Clinical Neuroscience

[LORETA (Low Resolution Electromagnetic Tomography): A three-dimensional EEG source localization method]

CLEMENS Béla

SEPTEMBER 23, 2011

Clinical Neuroscience - 2011;64(09-10)

[The author presents a brief overview of the EEG source localization method LORETA (Low Resolution Electromagnetic Tomography) with special reference to the not widely discussed data.]

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[It is not only the total curative resection of pathological tissue or the minimization of symptoms to be considered in epilepsy surgery or other neurosurgical procedures, it is equally desirable to maintain the best possible quality of life. Cortical mapping methods can help achieve this goal by delineating eloquent areas, i.e. brain regions that are vital for providing an acceptable quality of life, albeit not prone to compensatory reorganization. These areas include among others the Broca and Wernicke regions for speech, the primary motor, sensory and visual cortices. Functional MRI gained importance in the last decade as a non-invasive clinical cortical mapping technique. This method is capable of localizing cortical areas selectively activated by a given task condition. Thus, selecting appropriate tasks can help mapping eloquent brain regions. Using functional MRI provides information that is complementary to other mapping methods. Moreover, it can replace invasive methods such as the Wada test. Here, we explain the background of functional MRI, compare it to other clinical mapping methods, explain the intricacies of paradigm selection, and show the limitations of the technique while also pointing out alternative uses.]

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[Neuropsychological assessment makes its own valuable contribution to diagnosis in epilepsy, and the integration of those findings with the data from multiple sources provides the most comprehensive picture for a given epilepsy patient. The objective of this paper is to define and summarize the role and methods of neuropsychological assessment, and to present recommendations on themes for future clinical research in the neuropsychology of epilepsy.]

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[Neurophysiological research suggests that the so-called “standard” EEG analysis has been confronted with new diagnostic challenges. The findings mainly concern the occurrence, the neurophysiological and clinical significance of epileptiform EEG discharges in several neurological and psychiatric disorders. In addition to well-known interictal and ictal discharges, a growing number of recently recognized epileptiform phenomena have been described. The first reports suggested that they might be relevant for the comprehensive description of epileptic dysfunction and might contribute to diagnosis and treatment as well. However, considerable improvement of present-day “standard” EEG technique is necessary to give an appropriate answer to most challenges. Reliable registration and quantitative assessment of well-known epileptiform transients require extended electrode coverage of the head (high-density EEG) and long-term recordings including waking and sleep states to estimate frequency and dyna­mics of targeted activities. Computer-based automatic event detection is preferable to spare time and cost of the evaluation. The authors review recent progress concerning epidemiology, neurophysiology and clinical impact of well-known epileptiform transients and candidate epileptiform activities in neurological and psychiatric conditions. However, recent results need confirmation in large patient populations; therefore, research should not be restricted to a few central laboratories.]

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[EEG source localization using LORETA (Low Resolution Electromagnetic Tomography)]

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[Electroencephalography (EEG) has excellent temporal resolution, but the spatial resolution is poor. Different source localization methods exist to solve the so-called inverse problem, thus increasing the accuracy of spatial localization. This paper provides an overview of the history of source localization and the main categories of techniques are discussed. LORETA (Low Resolution Electromagnetic Tomography) is introduced in details: technical informations are discussed and localization properties of LORETA method are compared to other inverse solutions. Validation of the method with different imaging techniques is also discussed. This paper reviews several publications using LORETA both in healthy persons and persons with different neurological and psychiatric diseases. Finally future possible applications are discussed.]

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[EEG-based cerebral networks in 14 neurological disorders]

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[Background - Brain networks have not been systematically investigated yet in most neurological disorders. Purpose - To investigate EEG functional connectivity (EEGfC) networks in 14 neurological disorders. Patients - Potentially eligible patients were collected from clinical and EEG databases. All the available clinical data and EEG records were critically revised. All the patients who suffered of a single neurological disorder (out of the 14) and had a good quality EEG recording entered the study. Confoundig factors as comorbidity and CNS-active drug effects were eliminated as far as possible. EEG analysis - Three minutes of resting-state, waking EEG activity were selected for analysis. Current source density (CSD) values were computed for 2394 cortical voxels by Low Resolution Electromagnetic Tomography (LORETA). Thereafter, Pearson correlation coefficients were computed between all pairs of 23 cortical regions of interest (ROI) in each hemisphere (LORETA Source Correlation, LSC software). Computation was carried out for conventional EEG broad bands and very narrow bands (1 Hz bandwidth) between 1 and 25 Hz as well. Correlation coefficients of each group were statistically compared to our normative EEG (LSC) database by two-talied t-tests. Bonferroni-corrected p<0.05 values were accepted as statistically significant, and were graphically displayed as topographical networks. Results and conclusion - Group-specific networks were demonstrated. However, non-specific networks, charasteristic for most groups, were detected as well. Common finding were: decreased connectivity in the alpha band and increased connectivity in the delta, theta bands and upper-beta band. Decreased alpha-band connectivity presumably reflected primary lesional effects and on the other hand, non-specific vulnerability of “rich club connections”. Increased connectivity in the slow bands presumably indicated adaptive-compensatory activity of brain homeostasis. ]

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[Individual evaluation of loreta abnormalities in idiopathic generalized epilepsy]

CLEMENS Béla, PUSKÁS Szilvia, BESENYEI Mónika, KONKÁDOR István, HOLLÓDY Katalin, FOGARASI András, BENSE Katalin, EMRI Miklós, OPPOSITS Gábor, KOVÁCS Noémi Zsuzsanna, FEKETE István

[Background – Contemporary neuroimaging methods disclosed structural and functional cerebral abnormalities in idiopathic generalized epilepsies (IGEs). However, individual electrical (EEG) abnormalities have not been evaluated yet in IGE patients. IGE patients were investigated in the drug-free condition and after 3-6 month of antiepileptic treatment. To estimate the reproducibility of qEEG variables a retrospective recruited cohort of IGE patients was investigated. 19- channel resting state EEG activity was recorded. For each patient a total of 2 minutes EEG activity was analyzed by LORETA (Low Resolution Electromagnetic Tomography). Raw LORETA values were Z-transformed and projected to a MRI template. Z-values outside within the [+] 1. In drug-free condition, 41-50% of IGE patients showed abnormal LORETA values. 2. Abnormal LORETA findings showed great inter-individual variability. 3. Most abnormal LORETA-findings were symmetrical. 4. Most maximum Z-values were localized to frontal or temporal cortex. 5. Succesfull treatment was mostly coupled with disappearence of LORETA-abnormality, persistent seizures were accompanied by persistent LORETA abnormality. 1. LORETA abnormalities detected in the untreated condition reflect seizure-generating property of the cortex in IGE patients. 2. Maximum LORETA-Z abnormalities were topographically congruent with structural abnormalities reported by other research groups. 3. LORETA might help to investigate drug effects at the whole-brain level.]