Aberrant spatiotemporal activation profiles associated with math difficulties in children: a magnetic source imaging study.

The study investigates the relative degree and timing of cortical activation in parietal, temporal, and frontal regions during simple arithmetic tasks in children who experience math difficulties. Real-time brain activity was measured with magnetoencephalography during simple addition and numerosity judgments in students with math difficulties and average or above average reading skills (MD group, N = 14), students with below average scores on both math and basic reading tests (MD/RD group, N = 16) and students with above average scores on standardized math tests (control group, N = 25). Children with MD showed increased degree of neurophysiological activity in inferior and superior parietal regions in the right hemisphere compared to both controls and MD/RD students. Left hemisphere inferior parietal regions did not show the expected task-related changes and showed activity at a significant temporal delay. MD students also showed increased early engagement of prefrontal cortices. Taken together, these findings may indicate increased reliance on a network of right hemisphere parietal (and possibly frontal areas as well) for simple math calculations in students who experience math difficulties but perform within normal range in reading.

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.

Altering the brain circuits for reading through intervention: a magnetic source imaging study.

Intervention-related changes in spatiotemporal profiles of regional brain activation were examined by whole-head magnetoencephalography in 15 children with severe reading difficulties who had failed to show adequate progress to quality reading instruction during Grade 1. Intensive intervention initially focused on phonological decoding skills (for 8 weeks) and, during the subsequent 8 weeks, on rapid word recognition ability. Clinically significant improvement in reading skills was noted in 8 children who showed "normalizing" changes in their spatiotemporal profiles of regional brain activity (increased duration of activity in the left temporoparietal region and a shift in the relative timing of activity in temporoparietal and inferior frontal regions). Seven children who demonstrated "compensatory" changes in brain activity (increased duration of activity in the right temporoparietal region and frontal areas, bilaterally) did not show adequate response to intervention. Nonimpaired readers did not show systematic changes in brain activity across visits.

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 (Brodmann's 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.

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.

Reproducibility of measures of neurophysiological activity in Wernicke's area: a magnetic source imaging study.

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 (> 200 ms 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 8 mm across subjects. Intraparticipant variability (range) in the onset latency of activity was +/-50 ms, 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.

Early development of neurophysiological processes involved in normal reading and reading disability: a magnetic source imaging study.

This longitudinal study examined the development of the brain mechanism involved in phonological decoding in beginning readers using magnetic source imaging. Kindergarten students were assigned to 2 groups: those who showed mastery of skills that are important predictors of proficient reading (low-risk group) and those who initially did not show mastery but later benefited from systematic reading instruction and developed average-range reading skills at the end of Grade 1 (high-risk responders). Spatiotemporal profiles of brain activity were obtained during performance of letter-sound and pseudoword naming tasks before and after Grade 1 instruction. With few exceptions, low-risk children showed early development of brain activation profiles that are typical of older skilled readers. Provided that temporoparietal and visual association areas were recruited into the brain mechanism that supported reading, the majority of high-risk responder children benefited from systematic reading instruction and developed adequate reading abilities.

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.

Magnetocephalography: a noninvasive alternative to the Wada procedure.

OBJECT: In this study the authors evaluated the sensitivity and selectivity of a noninvasive language mapping procedure based on magnetoencephalography (MEG), for determining hemispheric dominance for language functions. METHODS: Magnetic activation profiles of the brain were obtained from 100 surgical candidates (age range 8-56 years) with medically intractable seizure disorder by using a whole-head MEG system within the context of a word recognition task. The degree of language-specific activity was indexed according to the number of consecutive sources (modeled as single, moving current dipoles) in perisylvian brain areas. Only activity sources that were observed with a high degree of spatial and temporal overlap in two split-half data sets were used to compute the MEG laterality index. Independently, all patients underwent Wada testing for the determination of hemispheric dominance for language. Independent clinical judgments based on MEG and Wada data showed a high degree of concordance (87%). Magnetoencephalography laterality judgments had an overall sensitivity of 98%, but a lower selectivity of 83%, which was due to the fact that MEG detected more activity in the nondominant hemisphere than was predicted based on the Wada test. A number of objective criteria were derived based on this large patient series to ensure data quality and bolster the clinical usefulness of MEG for language mapping. CONCLUSIONS: Although the availability of MEG is still limited across epilepsy surgery centers, this study method may be substituted for the Wada procedure in assessing hemispheric dominance for language in select cases.

Multimodality neuroimaging evaluation improves the detection of subtle cortical dysplasia in seizure patients.

The purpose of this study is to investigate if multimodality neuroimaging evaluation increases the detection of subtle focal cortical dysplasia as part of an epilepsy surgery evaluation. Three patients with normal magnetic resonance imaging and histopathological findings of focal cortical dysplasia were reviewed. Their magnetoencephalography recordings were performed on whole-head magnetoencephalography system. Magnetic resonance images were re-evaluated with special inspection in limited regions guided by magnetoencephalography spike localization. Two patients had ictal and interictal single photon emission computed tomography study after administration of Tc99m ECD. In two patients we found tiny focal abnormalities including slightly increased cortical thickness and blurred gray-white matter junction at the locations of interictal events after re-evaluation of the MR images indicating focal cortical dysplasia. The third patient showed focal atrophic change. All patients are seizure free after surgery. Both ictal and interictal single photon emission computed tomography showed hyperperfusion in the dysplastic cortex regions. Multimodality neuroimaging study can improve the detection of focal cortical dysplasia. Normal magnetic resonance images should be re-evaluated for subtle signs of focal cortical dysplasia especially when magnetoencephalography recording demonstrate focal epileptic discharges.

Contributions of magnetic source imaging to the understanding of dyslexia.

The problem in dyslexia is difficulty learning how the printed word maps onto spoken language. Magnetic Source Imaging protocols were used in three studies. The first study with dyslexic children showed greater activity in the right temporoparietal region. The second study showed the aberrant neural circuit was present in initial stages of reading acquisition. The third study demonstrated that the aberrant profile could be normalized following intensive instruction. These findings suggest that dyslexia represents a functional deficit in the neural circuit that mediates the conversion of print to sound, which is amenable to change given adequate instruction.

Temporal course of word recognition in skilled readers: a magnetoencephalography study.

The goal of the study was to investigate the neural circuit recruited by adult readers during performance of a lexical decision task by assessing the relative timing of neurophysiological activity in the brain regions that comprise this circuit. The time course of regional activation associated with lexical decision was studied in 17 adult volunteers using magnetoencephalography. Following activity in mesial occipital cortices, activation progressed to lateral and ventral occipito-temporal regions (often encompassing the posterior portion of the middle temporal gyrus), followed by activity in the superior temporal gyri (STGp), motor/premotor cortices, and the inferior frontal gyrus. The latency of STGp activation relative to the latency of the motor response to the word stimuli did not support critical involvement of this area in lexical decision. Timing, word length, and word frequency effects found for activity in each area are discussed in relation to the purported roles of each region into the brain circuit for reading.

Atypical language representation in patients with chronic seizure disorder and achievement deficits with magnetoencephalography.

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.

Integrating sensory and motor mapping in a comprehensive MEG protocol: clinical validity and replicability.

Considerable evidence supports the idea of magnetoencephalography (MEG) being a valuable noninvasive tool for presurgical mapping of sensory and motor functions. In this study, we test the validity and replicability of a new experimental paradigm for simultaneous sensory and motor mapping using MEG recordings. This comprehensive sensorimotor protocol (CSSMP), where external mechanic stimulation serves as a cue for voluntary movements, allows the recording of sensory and motor cortical responses during a single activation task. The stability and replicability of MEG-derived recordings during this paradigm were tested in a group of eight neurologically normal volunteers and six patients with perirolandic lesions. We found that a common sensorimotor cortical network, engaging sensory (S1, S2) and motor (M1) areas, was reliably activated in all subjects and patients and that the results remained exceptionally stable over time. Additionally, the clinical validity of the MEG-derived maps of activation was tested through intraoperative electrocortical stimulation mapping in the group of patients. The MEG-derived anatomical maps for specific sensory (S1) and motor (M1) responses were verified, by direct cortical mapping, and confirmed with the patient's surgical outcome. The results of this validation study show that the so-called CSSMP is a reliable and reproducible method for assessing simultaneously sensory and motor areas. This method minimizes methodological problems and improves our knowledge of the spatiotemporal organization of the sensorimotor cortical network and helps to optimize the surgical management of patients with perirolandic lesions.