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ABSTRACT Magnetoencephalography (MEG) is a neurophysiological technique that measures the magnetic fields associated with neuronal activity in the brain. It is closely related but distinct from its counterpart electroencephalography (EEG). The first MEG was recorded more than 50 years ago and has technologically evolved over this time. It is now well established in clinical practice particularly in the field of epilepsy surgery and functional brain mapping. However, underutilization and misunderstanding of the clinical applications of MEG is a challenge to more widespread use of this technology. A fundamental understanding of the neurophysiology and physics of MEG is discussed in this article as well as practical issues related to implementation, analysis, and clinical applications. The future of MEG and some potential clinical applications are briefly reviewed.
RESUMO A magnetoencefalografia (MEG) é uma técnica neurofisiológica que mede os campos magnéticos associados à atividade neuronal no cérebro. A técnica é distinta da eletroencefalografia (EEG), porém está intimamente relacionada a ela. A primeira foi registrada há mais de 50 anos e evoluiu tecnologicamente ao longo do tempo. Está agora bem estabelecida na prática clínica, particularmente nos campos da cirurgia de epilepsia e mapeamento cerebral funcional. No entanto, a subutilização e a incompreensão das aplicações clínicas da MEG são um desafio para o uso mais amplo dessa tecnologia. Suas bases neurofisiológica e física são discutidas neste artigo, bem como questões práticas relacionadas à sua implementação, análise e aplicações clínicas. O futuro da MEG e algumas aplicações clínicas potenciais são brevemente revistos.
ABSTRACT
La magnetoencefalografía es una técnica de neuroimagen no invasiva que mide, con gran exactitud temporal, los campos magnéticos en la superficie de la cabeza producidos por corrientes neuronales en regiones cerebrales. Esta técnica es sumamente útil en la investigación básica y clínica, porque además permite ubicar el origen de la actividad neural en el cerebro. En esta revisión se abordan aspectos básicos de la biofísica del método y se discuten los hallazgos sobre procesos como la percepción del habla, la atención auditiva y la integración de la información visual y auditiva, que son importantes en la investigación. Igualmente, se ilustran sus ventajas, sus limitaciones y las nuevas tendencias en la investigación con magnetoencefalografía.
Magnetoencephalography is a noninvasive imaging technique that measures the magnetic fields on the surface of the head --produced by neuronal currents in brain regions -- and provides highly accurate temporal information. Magnetoencephalography is extremely useful in basic and clinical research as it can also locate the sources of neural activity in the brain. This review chiefly approaches biophysics-related aspects of the method; findings are also discussed on issues such as speech perception, auditory attention and integration of visual-auditory information, which are quintessential in this type of research. Lastly, this review discusses the benefits and limitations of magnetoencephalography and outlines new trends in research with this technique.
ABSTRACT
In this study, we examined whether luminance processing in the human visual system would exhibit any history effect (i.e., inter-trial modulation) in psychophysical and magnetoencephalographic experiments. A disk was presented against a black background at various luminance levels in a randomized order. During the MEG recording, participants were instructed to rate the brightness of the disk (magnitude estimation) and to report it aloud during inter-stimulus interval. The MEG results showed that the neuromagnetic activation around 200-220 ms after the stimulus onset in the left occipito-temporal regions at a given trial was weaker when the disk luminance in the immediately prior trial was higher. An inverse inter-trial effect was also observed in the psychophysical experiment. These findings suggest that the neuromagnetic activity reflects the inter-trial modulation of luminance processing that correlates with the subjective perception of brightness.
En este estudio, se examinó si el procesamiento de iluminación en el sistema visual humano exhibie algún efecto de historia (es decir, modulación inter-ensayo) en experimentos psicofísicos y de magnetoencefalografía (MEG). Un disco se presentó contra un fondo negro en varios niveles de iluminación en un orden aleatorio. Durante el registro de MEG, los participantes fueron instruidos para clasificar el brillo del disco (estimación de magnitud) y reportarlo durante el intervalo inter-ensayo. Los resultados de MEG mostraron que la activación neuromagnetica alrededor 200-220 ms después de la aparición de estímulo en las regiones occipito-temporal izquierda en un ensayo dade fue más débil cuando la iluminación de disco en el ensayo inmediatamente antes fue mayor. También se observó un efecto inverso inter-ensayo en el experimento psicofísico. Estos hallazgos sugieren que la actividad neuromagnética refleja la modulación inter-ensayo de procesamiento de iluminación que se correlaciona con la percepción subjetiva de brillo.
Subject(s)
Perception , MagnetoencephalographyABSTRACT
Abstract ? The lifelong acquisition of cognitive skills shapes the biology of the brain. However, there are critical periods for the best use of the brain to process the acquired information. Objectives: To discuss the critical period of cognitive acquisition, the concept of cognitive reserve and the HAROLD (Hemispheric Asymmetry Reduction in Older adults) model. Methods: Seven women who learned how to read and to write after the age of 50 (ex-illiterates) and five women with 10 years of regular schooling (controls) were submitted to a language recognition test while brain activity was being recorded using magnetoencephalography. Spoken words were delivered binaurally via two plastic tubs terminating in ear inserts, and recordings were made with a whole head magnetometer consisting of 148 magnetometer coils. Results: Both groups performed similarly on the task of identifying target words. Analysis of the number of sources of activity in the left and right hemispheres revealed significant differences between the two groups, showing that ex-illiterate subjects exhibited less brain functional asymmetry during the language task. Conclusions: These results should be interpreted with caution because the groups were small. However, these findings reinforce the concept that poorly educated subjects tend to use the brain for information processing in a different way to subjects with a high educational level or who were schooled at the regular time. Finally, the recruiting of both hemispheres to tackle the language recognition test occurred to a greater degree in the ex-illiterate group where this can be interpreted as a sign of difficulty performing the task.
Resumo ? A aquisição ao longo da vida de competências cognitivas moldam a biologia do cérebro. No entanto, existem períodos críticos para o cérebro processar melhor as informações adquiridas. Objetivos: Discutir o período crítico de aquisição cognitiva, o conceito de reserva cognitiva e o modelo HAROLD (Redução da Assimetria Hemisférica em Idosos). Métodos: Sete mulheres que aprenderam a ler e escrever após a idade de 50 anos (ex-analfabetos) e cinco mulheres com 10 anos de escolaridade regular (controles) foram submetidas a um teste de reconhecimento de palavras enquanto a atividade cerebral estava sendo registrada mediante magnetoencefalografia. As palavras foram ouvidas com o emprego de dois tubos plásticos conectados a cada orelha e as gravações foram feitas com um magnetômetro de cabeça inteira com 148 bobinas de registro. Resultados: Ambos os grupos tiveram desempenho semelhante na identificação das palavras-alvo. Análise do número de fontes de atividade no hemisfério esquerdo e no hemisfério direito revelou diferenças significativas entre os dois grupos, mostrando que os ex-analfabetos tiveram menor assimetria cerebral funcional no desempenho da tarefa. Conclusões: Estes resultados devem ser interpretados com cautela, pois os grupos são pequenos. No entanto, reforçam o conceito que os indivíduos com baixa escolaridade tendem a usar o cérebro para processar informações de uma forma diferente do que indivíduos com nível educacional elevado ou que tenham o adquirido na época regular. Finalmente, o recrutamento de ambos os hemisférios para reconhecer as palavras ocorreu mais intensamente no grupo de ex-analfabetos e pode ser interpretado como um sinal de maior dificuldade na tarefa.
Subject(s)
Humans , Magnetoencephalography , Cognitive Reserve , Literacy , LanguageABSTRACT
El síndrome Dificultades del Aprendizaje (DA) fue descrito en 1963 por S. Kirk. Desde entonces, diversas escuelas en EE.UU., Canadá y España han afinado su concepto y clasificación. La UCM en España ha propuesto una definición descriptiva y totalizadora, y ha estudiado empíricamente distintas manifestaciones, intentando descubrir sus marcadores biológicos y las características neurológicas de sus principales manifestaciones (dislexia, discalculia, disortografia, TDA, TDAH, etc.). Se describen los hallazgos en DA a partir de estudios como la magnetoencefalografía (MEG), técnica inocua que recoge campos magnéticos generados naturalmente por el cerebro y analiza su distribución espacial para localizar los generadores neuronales responsables, proporcionando información simultánea sobre la estructura y la función cerebral en patrones de normalidad en el procesamiento cognitivo y patrones aberrantes propios de las particulares manifestaciones clínicas del síndrome DA.
The syndrome known as Learning Disabilities (LD) was described by S. Kirk in 1963. From that point on, institutions from the US, Canada and Spain have engaged in refining the concept and classification of LDs. The Complutense University in Spain, has proposed a descriptive and all-embracing definition, and has studied the different manifestations of LD, pursuing the description of biological markers and neurological features of LDs main expressions: dyslexia, dyscalculia, dysorthographia, Attention Deficit and Hyperactivity Disorder ADHD, and so forth. Findings in LD using functional neuroimaging techniques, namely Magnetoencephalography (MEG), are described. MEG is a non-invasive technique, which records magnetic fields naturally generated by the brain and their spatial distribution. It allows simultaneous functional and structural information. MEG is therefore used in the study of primary and superior cognitive functions, in surveillance of patterns of normal cognitive function and those specific to the different LD clinical manifestations.