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This chapter presents a summary of current notions regarding cortical specialization for language and a description of the methods employed for the assessment of that specialization. We distinguish between the "canonical" model of language specialization as it evolved from the early observations of Broca and Wernicke, implicating the inferior frontal gyrus and the posterior temporal cortex of the speech dominant hemisphere (usually the left) and its modern variants that are based on both detailed studies of lesion-symptom correlations and on the results of functional brain mapping methods. The latter fall into two categories. The first includes the invasive ones, namely the Wada procedure for assessing hemispheric dominance for speech and cortical stimulation mapping (whether intraoperative or extraoperative) for identifying cortical nodes or "hubs" of the neuronal network for language. The second category includes the noninvasive methods of functional magnetic resonance imaging, magnetoencephalography, and transcranial magnetic stimulation used for both assessment of hemispheric dominance for language and for localization of the cortical nodes of the language network. The advantages and the shortcomings of all methods are juxtaposed to facilitate selection of particular methods of assessment of the locus of the language network in particular cases., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Objective: The aim of the present study was to compare localization of the language cortex using cortical stimulation mapping (CSM), high gamma electrocorticography (hgECoG), functional magnetic resonance imaging (fMRI), and transcranial magnetic stimulation (TMS)., Methods: Language mapping using CSM, hgECoG, fMRI, and TMS were compared in nine patients with epilepsy. Considering CSM as reference, we compared language mapping approaches based on hgECoG, fMRI, and TMS using their sensitivity, specificity, and the results of receiver operating characteristic (ROC) analyses., Results: Our results show that areas involved in language processing can be identified by hgECoG, fMRI, and TMS. The average sensitivity/specificity of hgECoG, fMRI, and TMS across all patients was 100%/85%, 50%/80%, and 67%/66%, respectively. The average area under the ROC curve of hgECoG, fMRI, and TMS across CSM-positive patients was 0.98, 0.76, and 0.68, respectively., Conclusions: There is considerable concordance between CSM, hgECoG, fMRI, and TMS language mapping. Our results reveal that hgECoG, fMRI, and TMS are valuable tools for presurgical language mapping., Significance: Language mapping on the basis of hgECoG, fMRI, and TMS can provide important additional information, therefore, these methods can be used in conjunction with CSM or as an alternative, when the latter is deemed impractical., (Copyright © 2015 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.)

The efficacy of magnetoencephalography (MEG) as an alternative to invasive methods for investigating the cortical representation of language has been explored in several studies. Recently, studies comparing MEG to the gold standard Wada procedure have found inconsistent and often less-than accurate estimates of laterality across various MEG studies. Here we attempted to address this issue among normal right-handed adults (N=12) by supplementing a well-established MEG protocol involving word recognition and the single dipole method with a sentence comprehension task and a beamformer approach localizing neural oscillations. Beamformer analysis of word recognition and sentence comprehension tasks revealed a desynchronization in the 10-18Hz range, localized to the temporo-parietal cortices. Inspection of individual profiles of localized desynchronization (10-18Hz) revealed left hemispheric dominance in 91.7% and 83.3% of individuals during the word recognition and sentence comprehension tasks, respectively. In contrast, single dipole analysis yielded lower estimates, such that activity in temporal language regions was left-lateralized in 66.7% and 58.3% of individuals during word recognition and sentence comprehension, respectively. The results obtained from the word recognition task and localization of oscillatory activity using a beamformer appear to be in line with general estimates of left hemispheric dominance for language in normal right-handed individuals. Furthermore, the current findings support the growing notion that changes in neural oscillations underlie critical components of linguistic processing., (Published by Elsevier B.V.)

Although individuals born preterm are at risk for cognitive dysfunction, few studies have examined functional brain reorganization in these individuals. We used magnetoencephalography (MEG) to examine cortical reorganization related to preterm birth. Thirty-one adolescents systemically selected from a longitudinal study on child development based on gestational age, birth weight and medical complications (full term, low risk preterm, high-risk preterm) performed real-word and non-word auditory rhyme tasks during MEG recording. Equivalent current dipoles were localized every 4ms during the 50- to 250-ms period following the onset of the auditory stimulus. The association between the number of dipoles (NODs) and birth group, language task, latency and phonological skills was examined. Adolescents born preterm at high risk demonstrated a greater NODs in the left Broca's and prefrontal areas combined, left cingulate gyrus and left superior temporal gyrus and a fewer NODs in the right superior temporal gyrus as compared to those born preterm at low risk and term. A greater NODs in the left Broca's and prefrontal areas combined and fewer NODs in the left cingulate gyrus was associated with better phonological skills only in adolescents born preterm at high risk. These results suggest that the language networks are reorganized in adolescents born preterm at high risk. Increased prefrontal activity has also been seen in adolescents born preterm during a reading task and in young adults with a history of dyslexia who are well compensated for their disorder. We suggest that this increased prefrontal activation may represent increased top-down control of weak posterior language networks., (Copyright 2010 Elsevier B.V. All rights reserved.)

We compared functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) for the mapping of receptive language function. Participants performed the same language task in the two different imaging environments. MEG activation profiles showed prominent bilateral activity in superior temporal gyrus and left-lateralized activity in middle temporal gyrus. fMRI activation profiles revealed bilateral activity in prefrontal, superior temporal, middle temporal, and visual areas. Laterality quotients derived from the two modalities showed poor agreement between the two methods for commonly active regions of interest. Locations of peak activity also varied considerably within participants between the two methods.

Understanding the areas involved in language functions not only enables investigators to understand neuroanatomical structures, but may be a promising technique in the presurgical evaluation of epilepsy. The predictive power of various data reduction techniques was tested on language data obtained by magnetoencephalography (MEG) of 16 patients and 12 control subjects. Words were presented aurally in two phases: the study phase and the recognition phase. Subjects were asked to remember words from the study phase and indicate if they remembered those words during the recognition phase. Single equivalent-current dipoles were calculated to determine laterality indices and the neuroanatomical correlates of language function. For all patients, results indicated a concordance, sensitivity, and specificity of 0.75. After consideration of IQ scores and exclusion from the analysis of those patients with scores below the average range, the results indicated a concordance of 0.90, sensitivity of 0.86, and specificity of 1.00. These findings are consistent with previous MEG investigations of language function in comparison with the Wada technique and support the use of MEG language mapping in most patients with an IQ within or above the average range.

We review our experience with the application of magnetoencephalography (MEG) to the study of reorganization of the mechanisms supporting auditory language comprehension. In 3 studies, patient populations with cerebral insult of differing etiology, including epilepsy, surgical resection, and stroke, performed a running recognition task for spoken words while MEG data were collected using a whole-head magnetometer. Increased activation in the right hemisphere after left temporal lobectomy was associated with greater relative activation in that hemisphere preoperatively. Patients with chronic seizure disorder secondary to mesial temporal sclerosis exhibited a tendency toward an interhemispheric shift of language function, and those with epilepsy secondary to neoplasm showed a tendency toward an intrahemispheric shift. Patients with aphasia secondary to unilateral left-hemisphere stroke exhibited a more bilateral and diffuse overall profile of activation within the left hemisphere than control subjects of similar age. Taken together, results provide evidence that reorganization of cortex subserving auditory comprehension can occur well into the fifth and sixth decades and that the nature of the plastic response is dependent on variables such as premorbid language laterality, etiology, and, in specific groups, age at insult.

Purpose: To characterize the relation between hemispheric asymmetries in language-specific brain activity and reading/spelling achievement by using magnetoencephalography (MEG)., Methods: Patients (n = 83) with medically intractable complex partial seizures of either left- or right-hemisphere origin were classified as having reading and/or spelling deficits (RS) or as not impaired (NI) by using standard achievement tests. All patients had undergone noninvasive functional mapping of receptive language cortex by using MEG as part of a preoperative seizure surgery evaluation., Results: RS patients with left-hemisphere seizure onset exhibited relatively greater activation and earlier onset of late, language-specific MEG activity in posterior temporal and inferior parietal areas of the right as compared with the left hemisphere than did NI patients. These findings also were evident on an individual basis and were independent of global intellectual abilities., Conclusions: Reading and spelling achievement deficits in patients with complex partial seizures of left-hemisphere origin are associated with atypical language organization, possibly secondary to reorganization of language function to right-hemisphere areas that are not as efficient as homotopic areas in the left hemisphere in supporting reading and spelling functions.

Shift of the cortical mechanisms of language from the usually dominant left to the non-dominant right hemisphere has been demonstrated in the presence of large brain lesions. Here, we report a similar phenomenon in a patient with a cavernoma over the anterolateral superior temporal gyrus associated with epilepsy. Language mapping was performed by two complementary procedures, magnetoencephalography, and electrocorticography. The maps, indicated right temporal lobe dominance for receptive language and left frontal lobe dominance for expressive language. These results indicate that a small lesion, associated with epilepsy, may produce selective shifting of receptive language mechanisms as large lesions have been known to produce.

Advances in neuroimaging technologies over the last 15 years have prompted their relatively widespread use in the study of brain mechanisms supporting language function in children and adults. We reviewed reliability and external validity studies of 3 of the most common functional imaging methods, functional magnetic resonance imaging (fMRI), magnetoencephalography (MEG), and positron emission tomography (PET). Although reliability and validity reports for fMRI are generally quite favorable, significant variability was found across studies with respect to methodology, preventing in some cases either the assessment of the reliability of individual datasets, or cross-study comparisons. Reliability and validity reports of MEG are strong, yet methodological questions regarding optimal modeling techniques remain. PET investigators report good concordance of language maps with data from more invasive brain mapping techniques, but its use of radioactive tracers and poorer spatial and temporal resolution make it the least optimal of the 3 methods for language mapping. Investigations of the cortical networks supporting language function during development and into adulthood should be viewed in the context of the validity and reliability of the methods used, with careful attention to details regarding the methodologies employed in the acquisition and analysis of statistical maps.

This review consists of three main sections. In the first, the Introduction, the main theories of the neuronal mediation of linguistic operations, derived mostly from studies of the effects of focal lesions on linguistic performance, are summarized. These models furnish the conceptual framework on which the design of subsequent functional neuroimaging investigations is based. In the second section, the methods of functional neuroimaging, especially those of functional Magnetic Resonance Imaging (fMRI) and of Magnetoencephalography (MEG), are detailed along with the specific activation tasks employed in presurgical functional mapping. The reliability of these non-invasive methods and their validity, judged against the results of the invasive methods, namely, the "Wada" procedure and Cortical Stimulation Mapping (CSM), is assessed and their use in presurgical mapping is justified. In the third and final section, the applications of fMRI and MEG in basic research are surveyed in the following six sub-sections, each dealing with the assessment of the neuronal networks for (1) the acoustic and phonological, (2) for semantic, (3) for syntactic, (4) for prosodic operations, (5) for sign language and (6) for the operations of reading and the mechanisms of dyslexia. [ABSTRACT FROM AUTHOR]

Abstract: Magnetoencephalography (MEG) is a novel functional brain mapping technique capable of non-invasively measuring neurophysiological activity based on direct measures of the magnetic flux at the head surface associated with the synchronized electrical activity of neuronal populations. Among the most actively sought applications of MEG has been localization of language-specific cortex. This is in part due to its practical application for pre-surgical evaluation of patients with epilepsy or brain tumors. Until recently, comprehensive language mapping during surgical planning has relied on the application of invasive diagnostic methods, namely the Wada procedure and direct electrocortical stimulation mapping, often considered as the “gold standard” techniques for identifying language-specific cortex. In this review, we evaluate the utility of MEG as a tool for functional mapping of language in both clinical and normal populations. In particular, we provide a general description of MEG, with emphasis on facets of the technique related to language mapping. Additionally, we discuss the application of appropriate MEG language-mapping protocols developed to reliably generate spatiotemporal profiles of language activity, and address the validity of the technique against the “gold standards” of the Wada and electrocortical mapping procedures. [Copyright &y& Elsevier]

The type of syntactic operations that increase neuronal activation in humans as a result of syntactically erroneous, unexpected lexical items in hearing sentences has remained unclear. In the present study, we used recordings of magnetoencephalographic (MEG) activity to compare bare infinitive and full infinitive constructions in English. This research aims to identify the type of syntactic deviance that may trigger an early syntax-related mismatch field (MMF) component when unexpected words appear in sentences. Six speakers of English as a first language were presented with auditory stimuli of sentences or words in a passive odd-ball paradigm while watching a silent movie. The experimental protocol included four sessions, specifically investigating the sentential (structural) versions of full (with the ‘ to ’ infinitival particle) and bare infinitival structures (without the particle) and the lexical (non-structure) versions of the verb either with or without the particle to determine whether the structure processing of sentences was a more crucial factor in the detection of the MMF than the simple processing of lexical items in verb-only conditions. The amplitude analysis of the resulting evoked fields showed that the presence of the syntactic category error of bare infinitival structures against syntactic predictions evoked a significantly larger MMF activation with a peak latency of approximately 200 ms in the anterior superior temporal sulci in the left hemisphere, compared with the lexical items that did not have any syntactic status. These results clearly demonstrate that syntactically unexpected, illegal input in the bare infinitival structure is likely to be noticed more robustly in the brain while processing the structural information of the entire sentence than the corresponding verb-only items. [ABSTRACT FROM AUTHOR]

Abstract: Learning associations between people''s faces and names is a universal cognitive function with important social implications. The goal of the present study was to examine brain activity patterns associated with cross-modal encoding of names and faces. Learning face-name pairs was compared to unimodal learning tasks using the same visual and auditory stimuli. Spatiotemporal brain activation profiles were obtained with magnetoencephalography using an automated source estimation method. Results showed activation foci in left (for names) and right (for faces) temporal lobe perisylvian cortices, predominantly right-hemisphere occipital and occipitotemporal regions (for faces), and right hemisphere dorsolateral prefrontal regions during the encoding phase for both types of stimuli presented in isolation. Paired (face-name) stimulus presentation elicited bilateral prefrontal and temporal lobe perisylvian activity for faces and enhanced visual cortex activation in response to names (compared to names in the unpaired condition). These findings indicate distinct patterns of brain activation during the formation of associations between meaningful visual and auditory stimuli. [Copyright &y& Elsevier]

Abstract: Objective: The purpose of this study was to evaluate the reproducibility of estimates of neurophysiological activity obtained with Magnetic Source Imaging. Methods: Split-half data sets were obtained from 14 healthy volunteers during performance of a continuous recognition task for spoken words. The concurrent validity of spatiotemporal activation maps obtained with this task has been previously verified through comparisons with the Wada test and electrocortical stimulation mapping. Consecutive late activity sources (>200ms after stimulus onset) were modeled independently as equivalent current dipoles (ECDs) and used to identify the location of language-specific cortex in the left hemisphere (Wernicke''s area). Results: Linear displacement of the geometric center of the cluster of ECDs in this region ranged between 2 and 8mm across subjects. Intraparticipant variability (range) in the onset latency of activity was ±50ms, while the range of change in global field power for the entire set of ECDs in Wernicke''s area was less than 17% in all cases. Conclusions: The results indicate that despite its many conceptual limitations the ECD model can provide reliable estimates of regional cortical activity associated with the engagement of linguistic processes. Significance: The results highlight the need for reproducibility studies when research questions pose particular requirements for precision of estimates of regional neurophysiological activity. [Copyright &y& Elsevier]