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Highlights • MEG may aid in the localization of the epileptogenic zone in the parietal lobe. • SAMepi – a novel kurtosis beamformer – results in localizations similar to those of the ECD analysis. • A unifocal result in both the ECD and the SAMepi analysis is associated with a good clinical outcome., Objective To evaluate a novel analysis method (SAMepi) in the localization of interictal epileptiform magnetoencephalographic (MEG) activity in parietal lobe epilepsy (PLE) patients in comparison with equivalent current dipole (ECD) analysis. Methods We analyzed the preoperative interictal MEG of 17 operated PLE patients utilizing visual analysis and: (1) ECD with a spherical conductor model; (2) ECD with a boundary element method (BEM) conductor model; and (3) SAMepi – a kurtosis beamformer method. Localization results were compared between the three methods, to the location of the resection and to the clinical outcome. Results Fourteen patients had an epileptiform finding in the visual analysis; SAMepi detected spikes in 11 of them. A unifocal finding in both the ECD and in the SAMepi analysis was associated with a better chance of seizure-freedom (p = 0.02). There was no significant difference in the distances from the unifocal MEG localizations to the nearest border of the resection between the different analysis methods. Conclusions Localizations of unifocal interictal spikes detected by SAMepi did not significantly differ from the conventional ECD localizations. Significance SAMepi – a novel semiautomatic analysis method – is useful in localizing interictal epileptiform MEG activity in the presurgical evaluation of parietal lobe epilepsy patients.

Epilepsy is a neurological disease characterized by disturbances in the electric activity of the brain. Up to a third of epilepsy patients have an inadequate response to anti-epileptic medication. In these patients, the possibility of surgical treatment, that is, the surgical removal of the epileptogenic brain tissue, should be considered. Some patient subgroups, such as those with no potential epileptogenic lesion in the magnetic resonance imaging (MRI) of the brain, so called MRI-negative patients, are particularly challenging and typically require comprehensive noninvasive and invasive presurgical investigations. Magnetoencephalography (MEG) is a noninvasive method in which the magnetic fields generated by the cortical post-synaptic currents are recorded. The generators of the MEG signal are thought to be the same as the generators of the electroencephalographic (EEG) signal. However, the two methods differ in their sensitivity to source currents in different orientation and depth. Therefore, the two methods are complementary. One of the studies of this thesis examined the utility of MEG in patients whose epilepsy is caused by a focal cortical dysplasia (FCD). FCDs are highly epileptogenic malformations of cortical development, and often result in epilepsy that is refractory to anti-epileptic medication. They may often be invisible in the brain MRI, requiring extensive presurgical evaluation. In this study, we found that a more complete removal of the MEG interictal spike source cluster locations was associated with better surgical outcome. Additionally, the concordance between the interictal MEG spike localizations and the epileptogenic zone estimated by an intracranial recording was considered good in most of the MRI-negative patients, who were of a special interest in this study. The second study investigated the use of MEG as a tool to assess language lateralization in epilepsy patients. We introduced a simple auditory paradigm that had previously been shown to elicit differently lateralized MEG responses in healthy right-handed vs. left-handed subjects suggesting a potential value in studying language lateralization. We compared the lateralization results of MEG to the results of the intracarotid amobarbital procedure (IAP), also known as the Wada test. Although the lateralization results of MEG did not show a significant correlation to the IAP lateralization results, we discovered an MEG response pattern that was only present in patients with left hemisphere dominant IAP results. This pattern was seen in 67% to 78% of the patients with left dominant IAP results depending on the MEG analysis method utilized. In the third study of this thesis, we examined the utility of a novel kurtosis beamformer analysis method, SAMepi, in patients with parietal lobe epilepsy to localize the sources of interictal epileptiform activity. Parietal lobe epilepsies are rare in epilepsy surgery series, which may partially be explained by the difficulty of localizing the epileptogenic zone in this patient group. SAMepi is a semi-automatic analysis method, that would significantly reduce the workload and the subjective decision-making compared to the current clinical practice. We found that SAMepi analysis provided localization results that in most patients were similar to the results of the current clinical standard, that is, the equivalent current dipole (ECD) analysis. However, its sensitivity in detecting epileptiform discharges was somewhat lower than that of the current practice of visually reviewing the MEG signals. In this thesis, we have aimed to develop the utility of MEG in challenging epilepsy surgery patient groups, and to introduce a novel language paradigm as well as a novel semi-automatic analysis method for interictal spike localization. Although MEG already is quite an established tool in the presurgical evaluations of epilepsy surgery candidates in many epilepsy surgery centers, the continuous development of the method is crucial to maximize its diagnostic yield and to justify its higher cost compared to more conventional neurophysiological techniques, such as EEG. Epilepsia on neurologinen sairaus, jossa aivojen poikkeava sähköinen toiminta aiheuttaa oireita, jotka tyypillisesti esiintyvät kohtauksittain. Noin kolmasosalla epilepsiaa sairastavista potilaista epilepsialääkityksellä ei saavuteta kohtauksettomuutta. Silloin on tärkeää selvittää, onko epilepsian kirurginen hoito mahdollista. Erityisen laajoja kajoamattomia ja kajoavia tutkimuksia tarvitaan epilepsiakirurgian suunnittelussa niillä potilailla, joilla pään magneettikuvantamisessa (MRI) ei löydy epilepsian aiheuttajaksi sopivaa rakennemuutosta. Magnetoenkefalografiassa (MEG) aivojen sähköisen toiminnan tuottamia hyvin heikkoja magneettikenttiä mitataan pään ulkopuolelta kajoamattomasti. Tämän väitöskirjatyön tarkoituksena oli tutkia ja kehittää MEG:n käyttöä osana epilepsiakirurgiaa edeltäviä tutkimuksia. Yhdessä osatyössä tutkimme MEG:n käyttöä epilepsiakirurgiapotilailla, joiden epilepsian syynä oli paikallinen aivokuoren kehityshäiriö (engl. focal cortical dysplasia, FCD). FCD aiheuttaa varsin usein lääkehoidolle vastaamattoman epilepsian ja erityisesti pienet FCD:t jäävät melko usein näkymättä pään magneettikuvantamisessa (ns. MRI-negatiiviset potilaat). Tutkimuksen tulokset viittaavat siihen, että täydellisempi MEG:n kohtaustenvälisten piikkien lähdealueen poisto on yhteydessä parempaan leikkaustulokseen. Lisäksi totesimme, että suurimmalla osalla MRI-negatiivisista potilaista MEG:n paikannustulokset olivat yhteneviä kallonsisäisten elektroenkefalografia (EEG) -rekisteröintien avulla määritetyn epileptogeenisen, eli epilepsiaa aiheuttavan, alueen kanssa. Toisessa osatyössä tutkimme yksinkertaisen kuuloärsyketehtävän toimivuutta kielellisten toimintojen lateralisaation määrittämisessä. Vertasimme MEG:n tuloksia kajoavan ns. Wada-tutkimuksen tuloksiin. Totesimme tietynlaisten MEG-vasteiden esiintyvän vain niillä potilailla, joilla Wada-tutkimuksen tulokset viittasivat vasemman aivopuoliskon kielidominanssiin. Tosin tämä MEG-löydös todettiin vain osalla (67% tai 78% analyysimenetelmästä riippuen) näistä potilaista. Kolmannessa osatyössä tutkimme MEG:n käyttöä kohtaustenvälisten epileptiformisten ilmiöiden paikannuksessa päälaenlohkon epilepsiaa sairastavilla potilailla. Nykyisen kliinisessa käytössä olevan standardimenetelmän eli ns. virtadipolianalyysin lisäksi analysoimme MEG-tulokset käyttäen uutta puoliautomaattista keilanmuodostukseen (engl. beamforming) perustuvaa paikannusmenetelmää. Totesimme, että uuden menetelmän tuottamat paikannustulokset olivat suurimmalta osin samankaltaisia kuin nykyisen standardimenetelmän tuottamat tulokset. Epileptiformisten ilmiöiden toteamisessa uusi menetelmä ei kuitenkaan ollut aivan yhtä herkkä kuin perinteinen MEG-signaalien manuaaliseen tarkistamiseen perustuva analyysi. Tämän väitöskirjatyön tavoitteena on ollut kehittää MEG:n käyttöä haastavien potilasryhmien epilepsiakirurgia-arvioissa. MEG on vakiinnuttanut asemansa osana epilepsiakirurgiaa edeltäviä tutkimuksia monissa epilepsiakirurgiakeskuksissa, mutta sen jatkuva kehittäminen on välttämätöntä siitä saatavan hyödyn lisäämiseksi erityisesti huomioiden MEG-tutkimusten korkea hinta ja rajallinen saatavuus verrattuna EEG-tutkimuksiin.

Funding Information: This work was supported by Helsinki University Hospital, Neuroscience Center (HiLIFE; Univ. Helsinki), Aivosäätiö, Finnish Pediatric Foundation/ Lastentautiensäätiö (SV), Sigrid Juselius Foundation, and Academy of Finland (SV, 313242, 288220, 3104450). Funding Information: This work was supported by Helsinki University Hospital, Neuroscience Center (HiLIFE; Univ. Helsinki), Aivos??ti?, Finnish Pediatric Foundation/ Lastentautiens??ti? (SV), Sigrid Juselius Foundation, and Academy of Finland (SV, 313242, 288220, 3104450). Publisher Copyright: © 2021 International Federation of Clinical Neurophysiology Objective: To examine the usability of long-range temporal correlations (LRTCs) in non-invasive localization of the epileptogenic zone (EZ) in refractory parietal lobe epilepsy (RPLE) patients. Methods: We analyzed 10 RPLE patients who had presurgical MEG and underwent epilepsy surgery. We quantified LRTCs with detrended fluctuation analysis (DFA) at four frequency bands for 200 cortical regions estimated using individual source models. We correlated individually the DFA maps to the distance from the resection area and from cortical locations of interictal epileptiform discharges (IEDs). Additionally, three clinical experts inspected the DFA maps to visually assess the most likely EZ locations. Results: The DFA maps correlated with the distance to resection area in patients with type II focal cortical dysplasia (FCD) (p

Summary Objective Navigated transcranial magnetic stimulation (nTMS) is becoming increasingly popular in noninvasive preoperative language mapping, as its results correlate well enough with those obtained by direct cortical stimulation (DCS) during awake surgery in adult patients with tumor. Reports in the context of epilepsy surgery or extraoperative DCS in adults are, however, sparse, and validation of nTMS with DCS in children is lacking. Furthermore, little is known about the risk of inducing epileptic seizures with nTMS in pediatric epilepsy patients. We provide the largest validation study to date in an epilepsy surgery population. Methods We compared language mapping with nTMS and extraoperative DCS in 20 epilepsy surgery patients (age range 9‐32 years; 14 children and adolescents). Results In comparison with DCS, sensitivity of nTMS was 68 specificity 76 positive predictive value 27 and negative predictive value 95 Age, location of ictal‐onset zone near or within DCS‐mapped language areas or severity of cognitive deficits had no significant effect on these values. None of our patients had seizures during nTMS. Significance Our study suggests that nTMS language mapping is clinically useful and safe in epilepsy surgery patients, including school‐aged children and patients with extensive cognitive dysfunction. Similar to in tumor surgery, mapping results in the frontal region are most reliable. False negative findings may be slightly more likely in epilepsy than in tumor surgery patients. Mapping results should always be verified by other methods in individual patients.

INTRODUCTION Navigated transcranial magnetic stimulation (nTMS) is increasingly used for preoperative mapping of motor function, and clinical evidence for its benefit for brain tumor patients is accumulating. In respect to language mapping with repetitive nTMS, literature reports have yielded variable results, and it is currently not routinely performed for presurgical language localization. The aim of this project is to define a common protocol for nTMS motor and language mapping to standardize its neurosurgical application and increase its clinical value. METHODS The nTMS workshop group, consisting of highly experienced nTMS users with experience of more than 1500 preoperative nTMS examinations, met in Helsinki in January 2016 for thorough discussions of current evidence and personal experiences with the goal to recommend a standardized protocol for neurosurgical applications. RESULTS nTMS motor mapping is a reliable and clinically validated tool to identify functional areas belonging to both normal and lesioned primary motor cortex. In contrast, this is less clear for language-eloquent cortical areas identified by nTMS. The user group agreed on a core protocol, which enables comparison of results between centers and has an excellent safety profile. Recommendations for nTMS motor and language mapping protocols and their optimal clinical integration are presented here. CONCLUSION At present, the expert panel recommends nTMS motor mapping in routine neurosurgical practice, as it has a sufficient level of evidence supporting its reliability. The panel recommends that nTMS language mapping be used in the framework of clinical studies to continue refinement of its protocol and increase reliability.

Objective: The intracarotid amobarbital procedure (IAP) is the current “gold standard” in the preoperative assessment of language lateralization in epilepsy surgery candidates. It is, however, invasive and has several limitations. Here we tested a simple noninvasive language lateralization test performed with magnetoencephalography (MEG). Methods: We recorded auditory MEG responses to pairs of vowels and pure tones in 16 epilepsy surgery candidates who had undergone IAP. For each individual, we selected the pair of planar gradiometer sensors with the strongest N100m response to vowels in each hemisphere and —from the vector sum of signals of this gradiometer pair—calculated the vowel/tone amplitude ratio in the left (L) and right (R) hemisphere and, subsequently, the laterality index: LI = (L–R)/(L+R). In addition to the analysis using a single sensor pair, an alternative analysis was performed using averaged responses over 18 temporal sensor pairs in both hemispheres. Results: The laterality index did not correlate significantly with the lateralization data obtained from the IAP. However, an MEG pattern of stronger responses to vowels than tones in the left hemisphere and stronger responses to tones than vowels in the right hemisphere was associated with left-hemispheric language dominance in the IAP in all the six patients who showed this pattern. This results in a specificity of 100% and a sensitivity of 67% of this MEG pattern in predicting left-hemispheric language dominance (p = 0.01, Fisher’s exact test). In the analysis using averaged responses over temporal channels, one additional patient who was left-dominant in IAP showed this particular MEG pattern, increasing the sensitivity to 78% (p = 0.003). Significance: This simple MEG paradigm shows promise in feasibly and noninvasively confirming left-hemispheric language dominance in epilepsy surgery candidates. It may aid in reducing the need for the IAP, if the results are confirmed in larger patient samples.

SUMMARY Purpose: Ictal video‐electroencephalography (EEG) is commonly used to establish ictal onset-zone location. Recently software development has enabled systematic studies of ictal magnetoencephalography (MEG). In this article, we evaluate the ability of ictal MEG signals tolocalize the seizure-onset zone. Methods: Twenty-six patients underwent ictal MEG and epilepsy surgery. Prediction of seizure-onset zone by ictal and interictal MEG was retrospectively compared with ictal-onset area found by intracranial EEG in 12 patients. The specificity and sensitivity of the prediction were calculated at hemisphere-lobe (HL) and at hemisphere-lobesurface (HLS) levels. Key Findings: The sensitivity of ictal MEG source localization was 0.958 on HL and 0.706 on HLS levels, and its specificity was 0.900 on HL and 0.731 on HLS levels. The interictal MEG dipole cluster, defined as >10 dipoles on one lobar surface, had sensitivity of 0.400 and specificity of 0.769. Ictal MEG was equally sensitive and specific on dorsolateral and nondorsolateral neocortical surfaces up to a depth of 4 cm from the scalp. Significance: Sources of ictal-onset MEG signals and interictal dipole clusters are essentially equally specific in estimation of the ictal-onset zone on lobar surface resolution, but ictal MEG is more sensitive. On the lobe resolution, ictal MEG estimates ictal-onset zone with high sensitivity

Transcranial magnetic stimulation (TMS) is widely used both in basic research and in clinical practice. TMS has been utilized in studies of functional organization of speech in healthy volunteers. Navigated TMS (nTMS) allows preoperative mapping of the motor cortex for surgical planning. Recording behavioral responses to nTMS in the speech-related cortical network in a manner that allows off-line review of performance might increase utility of nTMS both for scientific and clinical purposes, e.g., for a careful preoperative planning. Four subjects participated in the study. The subjects named pictures of objects presented every 2-3s on a computer screen. One-second trains of 5 pulses were applied by nTMS 300ms after the presentation of pictures. The nTMS and stimulus presentation screens were cloned. A commercial digital camera was utilized to record the subject's performance and the screen clones. Delays between presentation, audio and video signals were eliminated by carefully tested combination of displays and camera. An experienced neuropsychologist studied the videos and classified the errors evoked by nTMS during the object naming. Complete anomias, semantic, phonological and performance errors were observed during nTMS of left fronto-parieto-temporal cortical regions. Several errors were detected only in the video classification. nTMS combined with synchronized video recording provides an accurate monitoring tool of behavioral TMS experiments. This experimental setup can be particularly useful for high-quality cognitive paradigms and for clinical purposes.

Creutzfeldt-Jakob disease (CJD) is a rare neurodegenerative disease with no effective therapy available. We recorded spontaneous magnetoencephalography and auditory evoked fields (AEFs) from a male patient with a rapidly progressive memory disorder, ataxia and myoclonus. Post-mortem examination confirmed sporadic CJD. Sources of the abnormal slow wave activity were localized with a beamformer software. Sources of sharp transients and AEFs were modeled with equivalent current dipoles. The estimated sources of spontaneous activity abnormalities were more dominant in the left hemisphere, in line with left-dominant abnormalities in diffusion-weighted MRI. Sources of AEFs were found in both temporal lobes. Magnetoencephalography measurements on CJD patients are feasible, and provide efficient means for localizing abnormal cortical activity in CJD.

The effects of alcohol (0.8 g/kg) on the prefrontal cortex were studied in nine healthy subjects using the technique of transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). A total of 120 magnetic pulses were delivered with a figure-of-eight coil to the left prefrontal cortex at the rate of 0.4-0.7 Hz. The EEG was recorded simultaneously with 60 scalp electrodes (41 electrodes were used for analysis); the TMS-evoked activation was estimated by the area under the global mean field amplitude (GMFA) time curve. TMS caused changes in EEG activity lasting up to 270 ms poststimulus. Alcohol decreased GMFA at 30-270 ms poststimulus (713+/-303 vs 478+/-142 microV ms; p=0.007). Alcohol-induced differences were most pronounced at anterior electrodes. These results suggest that alcohol reduces the excitability in the prefrontal cortex.

Summary Introduction Magnetoencephalography (MEG) measures magnetic fields generated by neuronal currents. MEG is complementary to EEG. Considerable body of evidence indicates that ictal MEG recordings can provide useful information for pre-surgical evaluation of epilepsy patients alongside the more established long-term ictal video-EEG. Ictal MEG is recorded in some epilepsy surgery centers. However, a wider adoption of ictal MEG is hampered by lack of tools for synchronized video-MEG recording similar to those of video-EEG. Methods We have augmented MEG with a synchronized behavioral video-recording system. To estimate its additional value in ictal recordings, we retrospectively analyzed recordings of 10 epilepsy patients with and without the video. Results In six patients out of ten, adding the video substantially changed the resulting interpretations. In all six cases the effect was considerable: the number of detected seizures changed by more than 50%. Conclusions Synchronized video and audio recording capabilities are important for effective ictal MEG recordings of epilepsy patients.

Video-EEG is the gold standard in clinical epilepsy diagnostics. MEG systems have traditionally lacked video capabilities. Here we describe the addition of a synchronized digital video recording (DVR) system to a standard Elekta Neuromag® 306-channel MEG device operated at BioMag laboratory of the Helsinki University Central Hospital. We also provide some insights obtained from using the system for ictal MEG recordings. Video and audio were recorded simultaneously with MEG, using a separate digital video recording station. Synchronization was achieved by timestamping the video, audio and MEG streams and synchronizing all parts of the system via Network Time Protocol (NTP). Video was recorded at 25 fps with an infrared-sensitive camera necessary to study patients that have seizures during sleep. MEG and video are synchronized to the maximum achievable accuracy of one video frame. At this time, five epileptic patients have undergone video-MEG recordings with the setup described above. In three patients, ictal recordings were obtained. In two cases the video con siderably facilitated the exact temporal localization of the seizure onset, thus reducing the amount of time required for data analysis and improving the reliability of the spatial analysis.

Creutzfeldt-Jakob Disease (CJD) is a rare neurodegenerative disease, which belongs to a group of spongiform encephalopathies termed prion diseases.

TMS combined with simultaneous EEG is a novel brain imaging tool allowing investigation local excitability of human cortex. As alcohol acts through increasing function of A-type gamma-aminobutyric acid receptors and attenuating the function of glutaminergic NMDA-receptors-related excitation, we tested whether TMS-evoked N100 response which is thought to reflect cortical inhibitory processes, might be affected by alcohol. Ten healthy subjects ingested alcohol (0.8 g/kg) and EEG responses from 60 channels before and after alcohol ingestion were recorded after left motor-cortex stimulation. Alcohol almost abolished TMS-evoked N100 response. Control experiments with a piece of plastic placed between the head and coil to exclude auditory artefacts were conducted. Alcohol effects were similar when EEG responses from control experiments were subtracted from real-TMS. Alcohol-induced decrease was similar at ipsilateral, contralateral and frontal EEG sites suggesting that alcohol may change cortico-cortical connectivity of motor cortex. Alternative explanation is that alcohol has overall suppression effect on motor cortex. N100 may provide a useful marker of neural inhibition of human cortex for drug research.

The stimulus intensity of prefrontal repetitive transcranial magnetic stimulation (rTMS) during depression treatment is usually determined by adjusting it with respect to the motor threshold (MT). There is some evidence that reactivity of the prefrontal cortex to transcranial magnetic stimulation (TMS) is lower than that of the motor cortex at MT stimulation. However, it is unknown whether this is true when other stimulus intensities are used. We investigated whether the magnitude and shape of the overall TMS-evoked electroencephalographic (EEG) responses differ between prefrontal and motor cortices. Magnetic pulses to the left motor and prefrontal cortices (the middle frontal gyrus identified from magnetic resonance images) were delivered at four intensities (60, 80, 100, and 120% of MT of the right abductor digiti minimi muscle) for six subjects. Simultaneously, EEG was recorded with 60 scalp electrodes. Global mean-field amplitudes (GMFAs) reflecting overall cortical activity were significantly smaller after prefrontal- than after motor-cortex TMS. A significant positive correlation (r s=0.84, p

The reactivity of the prefrontal cortex (PFC) was studied by measuring electroencephalographic (EEG) responses to transcranial magnetic stimulation (TMS) with different stimulus intensities. Focal TMS at intensities of 60%, 80%, 100%, and 120% of the motor threshold was delivered to the left middle frontal gyrus identified individually from magnetic resonance images (MRI) in seven healthy subjects. EEG was simultaneously recorded with 60 scalp electrodes. Stimulation evoked clear responses at all intensities. Left prefrontal TMS evoked an averaged EEG response consisting of five deflections at 27 ± 3 ms (peak I), 39 ± 3 ms (II), 52 ± 7 ms (III), 105 ± 14 ms (IV), and 193 ± 15 ms (V) at the Fz/FCz electrodes. The slope of the almost linear dependence of the overall response on stimulus intensity varied with latency. Potential distributions were relatively similar for the four intensities, suggesting that the same cortical structures may be activated. Intensity dependence function to TMS may be an indicator of cortical activation in humans.

Objective : The stimulus intensity of prefrontal transcranial magnetic stimulation (TMS) is usually determined with respect to the motor threshold (MT). However, the association between the excitability of the prefrontal and motor cortices is unknown. Methods : Magnetic pulses to the left motor and prefrontal cortices were delivered at the MT of the right abductor digiti minimi muscle for 9 subjects and at 4 different stimulus intensities (60, 80, 100, and 120% of MT) for two subjects. Simultaneously, EEG was recorded with 60 scalp electrodes. Results : Global mean field amplitudes of the TMS-evoked responses were significantly (32%) smaller after prefrontal than after motor cortex TMS, but they correlated positively. Conclusions : The reactivity to TMS is different between the motor and prefrontal cortices. However, an association between these reactivities suggests that MT may be used for determining the stimulus intensity of prefrontal TMS.

s of Oral Presentations / Clinical Neurophysiology 125, Supplement 1 (2014) S1–S339 S37 Results: The results showed changes in the morphology of the evoked potentials, following a circadian profile and reflecting times of drug delivery. Conclusion: The results demonstrated that we were able to measure and manipulate neural excitability in the circuit of Papez of canines. Under the assumption that neural hyper-excitability is a precusor to epileptic seizures, this new method will provide a systematic way to titrate therapies for effictive seizure prevention. Studying language: Insights from invasive and non-invasive mapping O28 Comparison of the language lateralizations suggested by magnetoencephalography and navigated transcranial magnetic stimulation in healthy subjects using a picture naming task J. Wilenius1,2, P. Lioumis2, R. Paetau1,3, J.P. Maekelae2 1Hospital District of Helsinki and Uusimaa, Department of Clinical Neurophysiology, Helsinki, Finland; 2Hospital District of Helsinki and Uusimaa, BioMag-laboratory, Helsinki, Finland; 3Hospital District of Helsinki and Uusimaa, Department of Pediatric Neurology, Helsinki, Finland Question: Magnetoencephalography (MEG) and navigated transcranial magnetic stimulation (nTMS) are promising non-invasive tools in the examination of language organization as part of presurgical studies before epilepsy or tumor surgery. This study aims to compare the language lateralization by these two techniques using a picture naming task in healthy subjects. Methods: Nineteen healthy subjects (age range 21–55 years) were studied with a picture naming task in MEG and nTMS. Results of five subjects are presented here. In MEG, sequential single equivalent current dipole source estimates were calculated separately over left and right temporal regions from 200 to 600 ms after the picture presentation. Laterality index (LI) was calculated by comparing the average source strength (dipole moment) on each hemisphere (LI = (left − right)/(left + right)). In nTMS, the same picture naming task was used and 1-s trains of five TMS stimuli (5Hz, starting at 300 ms after picture onset) were applied to interfere with the naming task performance. The number of naming errors on each hemisphere was used to calculate the laterality index in a similar manner as in MEG. Results: Both MEG and nTMS suggested left hemisphere language dominance (defined as LI>0.1) in four out of five subjects. In one subject, nTMS suggested right dominance (LI