Identification of language-specific brain activity using magnetoencephalography

The purpose of the present investigation was to explore the ability of magnetoencephalography (MEG) to identify brain areas involved in language comprehension. Event-related magnetic fields (ERFs) were recorded from 7 right-handed adults with no history of neurological disorder or learning disability as they engaged in an auditory and a visual word-recognition task. A face-recognition task served as control. During the later portion of the ERFs, activity sources from both language tasks tended to overlap in temporal and temporo-parietal cortices. There was a clear preponderance of such sources in the left compared to the right hemisphere in all participants. These findings demonstrate that MEG is a promising tool for identifying brain regions involved in the analysis of linguistic stimuli, in addition to the initial encoding of stimulus features.     

MEG language lateralization in partial epilepsy using dSPM of auditory event-related fields

ObjectiveMethods employed to determine hemispheric language dominance using magnetoencephalography (MEG) have differed significantly across studies in the choice of language-task, the nature of the physiological response studied, recording hardware, and source modeling methods. Our goal was to determine whether an analysis based on distributed source modeling can replicate the results of prior studies that have used dipole-modeling of event-related fields (ERFs) generated by an auditory word-recognition task to determine language dominance in patients with epilepsy.MethodsWe analyzed data from 45 adult patients with drug-resistant partial epilepsy who performed an auditory word-recognition task during MEG recording and also completed a language fMRI study as part of their evaluation for epilepsy surgery. Source imaging of auditory ERFs was performed using dynamic statistical parametric mapping (dSPM). Language laterality indices (LIs) were calculated for four regions of interest (ROIs) by counting above-threshold activations within a 300–600 ms time window after stimulus onset. Language laterality (LL) classifications based on these LIs were compared to the results from fMRI.ResultsThe most lateralized MEG responses to language stimuli were observed in a parietal region that included the angular and supramarginal gyri (AngSmg). In this region, using a half-maximal threshold, source activations were left dominant in 32 (71%) patients, right dominant in 8 (18%), and symmetric in 5 patients (11%). The best agreement between MEG and fMRI on the ternary classification of regional language dominance into left, right, or symmetric groups was also found at the AngSmg ROI (69%). This was followed by the whole-hemisphere and temporal ROIs (both 62%). The frontal ROI showed the least agreement with fMRI (51%). Gross discordances between MEG and FMRI findings were disproportionately of the type where MEG favored atypical right-hemispheric language in a patient with right-hemispheric seizure origin (p < 0.05 at three of the four ROIs).SignificanceIn a parietal region that includes the angular and supramarginal gyri, language laterality estimates based on dSPM of ERFs during auditory word-recognition shows a degree of MEG-fMRI concordance that is comparable to previously published estimates for MEG-Wada concordance using dipole counting methods and the same task. Our data also suggest that MEG language laterality estimates based on this task may be influenced by the laterality of epileptic networks in some patients. This has not been reported previously and deserves further study.     

Beta oscillations relate to the N400m during language comprehension

The relationship between the evoked responses (ERPs/ERFs) and the event‐related changes in EEG/MEG power that can be observed during sentence‐level language comprehension is as yet unclear. This study addresses a possible relationship between MEG power changes and the N400m component of the event‐related field. Whole‐head MEG was recorded while subjects listened to spoken sentences with incongruent (IC) or congruent (C) sentence endings. A clear N400m was observed over the left hemisphere, and was larger for the IC sentences than for the C sentences. A time–frequency analysis of power revealed a decrease in alpha and beta power over the left hemisphere in roughly the same time range as the N400m for the IC relative to the C condition. A linear regression analysis revealed a positive linear relationship between N400m and beta power for the IC condition, not for the C condition. No such linear relation was found between N400m and alpha power for either condition. The sources of the beta decrease were estimated in the LIFG, a region known to be involved in semantic unification operations. One source of the N400m was estimated in the left superior temporal region, which has been related to lexical retrieval. We interpret our data within a framework in which beta oscillations are inversely related to the engagement of task‐relevant brain networks. The source reconstructions of the beta power suppression and the N400m effect support the notion of a dynamic communication between the LIFG and the left superior temporal region during language comprehension. Hum Brain Mapp, 2012. © 2012 Wiley Periodicals, Inc.     

Top-down effects can modify the initially stimulus-driven auditory organization

We recorded event-related potentials (ERPs) and magnetic fields (ERFs) of the human brain to determine whether top-down control could modulate the initial organization of sound representations in the auditory cortex. We presented identical sound stimulation and manipulated top-down processes by instructing participants to either ignore the sounds (Ignore condition), to detect pitch changes (Attend-pitch condition), or to detect violations of a repeating tone pattern (Attend-pattern condition). The ERP results obtained in the Attend-pattern condition dramatically differed from those obtained with the other two task instructions. The magnetoencephalogram (MEG) findings were fully compatible, showing that the neural populations involved in detecting pattern violations differed from those involved in detecting pitch changes. The results demonstrate a top-down effect on the sound representation maintained in auditory cortex.     

Abstracts of the 5th joint meeting of the German, Austrian, and Swiss Sections of the International League Against Epilepsy, Basle, May 16-19, 2007

Presidential Symposium ¹ E. Pataraia
¹ University Hospital of Neurology (Vienna, A) Goals, methodology: Epilepsy surgery is defined as any neurosurgical intervention with the primary goal to relieve intractable epilepsy. On the other hand essential brain regions like primary motor and sensory cortex as well as brain areas supporting language and memory functions have to be spared to avoid neurological deficits caused by the operation. Thus, the exact localization of the epileptogenic zone and of essential brain regions is crucial for the successful surgical treatment of seizures that can only be accomplished during a thorough presurgical work‐up. A new noninvasive brain mapping procedure magnetoencephalography (MEG) was employed during the presurgical evaluation for localization of the epileptogenic zones and for the determination of hemispheric dominance and intrahemispheric localization of linguistic functions in patients with drug‐resistant focal epilepsies. Results: The role of MEG for the localization of the epileptogenic zone in the noninvasive evaluation of patients with focal drug‐resistant epilepsies: We evaluated the sensitivity and selectivity of interictal MEG versus prolonged ictal and interictal scalp video‐ EEG in order to identify patient groups that would benefit from preoperative MEG testing. One hundred thirteen consecutive patients with medically refractory epilepsy who underwent surgery were included. The epileptogenic region predicted by interictal and ictal Video‐EEG (V‐EEG) and MEG was defined in relation to the resected area as perfectly overlapping with the resected area, partially overlapping, or nonoverlapping. Using MEG, we were able to localize the resected region in a greater proportion of patients (72.3%) than with noninvasive V‐EEG (40%). MEG contributed to the localization of the resected region in 58.8% of the patients with a non‐localizing V‐EEG study and 72.8% of the patients for whom V‐EEG only partially identified the resected zone. Overall, MEG and V‐EEG results were equivalent in 32.3% of the cases, and additional localization information was obtained using MEG in 40% of the patients. MEG was most useful for presurgical planning in patients who had either partially or nonlocalizing V‐EEG results. Functional organization of interictal spike complex in medial temporal lobe epilepsies: Thirty patients with mesial temporal lobe epilepsy (MTLE) using combined MEG and EEG recordings were icluded. Spikes could be recorded in 14 patients (47%) during the 2‐ to 3‐h MEG/EEG recording session. The MEG and EEG spikes were subjected to separate dipole analyses and the spike dipole localizations were superimposed on MRI scans. All spike dipoles could be localized to the temporal lobe with a clear preponderance in the medial region. Based on dipole orientations in MEG, patients could be classified into two groups: patients with anterior medial vertical (AMV) dipoles, suggesting epileptic activity in the mediobasal temporal lobe and patients with anterior medial horizontal (AMH) dipoles, indicating involvement of the temporal pole and the anterior parts of the lateral temporal lobe. Whereas patients with AMV dipoles had strictly unitemporal interictal and ictal EEG changes during prolonged video‐EEG monitoring, 50% of patients with AMH dipoles showed evidence of bitemporal affection on interictal and ictal EEG. Nine patients underwent epilepsy surgery so far: all five patients with AMV dipoles became completely seizure‐free postoperatively (Class Ia) and two out of four patients with AMH dipoles experienced persistent auras (Class Ib). Plasticity of the brain mechanisms for receptive language in patients with mesial temporal lobe epilepsy and structural lesions: We examined brain activation profiles for receptive language function in patients with left hemisphere space occupying lesions and patients with left temporal lobe epilepsy due to mesial temporal sclerosis (MTS) to assess whether cross‐ and intrahemispheric plasticity for language varied as a function of lesion type or location. We evaluated 44 patients: 21 patients with MTS and 23 lesional patients. All patients underwent preoperative language mapping while performing a word recognition task. The location of the activity sources was subsequently determined by co‐registering them with MRIs. The number of clustered, contiguous activity sources located in temporal and inferior parietal regions (excluding sources in somatosensory cortices) was then assessed. Hemispheric lateralization of language‐specific magnetic activity was determined as left hemispheric, right hemispheric and bilateral according to relation of the acceptable late activity sources in left and right hemispheres. Patients were classified into two groups based on the location of the cluster(s) of language‐specific activity sources within the dominant hemisphere: typical localization of receptive language‐specific cortex (if the cluster of activity sources fell within the cortical region that is commonly identified as Wernicke's area) and atypical localization of receptive language‐specific cortex (if the cluster of activity sources did not overlap with Wernicke's area). A higher incidence of atypical language lateralization was noted among patients with MTS compared with lesional patients (43% vs. 13%). The majority of MTS patients with early seizure onset (before 5 years of age) showed atypical language lateralization. In contrast, the precise location of receptive language‐specific cortex within the dominant hemisphere was found to be outside of Wernicke's area in 30% of lesional patients and only 14% of MTS patients. There is an increased probability of a partial or total displacement of key components of the brain mechanisms responsible for receptive language function to the nondominant hemisphere in MTS patients. Early onset of seizures was strongly associated with atypical language lateralization. Lesions in the dominant hemisphere tend to result in an intrahemispheric reorganization of linguistic function. Organization of receptive language‐specific cortex before and after left temporal lobectomy: In the present study we documented the reorganization of brain areas mediating receptive language function in patients with left temporal lobe epilepsy after a standard anterior temporal lobe resection. We evaluated 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. The results of preoperative Wada‐testing and pre‐ and post‐operative neuropsychological testing and MEG language mapping were compared. Patients with atypical (bilateral) hemispheric dominance pre‐operatively were significantly 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 intra‐hemispheric changes involving a slight inferior shift of the putative location of Wernicke's area. Patients with bilateral representation tended to perform worse on neuropsychological test measures obtained both pre‐ and postoperatively. 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 Wernicke's area. Conclusions: Combined MEG/EEG dipole modeling can identify subcompartments of the temporal lobe involved in epileptic activity and may be helpful to differentiate between subtypes of mesial temporal lobe epilepsy noninvasively. MEG is most useful for presurgical planning in patients who have either partially or nonlocalizing V‐EEG results in the noninvasive evaluation phase. We predict the replacement of the more invasive procedure with MEG in the near future for temporal lobe epilepsies, subsequent to the optimization of the conditions under which preoperative MEG is performed. MEG can be especially helpful in the localization of language‐critical cortex in sites other than those expected within the dominant hemisphere. Our findings also suggest that not only structural elements, but also functional factors have an effect on receptive language organization in the brain. Factors influencing atypical language lateralization have theoretical importance for understanding the organization and reorganization of higher cognitive functions, as well as practical implications, especially in brain surgery and neurological rehabilitation. MEG is a useful method in clinical practice, as it has the capacity to provide reliable images of the working brain of individual subjects, and it is capable of capturing relevant aspects of brain activation by reflecting the actual participation of a particular area in the function under investigation. Finally, it is capable of capturing both the spatial as well as the temporal features of that activation.     

Effects of musical expertise and boundary markers on phrase perception in music

A neural correlate for phrase boundary perception in music has recently been identified in musicians. It is called music closure positive shift ("music CPS") and has an equivalent in the perception of speech ("language CPS"). The aim of the present study was to investigate the influence of musical expertise and different phrase boundary markers on the music CPS, using event-related brain potentials (ERPs) and event-related magnetic fields (ERFs). Musicians and nonmusicians were tested while listening to binary phrased melodies. ERPs and ERFs of both subject groups differed considerably from each other. Phrased melody versions evoked an electric CPS and a magnetic CPSm in musicians, but an early negativity and a less pronounced CPSm in nonmusicians, suggesting different perceptual strategies for both subject groups. Musicians seem to process musical phrases in a structured manner similar to language. Nonmusicians, in contrast, are thought to detect primarily discontinuity in the melodic input. Variations of acoustic cues in the vicinity of the phrase boundary reveal that the CPS is influenced by a number of parameters that are considered to indicate phrasing in melodies: pause length, length of the last tone preceding the pause, and harmonic function of this last tone. This is taken as evidence that the CPS mainly reflects higher cognitive processing of phrasing, rather than mere perception of pauses. Furthermore, results suggest that the ERP and MEG methods are sensitive to different aspects within phrase perception. For both subject groups, qualitatively different ERP components (CPS and early negativity) seem to reflect a top-down activation of general but different phrasing schemata, whereas quantitatively differing MEG signals appear to reflect gradual differences in the bottom-up processing of acoustic boundary markers.     

Simultaneous EEG and MEG source reconstruction in sparse electromagnetic source imaging

Electroencephalography (EEG) and magnetoencephalography (MEG) have different sensitivities to differently configured brain activations, making them complimentary in providing independent information for better detection and inverse reconstruction of brain sources. In the present study, we developed an integrative approach, which integrates a novel sparse electromagnetic source imaging method, i.e., variation‐based cortical current density (VB‐SCCD), together with the combined use of EEG and MEG data in reconstructing complex brain activity. To perform simultaneous analysis of multimodal data, we proposed to normalize EEG and MEG signals according to their individual noise levels to create unit‐free measures. Our Monte Carlo simulations demonstrated that this integrative approach is capable of reconstructing complex cortical brain activations (up to 10 simultaneously activated and randomly located sources). Results from experimental data showed that complex brain activations evoked in a face recognition task were successfully reconstructed using the integrative approach, which were consistent with other research findings and validated by independent data from functional magnetic resonance imaging using the same stimulus protocol. Reconstructed cortical brain activations from both simulations and experimental data provided precise source localizations as well as accurate spatial extents of localized sources. In comparison with studies using EEG or MEG alone, the performance of cortical source reconstructions using combined EEG and MEG was significantly improved. We demonstrated that this new sparse ESI methodology with integrated analysis of EEG and MEG data could accurately probe spatiotemporal processes of complex human brain activations. This is promising for noninvasively studying large‐scale brain networks of high clinical and scientific significance. Hum Brain Mapp, 2013. © 2010 Wiley Periodicals, Inc.     

Magnetoencephalographic localization of the basal temporal language area

Magnetoencephalography (MEG) recordings were made on 25 native English-speaking patients with localization-related epilepsy during a semantic language task (verb generation). Eighteen right-handed subjects with normal reading ability had MEG scans performed during the same language task. MEG data was analyzed by MR-FOCUSS, a current density imaging technique. Detectable MEG signals arising from activation in the left fusiform gyrus, also known as the basal temporal language area (BTLA), occurred at 167 +/- 18 ms (n = 43) in all subjects. The BTLA has been associated with a variety of language production and comprehension tasks involving processing of semantic, orthographic, and phonologic information. MEG may become an important tool in efforts to further define the linguistic operations of specific regions within this language area.     

Changes in language-specific brain activation after therapy for aphasia using magnetoencephalography: a case study

A patient with chronic aphasia underwent functional imaging during a language comprehension task using magnetoencephalography (MEG) before and after constraint induced language therapy (CILT). In the pre- and immediate post-treatment (TX) scans MEG activity sources were observed within right hemisphere only, and were located in areas homotopic to left hemisphere language areas. There was a significant increase in activation in these areas between the two sessions. This change was not observed in an age-matched patient with chronic aphasia who underwent sequential language testing and MEG scanning across a similar time period without being administered therapy. In the 3-month post-TX scan bilateral activation was observed, including significant activation within the left temporal lobe. The changes in the spatial parameters of the maps of receptive language function after therapy were accompanied by improvement in language function. Results provide support, in the same individual, for a role for both hemispheres in recovery of language function after therapy for chronic aphasia.     

Changes in Language-specific Brain Activation after Therapy for Aphasia using Magnetoencephalography: A Case Study

A patient with chronic aphasia underwent functional imaging during a language comprehension task using magnetoencephalography (MEG) before and after constraint induced language therapy (CILT). In the pre- and immediate post-treatment (TX) scans MEG activity sources were observed within right hemisphere only, and were located in areas homotopic to left hemisphere language areas. There was a significant increase in activation in these areas between the two sessions. This change was not observed in an age-matched patient with chronic aphasia who underwent sequential language testing and MEG scanning across a similar time period without being administered therapy. In the 3-month post-TX scan bilateral activation was observed, including significant activation within the left temporal lobe. The changes in the spatial parameters of the maps of receptive language function after therapy were accompanied by improvement in language function. Results provide support, in the same individual, for a role for both hemispheres in recovery of language function after therapy for chronic aphasia.     

Reliability of language mapping with magnetic source imaging in epilepsy surgery candidates

The external validity of a noninvasive language mapping protocol with magnetoencephalography (MEG) has been established through direct comparisons with invasive functional mapping techniques. This study examines the test-retest and interrater reliability of this protocol under realistic testing conditions in 21 epilepsy surgery candidates. Brain activation maps were obtained in the context of an auditory word recognition task and represented by temporally contiguous dipolar activity sources. Both the duration and strength of the associated magnetic flux were used as measures of the magnitude of regional brain activity. Hemispheric asymmetry indices based on these measures showed good interrater reliability and intraparticipant reproducibility. Similar findings were obtained with respect to the location of the geometric center of receptive language-specific cortex (Wernicke's) area in the dominant hemisphere. The results further support the adequacy of this MEG-based brain mapping protocol as a noninvasive tool for receptive language localization in epilepsy surgery candidates.     

Magnetoencephalographic study of event‐related fields and cortical oscillatory changes during cutaneous warmth processing

Thermoreception is an important cutaneous sense, which plays a role in the maintenance of our body temperature and in the detection of potential noxious heat stimulation. In this study, we investigated event‐related fields (ERFs) and neural oscillatory activities, which were modulated by warmth stimulation. We developed a warmth stimulator that could elicit a warmth sensation, without pain or tactile sensation, by using a deep‐penetrating 980‐nm diode laser. The index finger of each participant (n = 24) was irradiated with the laser warmth stimulus, and the cortical responses were measured using magnetoencephalography (MEG). The ERFs and oscillatory responses had late latencies (～1.3 s and 1.0–1.5 s for ERFs and oscillatory responses, respectively), which could be explained by a slow conduction velocity of warmth‐specific C‐fibers. Cortical sources of warmth‐related ERFs were seen in the bilateral primary and secondary somatosensory cortices (SI and SII), posterior part of the anterior cingulate cortex (pACC), ipsilateral primary motor, and premotor cortex. Thus, we suggested that SI, SII, and pACC play a role in processing the warmth sensation. Time–frequency analysis demonstrated the suppression of the alpha (8–13 Hz) and beta (18–23 Hz) band power in the bilateral sensorimotor cortex. We proposed that the suppressions in alpha and beta band power are involved in the automatic response to the input of warmth stimulation and sensorimotor interactions. The delta band power (1–4 Hz) increased in the frontal, temporal, and cingulate cortices. The power changes in delta band might be related with the attentional processes during the warmth stimulation.     

Executive control functions in task switching: evidence from brain injured patients

Executive mechanisms involved in task switching were studied in 18 brain injured patients. The patients had to rapidly switch back and forth between two visual classification tasks and the analyses focused on switch costs, (i.e., performance differences between switch and no-switch trials), and on interference effects, (i.e., processing costs imposed by the presence of interfering stimulus attributes). The patients were grouped according to the side of the brain lesion. Patients with left brain damage (LBD) showed higher switch costs than patients with right brain damage (RBD). These group differences were attributable to disproportionally high switch costs in patients with LBD and language or speech disorders. This result suggests that the efficiency of suppressing internal interference from a recently activated task set depends on the availability of verbal representations of the upcoming task. Patients with RBD showed higher interference from external task sets. This effect was not affected by language or speech disorders. The overall results argue for a fractionation of executive functions to protect against interference from internal and external sources in task switching.     

Executive Control Functions in Task Switching: Evidence from Brain Injured Patients

Executive mechanisms involved in task switching were studied in 18 brain injured patients. The patients had to rapidly switch back and forth between two visual classification tasks and the analyses focused on switch costs, (i.e., performance differences between switch and no-switch trials), and on interference effects, (i.e., processing costs imposed by the presence of interfering stimulus attributes). The patients were grouped according to the side of the brain lesion. Patients with left brain damage (LBD) showed higher switch costs than patients with right brain damage (RBD). These group differences were attributable to disproportionally high switch costs in patients with LBD and language or speech disorders. This result suggests that the efficiency of suppressing internal interference from a recently activated task set depends on the availability of verbal representations of the upcoming task. Patients with RBD showed higher interference from external task sets. This effect was not affected by language or speech disorders. The overall results argue for a fractionation of executive functions to protect against interference from internal and external sources in task switching.     

MEG imaging of recurrent gliomas reveals functional plasticity of hemispheric language specialization

In patients with gliomas, changes in hemispheric specialization for language determined by magnetoencephalography (MEG) were analyzed to elucidate the impact of treatment and tumor recurrence on language networks. Demonstration of reorganization of language networks in these patients has significant implications on the prevention of postoperative functional loss and recovery. Whole‐brain activity during an auditory verb generation task was estimated from MEG recordings in a group of 73 patients with recurrent gliomas. Hemisphere of language dominance was estimated using the language laterality index (LI), a measure derived from the task. The initial scan was performed prior to resection; patients subsequently underwent surgery and adjuvant treatment. A second scan was performed upon recurrence prior to repeat resection. The relationship between the shift in LI between scans and demographics, anatomic location, pathology, and adjuvant treatment was analyzed. Laterality shifts were observed between scans; the median percent change was 29.1% across all patients. Laterality shift magnitude and relative direction were associated with the initial position of language dominance; patients with increased lateralization experienced greater shifts than those presenting more bilateral representation. A change in LI from left or right to bilateral (or vice versa) occurred in 23.3% of patients; complete switch occurred in 5.5% of patients. Patients with tumors within the language‐dominant hemisphere experienced significantly greater shifts than those with contralateral tumors. The majority of patients with glioma experience shifts in language network organization over time which correlate with the relative position of language lateralization and tumor location.     

fMRI-constrained MEG source imaging and consideration of fMRI invisible sources

Recent studies on multimodal brain source imaging have shown that the use of functional MRI (fMRI) prior information could enhance spatial resolution of magnetoencephalography (MEG), while MEG could compensate poor temporal resolution of fMRI. This article deals with a multimodal imaging method, which combines fMRI and MEG for enhancing both spatial and temporal resolutions. Recent studies on the combination of fMRI and MEG have suggested that the fMRI prior information could be very easily implemented by just giving different weighting factors to the diagonal terms of source covariance matrix in linear inverse operator. We applied the fMRI constrained imaging method to several simulation data and experimental data (Japanese language lexical judgment experiment), and found that some MEG sources may be eliminated by the introduction of the fMRI weighting and the eliminated sources may affect source estimation in fMRI activation regions. In this article, in order to check whether the eliminated sources were fMRI invisible ones or just spurious ones, we placed small numbers of regional sources (rotating dipoles) around all possible activation regions and investigated their temporal changes. By investigating the results carefully, we could evaluate whether the missed sources were real or not.     

Across‐session consistency of context‐driven language processing: A magnetoencephalography study

Changes in brain organization following damage are commonly observed, but they remain poorly understood. These changes are often studied with imaging techniques that overlook the temporal granularity at which language processes occur. By contrast, electrophysiological measures provide excellent temporal resolution. To test the suitability of magnetoencephalography (MEG) to track language‐related neuroplasticity, the present study aimed at establishing the spectro‐temporo‐spatial across‐session consistency of context‐driven picture naming in healthy individuals, using MEG in two test–retest sessions. Spectro‐temporo‐spatial test–retest consistency in a healthy population is a prerequisite for studying neuronal changes in clinical populations over time. For this purpose, 15 healthy speakers were tested with MEG while performing a context‐driven picture‐naming task at two time points. Participants read a sentence missing the final word and named a picture completing the sentence. Sentences were constrained or unconstrained toward the picture, such that participants could either retrieve the picture name through sentence context (constrained sentences), or could only name it after the picture appeared (unconstrained sentences). The context effect (constrained versus unconstrained) in picture‐naming times had a strong effect size and high across‐session consistency. The context MEG results revealed alpha–beta power decreases (10–20 Hz) in the left temporal and inferior parietal lobule that were consistent across both sessions. As robust spectro‐temporo‐spatial findings in a healthy population are required for working toward longitudinal patient studies, we conclude that using context‐driven language production and MEG is a suitable way to examine language‐related neuroplasticity after brain damage.     

Parieto-occipital sources account for the increase in alpha activity with working memory load

The role of oscillatory alpha activity (8-13 Hz) in cognitive processing remains an open question. It has been debated whether alpha activity plays a direct role in the neuronal processing required for a given task or whether it reflects idling and/or functional inhibition. Recent electroencephalography (EEG) studies have demonstrated that alpha activity increases parametrically with load during retention in working memory paradigms. While it is known that the parieto-occipital cortex is involved in the generation of the spontaneous alpha oscillations, it remains unknown where the sources of the memory-dependent alpha activity are located. We recorded brain activity using magnetoencephalography (MEG) from human subjects performing a Sternberg memory task where faces were used as stimuli. Spectral analysis revealed a parametric increase in alpha activity with memory load over posterior brain areas. We then applied a source reconstruction technique that allowed us to map the parametric increase in alpha activity to the anatomical magnetic resonance (MR) images of the subject. The primary sources of the memory-dependent alpha activity were in the vicinity of the parieto-occipital sulcus. This region is not directly involved in working memory maintenance of faces. Our findings are consistent with the notion that alpha activity reflects disengagement or inhibition of the visual dorsal stream. We propose that the disengagement reflected in alpha power serves to suppress visual input in order to devote resources to structures responsible for working memory maintenance.     

Distributed source modeling of language with magnetoencephalography: Application to patients with intractable epilepsy

Purpose:   To examine distributed patterns of language processing in healthy controls and patients with epilepsy using magnetoencephalography (MEG), and to evaluate the concordance between laterality of distributed MEG sources and language laterality as determined by the intracarotid amobarbital procedure (IAP).
Methods:   MEG was performed in 10 healthy controls using an anatomically constrained, noise-normalized distributed source solution (dynamic statistical parametric map, dSPM). Distributed source modeling of language was then applied to eight patients with intractable epilepsy. Average source strengths within temporoparietal and frontal lobe regions of interest (ROIs) were calculated, and the laterality of activity within ROIs during discrete time windows was compared to results from the IAP.
Results:   In healthy controls, dSPM revealed activity in visual cortex bilaterally from ∼80 to 120 ms in response to novel words and sensory control stimuli (i.e., false fonts). Activity then spread to fusiform cortex ∼160–200 ms, and was dominated by left hemisphere activity in response to novel words. From ∼240 to 450 ms, novel words produced activity that was left-lateralized in frontal and temporal lobe regions, including anterior and inferior temporal, temporal pole, and pars opercularis, as well as bilaterally in posterior superior temporal cortex. Analysis of patient data with dSPM demonstrated that from 350 to 450 ms, laterality of temporoparietal sources agreed with the IAP 75% of the time, whereas laterality of frontal MEG sources agreed with the IAP in all eight patients.
Discussion:   Our results reveal that dSPM can unveil the timing and spatial extent of language processes in patients with epilepsy and may enhance knowledge of language lateralization and localization for use in preoperative planning.     

Colour or shape: examination of neural processes underlying mental flexibility in posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is a mental disorder that stems from exposure to one or more traumatic events. While PTSD is thought to result from a dysregulation of emotional neurocircuitry, neurocognitive difficulties are frequently reported. Mental flexibility is a core executive function that involves the ability to shift and adapt to new information. It is essential for appropriate social-cognitive behaviours. Magnetoencephalography (MEG), a neuroimaging modality with high spatial and temporal resolution, has been used to track the progression of brain activation during tasks of mental flexibility called set-shifting. We hypothesized that the sensitivity of MEG would be able to capture the abnormal neurocircuitry implicated in PTSD and this would negatively impact brain regions involved in set-shifting. Twenty-two soldiers with PTSD and 24 matched control soldiers completed a colour–shape set-shifting task. MEG data were recorded and source localized to identify significant brain regions involved in the task. Activation latencies were obtained by analysing the time course of activation in each region. The control group showed a sequence of activity that involved dorsolateral frontal cortex, insula and posterior parietal cortices. The soldiers with PTSD showed these activations but they were interrupted by activations in paralimbic regions. This is consistent with models of PTSD that suggest dysfunctional neurocircuitry is driven by hyper-reactive limbic areas that are not appropriately modulated by prefrontal cortical control regions. This is the first study identifying the timing and location of atypical neural responses in PTSD with set-shifting and supports the model that hyperactive limbic structures negatively impact cognitive function.