semantic cognition

The neural foundations underlying semantic processing have been extensively investigated, highlighting a pivotal role of the ventral stream. However, although studies concerning the involvement of the left ventral route in verbal semantics are proficient, the potential implication of the right ventral pathway in non-verbal semantics has been to date unexplored. To gain insights on this matter, we used an intraoperative direct electrostimulation to map the structures mediating the non-verbal semantic system in the right hemisphere. Thirteen patients presenting with a right low-grade glioma located within or close to the ventral stream were included. During the 'awake' procedure, patients performed both a visual non-verbal semantic task and a verbal (control) task. At the cortical level, in the right hemisphere, we found non-verbal semantic-related sites (n = 7 in 6 patients) in structures commonly associated with verbal semantic processes in the left hemisphere, including the superior temporal gyrus, the pars triangularis, and the dorsolateral prefrontal cortex. At the subcortical level, we found non-verbal semantic-related sites in all but one patient (n = 15 sites in 12 patients). Importantly, all these responsive stimulation points were located on the spatial course of the right inferior fronto-occipital fasciculus (IFOF). These findings provide direct support for a critical role of the right IFOF in non-verbal semantic processing. Based upon these original data, and in connection with previous findings showing the involvement of the left IFOF in non-verbal semantic processing, we hypothesize the existence of a bilateral network underpinning the non-verbal semantic system, with a homotopic connectional architecture.

The Influence of Frontal Lobe Tumors and Surgical Treatment on Advanced Cognitive Functions

Brain cognitive functions affect patient quality of life. The frontal lobe plays a crucial role in advanced cognitive functions, including executive function, meta-cognition, decision-making, memory, emotion, and language. Therefore, frontal tumors can lead to serious cognitive impairments. Currently, neurosurgical treatment is the primary method to treat brain tumors; however, the effects of the surgical treatments are difficult to predict or control. The treatment may both resolve the effects of the tumor to improve cognitive function or cause permanent disabilities resulting from damage to healthy functional brain tissue. Previous studies have focused on the influence of frontal lesions and surgical treatments on patient cognitive function. Here, we review cognitive impairment caused by frontal lobe brain tumors.

Language function distribution in left-handers: A navigated transcranial magnetic stimulation study

Recent studies suggest that in left-handers, the right hemisphere (RH) is more involved in language function when compared to right-handed subjects. Since data on lesion-based approaches is lacking, we aimed to investigate language distribution of left-handers by repetitive navigated transcranial magnetic stimulation (rTMS). Thus, rTMS was applied to the left hemisphere (LH) and RH in 15 healthy left-handers during an object-naming task, and resulting naming errors were categorized. Then, we calculated error rates (ERs=number of errors per number of stimulations) for both hemispheres separately and defined a laterality score as the quotient of the LH ER - RH ER through the LH ER + RH ER (abbreviated as (L-R)/(L+R)). In this context, (L-R)/(L+R)>0 indicates that the LH is dominant, whereas (L-R)/(L+R)<0 shows that the RH is dominant. No significant difference in ERs was found between hemispheres (all errors: mean LH 18.0±11.7%, mean RH 18.1±12.2%, p=0.94; all errors without hesitation: mean LH 12.4±9.8%, mean RH 12.9±10.0%, p=0.65; no responses: mean LH 9.3±9.2%, mean RH 11.5±10.3%, p=0.84). However, a significant difference between the results of (L-R)/(L+R) of left-handers and right-handers (source data of another study) for all errors (mean 0.01±0.14 vs. 0.19±0.20, p=0.0019) and all errors without hesitation (mean -0.02±0.20 vs. 0.19±0.28, p=0.0051) was revealed, whereas the comparison for no responses did not show a significant difference (mean: -0.004±0.27 vs. 0.09±0.44, p=0.64). Accordingly, left-handers present a comparatively equal language distribution across both hemispheres with language dominance being nearly equally distributed between hemispheres in contrast to right-handers.

Modulation of the inter-hemispheric processing of semantic information during normal aging. A divided visual field experiment

We evaluated the effect of normal aging on the inter-hemispheric processing of semantic information by using the divided visual field (DVF) method, with words and pictures. Two main theoretical models have been considered, (a) the HAROLD model which posits that aging is associated with supplementary recruitment of the right hemisphere (RH) and decreased hemispheric specialization, and (b) the RH decline theory, which assumes that the RH becomes less efficient with aging, associated with increased LH specialization. Two groups of subjects were examined, a Young Group (YG) and an Old Group (OG), while participants performed a semantic categorization task (living vs. non-living) in words and pictures. The DVF was realized in two steps: (a) unilateral DVF presentation with stimuli presented separately in each visual field, left or right, allowing for their initial processing by only one hemisphere, right or left, respectively; (b) bilateral DVF presentation (BVF) with stimuli presented simultaneously in both visual fields, followed by their processing by both hemispheres. These two types of presentation permitted the evaluation of two main characteristics of the inter-hemispheric processing of information, the hemispheric specialization (HS) and the inter-hemispheric cooperation (IHC). Moreover, the BVF allowed determining the driver-hemisphere for processing information presented in BVF. Results obtained in OG indicated that: (a) semantic categorization was performed as accurately as YG, even if more slowly, (b) a non-semantic RH decline was observed, and (c) the LH controls the semantic processing during the BVF, suggesting an increased role of the LH in aging. However, despite the stronger involvement of the LH in OG, the RH is not completely devoid of semantic abilities. As discussed in the paper, neither the HAROLD nor the RH decline does fully explain this pattern of results. We rather suggest that the effect of aging on the hemispheric specialization and inter-hemispheric cooperation during semantic processing is explained not by only one model, but by an interaction between several complementary mechanisms and models.

Disrupting the right pars opercularis with electrical stimulation frees the song: case report

The authors report the first case of a strikingly unusual speech impairment evoked by intraoperative electrostimulation in a 36-year-old right-handed patient, a well-trained singer, who underwent awake surgery for a right fronto-temporo-insular low-grade glioma. Functionally disrupting the pars opercularis of the right inferior frontal gyrus led the patient to automatically switch from a speaking to a singing mode of language production. Given the central role of the right pars opercularis in the inhibitory control network, the authors propose that this finding may be interpreted as possible evidence for a competitive and independent neurocognitive subnetwork devoted to the melodically intoned articulation of words (normal language-based vs singing-based) in subjects with high expertise. From a more clinical perspective, such data may have implications for awake neurosurgery, especially to preserve the quality of life for singers.

Tractography of Association Fibers Associated with Language Processing

INTRODUCTION

Several major association fiber tracts are known to be part of the language processing system. There is evidence that high angular diffusion-based MRI is able to separate these fascicles in a constant way. In this study, we wanted to proof this thesis using a novel whole brain "global tracking" approach and to test for possible lateralization.

METHODS

Global tracking was performed in six healthy right-handed volunteers for the arcuate fascicle (AF), the medial longitudinal fascicle (MdLF), the inferior fronto-occipital fascicle (IFOF), and the inferior longitudinal fascicle (ILF). These fiber tracts were characterized quantitatively using the number of streamlines (SL) and the mean fractional anisotropy (FA).

RESULTS

We were able to characterize the AF, the MdLF, the IFOF, and the ILF consistently in six healthy volunteers using global tracking. A left-sided dominance (LI > 0.2) for the AF was found in all participants. The MdLF showed a left-sided dominance in four participants (one female, three male). Regarding the FA, no lateralization (LI > 0.2) could be shown in any of the fascicles.

CONCLUSION

Using a novel global tracking algorithm we confirmed that the courses of the primary language processing associated fascicles can consistently be differentiated. Additionally we were able to show a streamline-based left-sided lateralization in the AF of all right-handed healthy subjects.

Current and potential utility of transcranial magnetic stimulation in the diagnostics before brain tumor surgery

This article describes the evolution and state-of-the-art of navigated transcranial magnetic stimulation for evaluation of patients with brain tumors in presumed eloquent location. Alternative noninvasive technologies for functional brain mapping are described and assessed in the context of their usability and clinical needs. In addition to the description of the current validation level and clinical application of navigated transcranial magnetic stimulation for motor and language mapping, the manuscript highlights ongoing research efforts and provides an outlook on upcoming developments in the field of noninvasive brain mapping. Finally, the clinical rationale for presurgical noninvasive brain mapping is discussed in the light of current developments in neurosurgery.

The semantic anatomical network: Evidence from healthy and brain-damaged patient populations

Semantic processing is central to cognition and is supported by widely distributed gray matter (GM) regions and white matter (WM) tracts. The exact manner in which GM regions are anatomically connected to process semantics remains unknown. We mapped the semantic anatomical network (connectome) by conducting diffusion imaging tractography in 48 healthy participants across 90 GM "nodes," and correlating the integrity of each obtained WM edge and semantic performance across 80 brain-damaged patients. Fifty-three WM edges were obtained whose lower integrity associated with semantic deficits and together with their linked GM nodes constitute a semantic WM network. Graph analyses of this network revealed three structurally segregated modules that point to distinct semantic processing components and identified network hubs and connectors that are central in the communication across the subnetworks. Together, our results provide an anatomical framework of human semantic network, advancing the understanding of the structural substrates supporting semantic processing.

Subcortical language and non-language mapping in awake brain surgery: the use of multimodal tests

BACKGROUND

Awake craniotomy is currently considered the gold standard to maximise the extent of resection and to minimise postoperative deficits in patients with supratentorial tumours near eloquent areas. In addition to direct electrical stimulation (DES) of the cortex, intraoperative subcortical mapping is increasingly used as it optimises the benefit-to-risk ratio by decreasing (permanent) postoperative neurological deficits. However, only little attention has been paid to subcortical mapping procedures and especially the tasks to be used.

METHODS

In this article, language and non-language testing at the subcortical level is described and discussed by means of three right-handed cases with a glioma in the left hemisphere. To assess subcortical functions, a multimodal test named the Quick Mixed Test was developed (QMT). Pre-, intra- and postoperative test results are described and discussed in detail.

RESULTS

Based on the analysis of these preliminary observations, a number of clinical recommendations for intraoperative subcortical mapping may be made: (1) the selection of a set of language and non-language tests needs to be tailored according to the functional corticosubcortical regions affected by the tumoral lesion and the patient's characteristics (job/hobby/daily life activities); (2) language and non-language tests should be presented in a multimodal and alternating way during subcortical stimulation since this approach enables screening various functions simultaneously or in a very short period of time and (3) spontaneous speech is a useful adjunct to standardised tests since it most resembles daily life conversation.

CONCLUSION

Administration of multimodal tests during subcortical DES such as the experimental QMT may facilitate identification of eloquent pathways leading to avoidance of permanent neurological impairments.

Contemporary model of language organization: an overview for neurosurgeons

Classic models of language organization posited that separate motor and sensory language foci existed in the inferior frontal gyrus (Broca's area) and superior temporal gyrus (Wernicke's area), respectively, and that connections between these sites (arcuate fasciculus) allowed for auditory-motor interaction. These theories have predominated for more than a century, but advances in neuroimaging and stimulation mapping have provided a more detailed description of the functional neuroanatomy of language. New insights have shaped modern network-based models of speech processing composed of parallel and interconnected streams involving both cortical and subcortical areas. Recent models emphasize processing in "dorsal" and "ventral" pathways, mediating phonological and semantic processing, respectively. Phonological processing occurs along a dorsal pathway, from the posterosuperior temporal to the inferior frontal cortices. On the other hand, semantic information is carried in a ventral pathway that runs from the temporal pole to the basal occipitotemporal cortex, with anterior connections. Functional MRI has poor positive predictive value in determining critical language sites and should only be used as an adjunct for preoperative planning. Cortical and subcortical mapping should be used to define functional resection boundaries in eloquent areas and remains the clinical gold standard. In tracing the historical advancements in our understanding of speech processing, the authors hope to not only provide practicing neurosurgeons with additional information that will aid in surgical planning and prevent postoperative morbidity, but also underscore the fact that neurosurgeons are in a unique position to further advance our understanding of the anatomy and functional organization of language.

Differences between patterns of brain activity associated with semantics and those linked with phonological processing diminish with age

It is widely believed that language function tends to show little age-related performance decline. Indeed, some older individuals seem to use compensatory mechanisms to maintain a high level of performance when submitted to lexical tasks. However, how these mechanisms affect cortical and subcortical activity during semantic and phonological processing has not been extensively explored. The purpose of this study was to look at the effect of healthy aging on cortico-subcortical routes related to semantic and phonological processing using a lexical analogue of the Wisconsin Cart-Sorting Task. Our results indicate that while young adults tend to show increased activity in the ventrolateral prefrontal cortex, the dorsolateral prefrontal cortex, the fusiform gyrus, the ventral temporal lobe and the caudate nucleus during semantic decisions and in the posterior Broca's area (area 44), the temporal lobe (area 37), the temporoparietal junction (area 40) and the motor cortical regions during phonological decisions, older individuals showed increased activity in the dorsolateral prefrontal cortex and motor cortical regions during both semantic and phonological decisions. Furthermore, when semantic and phonological decisions were contrasted with each other, younger individuals showed significant brain activity differences in several regions while older individuals did not. Therefore, in older individuals, the semantic and phonological routes seem to merge into a single pathway. These findings represent most probably neural reserve/compensation mechanisms, characterized by a decrease in specificity, on which the elderly rely to maintain an adequate level of performance.

A re-examination of neural basis of language processing: proposal of a dynamic hodotopical model from data provided by brain stimulation mapping during picture naming

From recent findings provided by brain stimulation mapping during picture naming, we re-examine the neural basis of language. We studied structural-functional relationships by correlating the types of language disturbances generated by stimulation in awake patients, mimicking a transient virtual lesion both at cortical and subcortical levels (white matter and deep grey nuclei), with the anatomical location of the stimulation probe. We propose a hodotopical (delocalized) and dynamic model of language processing, which challenges the traditional modular and serial view. According to this model, following the visual input, the language network is organized in parallel, segregated (even if interconnected) large-scale cortico-subcortical sub-networks underlying semantic, phonological and syntactic processing. Our model offers several advantages (i) it explains double dissociations during stimulation (comprehension versus naming disorders, semantic versus phonemic paraphasias, syntactic versus naming disturbances, plurimodal judgment versus naming disorders); (ii) it takes into account the cortical and subcortical anatomic constraints; (iii) it explains the possible recovery of aphasia following a lesion within the "classical" language areas; (iv) it establishes links with a model executive functions.

The persistent crucial role of the left hemisphere for language in left-handers with a left low grade glioma: a stimulation mapping study

BACKGROUND

Left-handers have a more bilateral language representation than right-handers. Therefore, in left-handers with a low-grade glioma (LGG) in the left hemisphere (LH), one could hypothesize that the right hemisphere (RH) might allow language compensation, at least partly, with no or only a minor persistent role of the LH in speech. However, although LGG induces language reorganization in right-handed patients, little is known in left-handers. Here, we report the first series of left-handers who underwent awake surgery for a left LGG using intraoperative mapping, in order to investigate whether there was still an involvement of LH in language.

METHOD

Ten consecutive left-handed patients were operated for a left LGG (three frontal, four paralimbic, one parietal, one temporal, one parieto-temporal tumor) using an awake procedure with intraoperative electrical language mapping.

RESULTS

Intraoperative language disorders were elicited in all cases but one by electrostimulation in the LH. Cortical language sites were detected in nine cases. Subcortical stimulation also demonstrated the crucial role of left white matter pathways in language, including the inferior occipital-frontal fascicle, arcuate fascicle, lateral segment of the superior longitudinal fascicle and fibers from the ventral premotor cortex. Moreover, stimulation of deep gray nuclei generated language disturbances in four patients. These nine patients experienced transient postoperative language worsening, supporting the persistent critical role of LH in speech. In only one patient, no language deficit was evoked intraoperatively and postoperatively. The ten patients returned to a normal life. Total or subtotal resection was achieved in all cases but one.

CONCLUSIONS

Our results suggest that, even though the RH may participate in language compensation, the LH in left-handers still plays a crucial role, despite a left slow-growing LGG. Thus, we propose to routinely consider awake surgery for left LGG removal in left-handers patients, to optimize the extent of resection while preserving language.

Selection of intraoperative tasks for awake mapping based on relationships between tumor location and functional networks

Intraoperative electrical brain mapping is currently the most reliable method to identify eloquent cortical and subcortical structures at the individual level and to optimize the extent of resection of intrinsic brain tumors. The technique allows the preservation of quality of life, not only allowing avoidance of severe neurological deficits but also facilitating preservation of high neurocognitive functions. To accomplish this goal, however, it is crucial to optimize the selection of appropriate intraoperative tasks, given the limited intrasurgical awake time frame. In this review, the authors' aim was to propose specific parameters that could be used to build a personalized protocol for each patient. They have focused on lesion location and relationships with functional networks to guide selection of intrasurgical tasks in an effort to increase reproducibility among neurooncological centers.

Mapping the connectivity underlying multimodal (verbal and non-verbal) semantic processing: a brain electrostimulation study

Accessing the meaning of words, objects, people and facts is a human ability, made possible thanks to semantic processing. Although studies concerning its cortical organization are proficient, the subcortical connectivity underlying this semantic network received less attention. We used intraoperative direct electrostimulation, which mimics a transient virtual lesion during brain surgery for glioma in eight awaken patients, to map the anatomical white matter substrate subserving the semantic system. Patients performed a picture naming task and a non-verbal semantic association test during the electrical mapping. Direct electrostimulation of the inferior fronto-occipital fascicle, a poorly known ventral association pathway which runs throughout the brain, induced in all cases semantic disturbances. These transient disorders were highly reproducible, and concerned verbal as well as non-verbal output. Our results highlight for the first time the essential role of the left inferior fronto-occipital fascicle in multimodal (and not only in verbal) semantic processing. On the basis of these original findings, and in the lights of phylogenetic considerations regarding this fascicle, we suggest its possible implication in the monitoring of the human level of consciousness related to semantic memory, namely noetic consciousness.

A new lexical card-sorting task for studying fronto-striatal contribution to processing language rules

The role of fronto-striatal regions in processing different language rules such as semantic and (grapho) phonological ones is still under debate. We have recently developed a lexical analog of the Wisconsin card sorting task which measures set-shifting abilities where the visual rules color, number, shape were replaced by three language ones: semantic, rhyme and syllable onset (attack). In the present study we aimed to compare fronto-striatal activations between the different lexical rules that are required for matching the test words to the response ones. Using functional magnetic resonance imaging (fMRI), fourteen healthy, native French-speaking participants were scanned. The results showed that some regions within the brain language network are differentially involved in semantic and phonological processes. Semantic decisions activated significantly the ventrolateral prefrontal cortex, the dorsolateral prefrontal cortex, the fusiform gyrus, the ventral temporal lobe and the caudate nucleus, while phonological decisions produced significant activation in posterior Broca's area (area 44), the temporoparietal junction and motor cortical regions. These findings provide critical support for the existence of a ventral subcortical semantic pathway and a more dorsal phonological stream as proposed by Duffau, Leroy, and Gatignol (2008). Furthermore, we propose that the strong involvement of area 47/12 of the ventrolateral prefrontal cortex and caudate nucleus observed in semantic processing, is not specific to language, but to the fact that a category or a rule has to be retrieved amongst competing ones in memory, similarly to what is observed when planning a set-shift in the original (non-lexical) version of the Wisconsin card sorting task.

Asymmetries of the arcuate fasciculus in monozygotic twins: genetic and nongenetic influences

We assessed cerebral asymmetry for language in 35 monozygotic twin pairs. Using DTI, we reconstructed the arcuate fasciculus in each twin. Among the male twins, right-handed pairs showed greater left-sided asymmetry of connectivity in the arcuate fasciculus than did those with discordant handedness, and within the discordant group the right-handers had greater left-sided volume asymmetry of the arcuate fasciculus than did their left-handed co-twins. There were no such effects in the female twins. Cerebral asymmetry for language showed more consistent results, with the more left-cerebrally dominant twins also showing more leftward asymmetry of high anisotropic fibers in the arcuate fasciculus, a result applying equally to female as to male twins. Reversals of arcuate fasciculus asymmetry were restricted to pairs discordant for language dominance, with the left-cerebrally dominant twins showing leftward and the right-cerebrally dominant twins rightward asymmetry of anisotropic diffusion in the arcuate fasciculus. Because monozygotic twin pairs share the same genotype, our results indicate a strong nongenetic component in arcuate fasciculus asymmetry, particularly in those discordant for cerebral asymmetry.