Rebecca Billingsley-Marshall

Reorganization of language-specific cortex in patients with lesions or mesial temporal epilepsy

Objective: To examine brain activation profiles for receptive language function, using magnetoencephalography (MEG), in patients with left hemisphere space-occupying lesions and patients with left temporal lobe epilepsy due to mesial temporal sclerosis (MTS) and to evaluate whether cross- and intrahemispheric plasticity for language varied as a function of lesion type or location. Methods: Twenty-one patients with MTS and 23 lesional patients underwent preoperative language mapping while performing a word recognition task. The anatomic location of late activity sources was determined by co-registering MEG coordinates onto structural MRI scans. A language laterality index was calculated based on the number of activity sources in each hemisphere. The location of language-specific activity was examined in relation to its proximity or overlap with Wernicke’s area. Results: A higher incidence of atypical language lateralization was noted among patients with MTS than lesional patients (43 vs 13%). The majority of MTS patients with early seizure onset (before age 5) showed atypical language lateralization. In contrast, the precise location of receptive language-specific cortex within the dominant hemisphere was found to be atypical (outside of Wernicke’s area) in 30% of lesional patients and only 14% of MTS patients. Conclusions: There is an increased probability of a partial or total displacement of key components of the brain mechanism responsible for receptive language function to the nondominant hemisphere in mesial temporal sclerosis patients. Early onset of seizures is strongly associated with atypical language lateralization. Lesions in the dominant hemisphere tend to result in an intrahemispheric reorganization of linguistic function.

Toward the substitution of invasive electroencephalography in epilepsy surgery.

The authors compared the localization accuracy of interictal magnetoencephalography (MEG) with ictal and interictal invasive video electroencephalography (VEEG) in identifying the epileptogenic zone in epilepsy surgery candidates. Forty-one patients, 29 with temporal lobe epilepsy (TLE) and 12 with extratemporal lobe epilepsy (ETLE), participated. Only patients with interictal changes during the MEG recordings were included. A comparison of the accuracy of invasive VEEG and MEG seizure zone identification was based on the degree of overlap between the location of the actual surgical resection and the zone identified by each method, and the success of surgery in reducing seizure activity. No statistical differences were observed between the accuracy of invasive VEEG and MEG in determining the location of the seizure zone across TLE and ETLE cases. Invasive VEEG and MEG localization judgments were correct in 54% and 56% of the cases, respectively. Separate group analyses suggested that MEG may be less beneficial relative to invasive VEEG in ETLE than TLE cases. MEG is of statistically equivalent accuracy to invasive VEEG, despite the fact that its use has not reached optimal conditions. The authors predict the replacement of the more invasive procedure with MEG in the near future for TLE cases, subsequent to the optimization of the conditions under which preoperative MEG is performed.

Functional neuroimaging with MEG: Normative language profiles

The reliability of language-specific brain activation profiles was assessed using Magnetoencephalography (MEG) in five experiments involving ninety-seven normal volunteers of both genders ranging in age from seven to eighty-four years. MEG data were analyzed with a fully automated method to eliminate subjective judgments in the process of deriving the activation profiles. Across all experiments, profiles were characterized by significant bilateral activity centered in the superior temporal gyrus, and in activity lateralized to the left middle temporal gyrus. These features were invariant across age, gender, variation in task characteristics, and mode of stimulus presentation. The absolute amount of activation, however, did decline with age in the auditory tasks. Moreover, contrary to the commonly held belief that left hemisphere dominance for language is greater in men than in women, our data revealed an opposite albeit a not consistently significant trend.

Intensive Instruction Affects Brain Magnetic Activity Associated with Oral Word Reading in Children with Persistent Reading Disabilities

Fifteen children ages 7 to 9 years who had persistent reading difficulties despite adequate instruction were provided with intensive tutorial interventions. The interventions targeted deficient phonological processing and decoding skills for 8 weeks (2 hours per day) followed by an 8-week, 1-hour-per-day intervention that focused on the development of reading fluency skills. Spatiotemporal brain activation profiles were obtained at baseline and after each 8-week intervention program using magnetoencephalography during the performance of an oral sight-word reading task. Changes in brain activity were found in the posterior part of the middle temporal gyrus (Brodmanns Area [BA] 21: increased degree of activity and reduced onset latency), the lateral occipitotemporal region (BA 19/37: decreased onset latency of activation), and the premotor cortex (increased onset latency). Overall changes associated with the intervention were primarily normalizing, as indicated by (a) increased activity in a region that is typically involved in lexical—semantic processing (BA 21) and (b) a shift in the relative timing of regional activity in temporal and frontal cortices to a pattern typically seen in unimpaired readers. These findings extend previous results in demonstrating significant changes in the spatiotemporal profile of activation associated with word reading in response to reading remediation.

Organization of receptive language-specific cortex before and after left temporal lobectomy

Objective: To examine brain activation associated with receptive language in patients with left temporal lobe epilepsy (TLE) before and after an anterior temporal lobectomy using magnetoencephalography (MEG), and to evaluate which patients were most likely to show a change in the lateralization and localization of the mechanisms supporting receptive language and if such changes were associated with neuropsychological function. Methods: Twelve patients with left TLE underwent preoperative Wada testing, and pre- and postoperative neuropsychological testing and MEG language mapping. The anatomic location of receptive language-related activity sources observed with MEG was determined by coregistering MEG data with structural MRI scans. Language laterality indices were calculated based on the number of reproducible activity sources in each hemisphere. The proximity of language-specific activity sources to Wernickes area was also examined. Results: Although the small sample size precluded formal statistical analyses, patients with atypical (bilateral) hemispheric dominance preoperatively were more likely than patients with typical (left-hemisphere) dominance to show evidence of increased right hemisphere participation in language functions after surgery. Patients with left hemispheric dominance preoperatively were more likely to show intrahemispheric changes involving a slight inferior shift of the putative location of Wernickes area. Patients with bilateral representation tended to perform worse on neuropsychological test measures obtained both pre- and postoperatively. Conclusion: Interhemispheric functional reorganization of language-specific areas may occur in patients undergoing left anterior temporal lobectomy. Intrahemispheric reorganization may take place even when the resection does not directly impinge upon Wernickes area.

A comparison of functional MRI and magnetoencephalography for receptive language mapping

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.

Atypical Language Representation in Patients with Chronic Seizure Disorder and Achievement Deficits with Magnetoencephalography

Summary:  Purpose: To characterize the relation between hemispheric asymmetries in language‐specific brain activity and reading/spelling achievement by using magnetoencephalography (MEG).

Reliability and validity of functional neuroimaging techniques for identifying language-critical areas in children and adults.

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.

Magnetic Source Imaging Studies of Dyslexia Interventions

Rapidly accumulating evidence from functional brain imaging studies indicates that developmental reading disability is associated with a functional disruption of the brain circuits that normally develop to support reading-related processes. This article briefly overviews recent advances in methods that capture the anatomical outline and temporal (dynamic) features of regional brain activation during performance of reading tasks. One of these methods, magnetoencephalography (MEG) or magnetic sources imaging (MSI) is described in more detail in the context of investigations of changes in spatiotemporal patterns of brain activity associated with improvement in reading skills in response to various types of educational interventions.

A review of clinical applications of magnetoencephalography.

Publisher Summary Magnetoencephalography (MEG) is a non-invasive brain imaging method that allows for the real-time investigation of cortical activity. The millisecond temporal resolution of MEG, combined with its excellent spatial resolution, distinguish it from functional brain imaging techniques that rely on hemodynamic measures, such as positron emission tomography (PET) and functional magnetic resonance imaging (f MRI). The temporal and spatial resolution of MEG, as well as its demonstrated reliability and external validity, have led to its routine use in many neurosurgery centers around the world. This chapter discusses the primary clinical applications of MEG. These applications, namely the identification of epileptogenic foci in candidates for epilepsy surgery and the mapping of function specific cortex prior to resection, are presented following a brief description of MEG methodology. MEG involves the measurement of neuromagnetic signals emanating from the brain. Magnetic activity measured outside the head is produced primarily by intracellular electrical currents within the dendrites of pyramidal cells in neo- and archeo-cortical brain structures. Unlike the secondary volume currents detected outside the head with electroencephalography (EEG) (and event-related potentials), the magnetic flux detected at the surface of the head with MEG penetrates the skull and tissues without significant distortion.