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Ressonância Magnética Funcional (RMf): princípios e aplicações para a neuropsicologia e neurociências
Abstract
fMRI is a very important tool both in the field of Clinical Neuropsychology and Cognitive Neurocience. In the clinical domain, it is mostly used to locate functional regions such as those associated with motor or language processes, although new applications are being developed. Its use in research has permitted the confirmation, extension and rejection of theoretical models about brain functions. The rapid growth in the use of this technique in both clinical environment and in research, together with its fast development, generates the need to understand the advantages and limits of this technique in order to allow for more appropriate and objective conclusions. To achieve so, it is vital to properly understand how the images are generated and analyzed, in order to avoid some common mistakes regarding their interpretation. Thus, the main objective of this article is to present a general yet detailed review of the various steps involved in the design, acquisition, analysis and interpretation of fMRI experiments.
... Estas distintas propiedades conllevan a que se generen diferencias en cuanto a las inhomogeneidades en el campo magnético local de las zonas en las que hay consumo de oxígeno y en aquellas en las que no, lo cual afecta al tiempo de relajación transversal llamado T2*. De este modo, si se compara la señal (proporcional al tiempo de relajación T2*) en una misma región bajo dos condiciones, una de reposo y otra en la que esa región está activa, se encuentran distintos valores (Armony, Trejo-Martínez, & Hernández, 2012;Poldrack, Mumford, & Nichols, 2011). ...
... Estas distintas propiedades conllevan a que se generen diferencias en cuanto a las inhomogeneidades en el campo magnético local de las zonas en las que hay consumo de oxígeno y en aquellas en las que no, lo cual afecta al tiempo de relajación transversal llamado T2*. De este modo, si se compara la señal (proporcional al tiempo de relajación T2*) en una misma región bajo dos condiciones, una de reposo y otra en la que esa región está activa, se encuentran distintos valores (Armony, Trejo-Martínez, & Hernández, 2012;Poldrack, Mumford, & Nichols, 2011). El resonador magnético posibilita de esta manera un registro indirecto de la actividad neuronal a partir de sus demandas metabólicas de oxígeno. ...
La resonancia magnética funcional es una de las técnicas de neuroimagen más difundidas en las neurociencias cognitivas. Su influencia tuvo un rol importante en la configuración del costado experimental de este campo. Frente a esto, consideramos que su estatus como evidencia no sido suficientemente discutido en la literatura filosófica. Aquí nos centramos sobre este punto desde la óptica de la tensión clásica sobre el alcance de los enfoques localizacionistas en neurociencias. Atendemos al modo en que esta tensión se manifiesta hoy. Tomamos en cuenta un número de limitaciones que presenta la resonancia magnética funcional y analizamos el modo en que los investigadores les hacen frente. En función de esto, sostenemos que existe una medida importante en que la técnica puede ser orientada sobre la base de supuestos y consideraciones teóricas generales. Para sustentar nuestras ideas, tomamos algunos ejemplos recientes de abordajes neurocientíficos caracterizados por abordar de manera directa el carácer dinámico de la actividad a gran escala en el cerebro.
... Se han tenido nuevos alcances científicos para la comprensión del desarrollo cognitivo del ser humano; por ejemplo, estudios que usan las técnicas de imágenes cerebrales como resonancia magnética han dado más evidencia a la neurociencia cognitiva (Armony et al., 2012). Los estudios señalan que el desarrollo adecuado de las funciones ejecutivas está ligado a la creatividad (Ramirez-Villen et al., 2017). ...
... Se han tenido nuevos alcances científicos para la comprensión del desarrollo cognitivo del ser humano; por ejemplo, estudios que usan las técnicas de imágenes cerebrales como resonancia magnética han dado más evidencia a la neurociencia cognitiva (Armony et al., 2012). Los estudios señalan que el desarrollo adecuado de las funciones ejecutivas está ligado a la creatividad (Ramirez-Villen et al., 2017). ...
... Accordingly, there has been an exponential increase in methods for the study of neuronal activity arising from modern techniques such as electroencephalography (EEG), magnetoencephalography (MEG), and magnetic resonance imaging (MRI) (Rossini et al., 2019). MRI is widely used for its non-invasive nature, for not having ionizing radiation, and for its high spatial resolution (Armony et al., 2012). Besides, the approach to the resting-state paradigm, known as resting state functional magnetic resonance imaging (rs-fMRI), has delivered good results in terms of detecting neural circuits, as is the case of the default mode network (Raichle, 2015) that allows functional differences in complex pathologies like Alzheimer's (Qian et al., 2019) or Parkinson's (Franciotti et al., 2019) to be detected, amongst other applications (Lang et al., 2014;Pearlson, 2017). ...
In the last decade, neurosciences have had an increasing interest in resting state functional magnetic resonance imaging (rs-fMRI) as a result of its advantages, such as high spatial resolution, compared to other brain exploration techniques. To improve the technique, the elimination of artifacts through Independent Components Analysis (ICA) has been proposed, as this can separate neural signal and noise, opening possibilities for automatic classification. The main classification techniques have focused on processes based on typical machine learning. However, there are currently more robust approaches such as convolutional neural networks, which can deal with complex problems directly from the data without feature selection and even with data that does not have a simple interpretation, being limited by the amount of data necessary for training and its high computational cost. This research focused on studying four methods of volume reduction mitigating the computational cost for the training of 3 models based on convolutional neural networks. One of the reduction techniques is a novel approach that we call Reduction by Consecutive Binary Patterns (RCBP), which was shown to preserve the spatial features of the independent components. In addition, the RCBP showed networks in components associated with neuronal activity more clearly. The networks achieved accuracy above 98 % in classification, and one network was even found to be over 99 % accurate, outperforming most machine learning-based classification algorithms.
... En este sentido, mientras más se enfrente el algoritmo a los casos en estudio, mayor precisión logrará 47 , de manera similar a lo que ocurre con el especialista. Es por este motivo que, antes de pensar en las limitaciones propias de cada técnica de imagen, deben considerarse otros factores asociados al procesamiento de los datos y su interpretación, tales como la duración y condiciones que permitan comparar los estudios o el tipo de software utilizado 49 . ...
Las crecientes cifras mundiales de prevalencia e incidencia de la Enfermedad de Alzheimer exigen el desarrollo de métodos diagnósticos cada vez más precoces. La enfermedad suele diagnosticarse cuando la patología ya está avanzada y poco se puede hacer terapéuticamente, conllevando una gran pérdida de años de vida y altos costos sociales y familiares. Considerando esta patología como una disrupción a gran escala de las redes neuronales del cerebro humano, distintos estudios han propuesto biomarcadores basados en resonancia magnética y tomografía por emisión de positrones. El presente artículo revisa sistemáticamente dichos estudios considerando un abordaje desde la ciencia de redes neuronales.
The increased global prevalence and incidence of Alzheimer’s Disease demands the development of early diagnostic methods. This disease is usually diagnosed when the pathology is already advanced and therapeutically, there’s not much to do, leading to a great loss of years of life, high socials and family costs. Considering this pathology as a large-scale disruption of neural networks of the human brain, different studies have proposed biomarkers based on functional magnetic resonance imaging and positron emission tomography. This article systematically reviews these studies considering an approach of neural networks science.
... Entre mayor sea el nivel educativo y por ende la reserva cognitiva se observará mayor actividad en el estado de reposo (48). Tercero, hay que tener en cuenta que el hecho de que un área se active durante la ejecución de una tarea no implica que esa área sea indispensable para la tarea en cuestión (49). Cuarto, inversamente, el que no se activen algunas áreas esperadas durante la ejecución de una tarea, no permite asumir que no participan en el proceso; es posible que participen pero que esto no sea capturado (50). ...
To highlight the capacity of functional magnetic resonance imaging (fMRI) with tasks in order to measure different higher brain functions by running paradigms that produce brain activation in the regions involved. Highlighting the importance of the neuropsychologist in creating paradigms and interpreting results. A non-systematic review of the scientific literature contained in the databases was conducted: Rev Neurol, Neurology, Radiologia, Neuroimage, J Neuroimaging, Science, Brain, Neuroscience and biobehavioral reviews, journal of neuroscience, Eur J Radiol, Magnetic Resonance in Medicine, Neurosurgery, Neuroimagingclin, Neuropsicologia Latinoamericana, International Journal of Neuroscience, Biol Psychiatry, Psychol Med, Arch Gen Psychiatry, Psychiatry Res Neuroimaging, Neuro Report, Neuron, J ClinExpNeuropsychol , Proc Natl Acad Sci U S A, Ann Neurol Neurobiol Aging, Neurosci Lett, Journal of Neuroscience. The descriptors used were: "functional MRI", "Paradigms" and "Neuropsychology". Papers in Spanish and English of any kind were selected since the start of indexing the primary source until November 2014. 42 articles were retrieved. The following concepts were analyzed: functional magnetic resonance imaging, neuropsychology, higher brain functions, activated brain areas paradigms. The neural activation maps confirm the simultaneous involvement of different brain areas, even distant ones, during the execution of paradigms. Neuropsychologist participation within the multidisciplinary team is very important for its deep understanding of the factors involved in the performance of different cognitive tasks potentially assessable by fMRI.
... Em estudo recente de revisão teórica da literatura, Ramos e Hamdan (2014) explicitam que os dados de neuroimagem, quando interpretados de forma equivocada ou utilizados inapropriadamente, podem contribuir para a consolidação de mitos sobre como o cérebro humano funciona, principalmente quando se considera que o resultado final da neuroimagem é uma "foto estática" da atividade cerebral. Por outro lado, enquanto técnica que permite identificar áreas cerebrais envolvidas no desempenho de um determinado processo mental, a Ressonância Magnética Funcional (RMf ), por exemplo, é um método complementar no processo de ANP que permite identificar a localização de tumores cerebrais, averiguar a integridade ou prejuízo de uma função em caso de lesão hemisférica e proporcionar "informação sobre estratégias ou mecanismos compensatórios na execução de uma tarefa cognitiva" (Armony, Trejo-Martínez, & Hernández, 2012). Por sua vez, a ANP procura esclarecer correlatos comportamentais de lesões detectadas por neuroimagem, estabelecer o prognóstico do paciente e averiguar a existência de distúrbios cognitivos, principalmente em seus estágios iniciais, quando ainda não há um quadro neurológico evidente (Capovilla, 2007). ...
Pesquisas científicas relacionadas à avaliação neuropsicológica têm despertado interesse crescente nos últimos anos. Este estudo objetivou realizar uma revisão sistemática sobre avaliação neuropsicológica no Brasil e foi efetuado nas bases de dados SciELO, PePSIC, LILACS e BDTD, no período de setembro/2012 a novembro/2012, utilizando as palavras-chave avaliação neuropsicológica. A amostra final resultou em 241 trabalhos empíricos originais formados por artigos científicos (n=131), dissertações de mestrado (n=68) e teses de doutorado (n=42), abrangendo o período de 1993 a 2012. Os resultados evidenciaram que: a) mais de 60% da produção científica concentra-se no período de 2007 a 2012, com média de produção de 12,7% por ano; b) o número de artigos científicos soma pouco mais de 50% se comparado com o número de teses e dissertações; c) a publicação de pesquisas sobre a temática se dá majoritariamente em periódicos médicos (47,7%); d) o principal eixo temático que emprega Avaliação Neuropsicológica são distúrbios neuropsiquiátricos (49,7%); e) a Região Sudeste concentra mais de 50% da produção científica nacional. Conclui-se que esta pesquisa revela um panorama atualizado da produção científica sobre o tema, enfatizando a necessidade de mais pesquisas psicométricas e de outras revisões sistemáticas envolvendo temas de interesse da neuropsicologia, como reabilitação neuropsicológica.
En este libro nos hemos propuesto explicar qué es el análisis cualitativo utilizado en el diagnóstico clínico neuropsicológico y el desarrollo psicológico. Incluye sus fundamentos teóricos, metodológicos, epistemológicos y prácticos. Explicamos cómo
se realiza la evaluación cualitativa, los aspectos prácticos en su implementación, las diferencias fundamentales con la medición de tareas específicas propias de las pruebas psicométricas (test), o la cuantificación de signos y síntomas de las escalas clínicas usadas en psicología, psiquiatría, y psicopedagogía, así como en el ámbito judicial- forense. Argumentamos la importancia de distinguir el diagnóstico clínico, de la medición psicométrica que, cuando se confunden, se reduce el diagnóstico a la medición psicométrica llevando al error de creer que se hace diagnóstico clínico con medir qué tan diferente es la persona de la norma de una muestra poblacional en tal o cual tarea, o conjunto de ellas. La psicometría mide y cuantifica, pero no diagnostica.
El presente trabajo efectúa una reflexión crítica sobre el delito de incitación al odio como tipo delictivo autónomo, pues son los efectos discriminatorios aquello que, en atención a los principios que informan el Derecho penal, se debería atender en esta categoría delictiva. Estos comportamientos discriminatorios, más que fundamentados en un sentimiento como es el odio, como los últimos estudios de neuroimagen señalan, descansan en la asunción de prejuicios estereotipados capaces de ser controlados.
La resonancia magnética funcional en estado de reposo (rs-fMRI) es una de las técnicas más relevantes en exploración cerebral. No obstante, la misma es susceptible a muchos factores externos que pueden ocluir la señal de interés. En este orden de ideas, las imágenes rs-fMRI han sido estudiadas desde diferentes enfoques, existiendo un especial interés en las técnicas de eliminación de artefactos a través del Análisis de Componentes Independientes (ICA por sus siglas en inglés). El enfoque es una herramienta poderosa para la separación ciega de fuentes donde es posible eliminar los elementos asociados a ruido. Sin embargo, dicha eliminación está sujeta a la identificación o clasificación de las componentes entregadas por ICA. En ese sentido, esta investigación se centró en encontrar una estrategia alternativa para la clasificación de las componentes independientes. El problema se abordó en dos etapas. En la primera de ellas, se redujeron las componentes (volúmenes 3D) a imágenes mediante el Análisis de Componentes Principales (PCA por sus siglas en inglés) y con la obtención de los planos medios. Los métodos lograron una reducción de hasta dos órdenes de magnitud en peso de los datos y, además, demostraron conservar las características espaciales de las componentes independientes. En la segunda etapa, se usaron las reducciones para entrenar seis modelos de redes neuronales convolucionales. Las redes analizadas alcanzaron precisiones alrededor de 98 % en la clasificación e incluso se encontró una red con una precisión del 98.82 %, lo cual refleja la alta capacidad de discriminación de las redes neuronales convolucionales.
Los paradigmas son las tareas experimentales que se usan para mapear el cerebro a través de Resonancia Magnética funcional. Su validación previa es fundamental para garantizar un registro exitoso de la actividad cognitiva a investigar. Se validó un paradigma de razonamiento abstracto formado por una tarea de analogías semánticas y una tarea de analogías visuales en jóvenes-adultos entre 18 y 30 años de edad de la ciudad de Cuenca-Ecuador. Se programó el paradigma en el software libre PsychoPy 3 para experimentos de neurociencia. Los resultados indicaron que el paradigma fue comprensible y sencillo de resolver. También se halló que el tiempo promedio de respuesta en la tarea de razonamiento semántico fue significativamente menor al tiempo promedio de resolución de la tarea de razonamiento visual. Por lo tanto, se pudo establecer el número de estímulos necesarios y su tiempo promedio de exposición para garantizar la potencia de contraste y significancia estadística al utilizarlo en un Resonador Magnético.
La Resonancia Magnética Funcional (RMF) constituye una poderosa herramienta de diagnóstico por imágenes no invasivas, que permite detectar cambios en el cerebro cuando se realizan de-terminadas tareas. De esta forma se pueden estudiar áreas del cerebro humano como el lenguaje, motricidad, etc.
Functional magnetic resonance imaging is one of the most commonly used neuroimaging techniques in cognitive neuroscience. Its influence had a central role in establishing the experimental side of the field. Given this, we consider that its status as a source of evidence has not been sufficiently dealt within the philosophical literature. We focus on this issue from the standpoint of the classical problem of defining the scope of localizationist approaches in neuroscience. We attend to the way this tension unfolds today, considering some recent examples of neuroscientific approaches that tackle the dynamic character of the brain's large scale activity. We take into account a number of limitations that functional magnetic resonance imaging presents, distinguishing those of them whose treatment involves not merely technical issues. On the basis of an analysis of some ways researchers deal with them, we claim that there is a considerable extent in which this kind of neuroimaging studies can be oriented according to general assumptions and theoretical considerations. We conclude that this particular theoretical permeability is a main factor affecting the technique's status as neuroscientific evidence.
Attention deficit disorder (ADD) refers to a neurobehavioral condition commonly initiated in childhood. Its clinical diagnosis involves poor attention, distractibility, difficulties to inhibit motor behaviors and cognitive impulsivity.
Background: Speech‐language pathologists are meeting an increasing number of bilingual clients. This poses a special challenge to clinical practice, given that bilingualism adds to the complexity of aphasia patterns and clinical decisions must be made accordingly. One question that has come to the attention of clinical aphasiologists is that of the language in which therapy should be administered. This issue becomes particularly relevant in cases of involuntary language switching, when choosing between L1 and L2 implies inhibiting one of the languages. Models of lexical selection in bilingual people offer a rationale for language choice based on the specificities of bilingual aphasia within each client.Aims: To provide evidence for model‐based intervention in bilingual aphasia, particularly in cases of pathological language switching.Methods & Procedures: This paper reports a model‐driven intervention in a case of involuntary language switching following aphasia in a Spanish–English bilingual client.Outcomes & Results: Intervention tailored to the client's strengths resulted in improved communication skills thanks to the implementation of a self‐regulated strategy to overcome involuntary language switching.Conclusions: Model‐driven descriptions of bilingual aphasia contribute to efficient intervention by identifying therapy approaches that take account of each client's language abilities. Further, clinical data analysed within models of bilingual language processing can provide evidence for dissociations between components of the bilingual lexical system.
Over the past 20 years, neuroimaging has become a predominant technique in systems neuroscience. One might envisage that over the next 20 years the neuroimaging of distributed processing and connectivity will play a major role in disclosing the brain's functional architecture and operational principles. The inception of this journal has been foreshadowed by an ever-increasing number of publications on functional connectivity, causal modeling, connectomics, and multivariate analyses of distributed patterns of brain responses. I accepted the invitation to write this review with great pleasure and hope to celebrate and critique the achievements to date, while addressing the challenges ahead.
Patients surviving severe brain injury may regain consciousness without recovering their ability to understand, move and communicate. Recently, electrophysiological and neuroimaging approaches, employing simple sensory stimulations or verbal commands, have proven useful in detecting higher order processing and, in some cases, in establishing some degree of communication in brain-injured subjects with severe impairment of motor function. To complement these approaches, it would be useful to develop methods to detect recovery of consciousness in ways that do not depend on the integrity of sensory pathways or on the subject's ability to comprehend or carry out instructions. As suggested by theoretical and experimental work, a key requirement for consciousness is that multiple, specialized cortical areas can engage in rapid causal interactions (effective connectivity). Here, we employ transcranial magnetic stimulation together with high-density electroencephalography to evaluate effective connectivity at the bedside of severely brain injured, non-communicating subjects. In patients in a vegetative state, who were open-eyed, behaviourally awake but unresponsive, transcranial magnetic stimulation triggered a simple, local response indicating a breakdown of effective connectivity, similar to the one previously observed in unconscious sleeping or anaesthetized subjects. In contrast, in minimally conscious patients, who showed fluctuating signs of non-reflexive behaviour, transcranial magnetic stimulation invariably triggered complex activations that sequentially involved distant cortical areas ipsi- and contralateral to the site of stimulation, similar to activations we recorded in locked-in, conscious patients. Longitudinal measurements performed in patients who gradually recovered consciousness revealed that this clear-cut change in effective connectivity could occur at an early stage, before reliable communication was established with the subject and before the spontaneous electroencephalogram showed significant modifications. Measurements of effective connectivity by means of transcranial magnetic stimulation combined with electroencephalography can be performed at the bedside while by-passing subcortical afferent and efferent pathways, and without requiring active participation of subjects or language comprehension; hence, they offer an effective way to detect and track recovery of consciousness in brain-injured patients who are unable to exchange information with the external environment.
Functional magnetic resonance imaging (fMRI) is one of the most widely used tools to study the neural underpinnings of human cognition. Standard analysis of fMRI data relies on a general linear model (GLM) approach to separate stimulus induced signals from noise. Crucially, this approach relies on a number of assumptions about the data which, for inferences to be valid, must be met. The current paper reviews the GLM approach to analysis of fMRI time-series, focusing in particular on the degree to which such data abides by the assumptions of the GLM framework, and on the methods that have been developed to correct for any violation of those assumptions. Rather than biasing estimates of effect size, the major consequence of non-conformity to the assumptions is to introduce bias into estimates of the variance, thus affecting test statistics, power, and false positive rates. Furthermore, this bias can have pervasive effects on both individual subject and group-level statistics, potentially yielding qualitatively different results across replications, especially after the thresholding procedures commonly used for inference-making.
Ability to non-invasively map the hemodynamic changes occurring focally in areas of brain involved in various motor, sensory and cognitive functions by functional magnetic resonance imaging (fMRI) has revolutionized research in neuroscience in the last two decades. This technique has already gained clinical use especially in pre-surgical evaluation of epilepsy and neurosurgical planning of resection of mass lesions adjacent to eloquent cortex. In this review we attempt to illustrate basic principles and techniques of fMRI, its applications, practical points to consider while performing and evaluating clinical fMRI and its limitations.
Focal brain lesions are assumed to produce language deficits by two basic mechanisms: local cortical dysfunction at the lesion site, and remote cortical dysfunction due to disruption of the transfer and integration of information between connected brain regions. However, functional imaging studies investigating language outcome after aphasic stroke have tended to focus only on the role of local cortical function. In this positron emission tomography functional imaging study, we explored relationships between language comprehension performance after aphasic stroke and the functional connectivity of a key speech-processing region in left anterolateral superior temporal cortex. We compared the organization of left anterolateral superior temporal cortex functional connections during narrative speech comprehension in normal subjects with left anterolateral superior temporal cortex connectivity in a group of chronic aphasic stroke patients. We then evaluated the language deficits associated with altered left anterolateral superior temporal cortex connectivity in aphasic stroke. During normal narrative speech comprehension, left anterolateral superior temporal cortex displayed positive functional connections with left anterior basal temporal cortex, left inferior frontal gyrus and homotopic cortex in right anterolateral superior temporal cortex. As a group, aphasic patients demonstrated a selective disruption of the normal functional connection between left and right anterolateral superior temporal cortices. We observed that deficits in auditory single word and sentence comprehension correlated both with the degree of disruption of left-right anterolateral superior temporal cortical connectivity and with local activation in the anterolateral superior temporal cortex. Subgroup analysis revealed that aphasic patients with preserved positive intertemporal connectivity displayed better receptive language function; these patients also showed greater than normal left inferior frontal gyrus activity, suggesting a possible 'top-down' compensatory mechanism. These results demonstrate that functional connectivity between anterolateral superior temporal cortex and right anterior superior temporal cortex is a marker of receptive language outcome after aphasic stroke, and illustrate that language system organization after focal brain lesions may be marked by complex signatures of altered local and pathway-level function.
En este artículo se muestran las nuevas tendencias en la evaluación del trastorno por déficit de atención con hiperactividad (TDAH). Partiendo de la valoración más tradicional basada en estimaciones comportamentales realizadas en contextos naturales por padres y profesores, pasando por la evaluación neuropsicológica de las funciones ejecutivas y planteando los retos de la Resonancia Magnética Funcional (RMf).
Traumatic brain injury (TBI) patients have working memory deficits and altered patterns of brain activation during this function. The evolution of the impairment has not been examined to date. This study investigated longitudinal changes in brain activation during a working memory task. Twelve patients with severe and diffuse TBI and ten healthy matched controls were fMRI scanned twice at a 6-month interval during an n-back task (0-, 2- and 3-back). All the TBI patients selected presented signs of diffuse axonal injury on CT but had no evidence of focal lesions on MRI clinical examination. Significant changes in brain activation over time were observed in patients, but not in controls. During the first examination, though both groups engaged bilateral fronto-parietal regions known to be involved in working memory, activation of the right superior frontal gyrus was low in the TBI group. However, the difference between TBI and controls had decreased significantly after 6 months. A factor analysis confirmed the greater increase in activation in the right superior frontal cortex in the TBI group than in healthy controls, leading to normalization of the brain activation pattern. In conclusion, this longitudinal study provides evidence of a progressive normalization of the working memory activation pattern after diffuse axonal injury in severe TBI, coinciding with an improvement in performance on this function.
Neuronal activity causes local changes in cerebral blood flow, blood volume, and blood oxygenation. Magnetic resonance imaging (MRI) techniques sensitive to changes in cerebral blood flow and blood oxygenation were developed by high-speed echo planar imaging. These techniques were used to obtain completely noninvasive tomographic maps of human brain activity, by using visual and motor stimulus paradigms. Changes in blood oxygenation were detected by using a gradient echo (GE) imaging sequence sensitive to the paramagnetic state of deoxygenated hemoglobin. Blood flow changes were evaluated by a spin-echo inversion recovery (IR), tissue relaxation parameter T1-sensitive pulse sequence. A series of images were acquired continuously with the same imaging pulse sequence (either GE or IR) during task activation. Cine display of subtraction images (activated minus baseline) directly demonstrates activity-induced changes in brain MR signal observed at a temporal resolution of seconds. During 8-Hz patterned-flash photic stimulation, a significant increase in signal intensity (paired t test; P less than 0.001) of 1.8% +/- 0.8% (GE) and 1.8% +/- 0.9% (IR) was observed in the primary visual cortex (V1) of seven normal volunteers. The mean rise-time constant of the signal change was 4.4 +/- 2.2 s for the GE images and 8.9 +/- 2.8 s for the IR images. The stimulation frequency dependence of visual activation agrees with previous positron emission tomography observations, with the largest MR signal response occurring at 8 Hz. Similar signal changes were observed within the human primary motor cortex (M1) during a hand squeezing task and in animal models of increased blood flow by hypercapnia. By using intrinsic blood-tissue contrast, functional MRI opens a spatial-temporal window onto individual brain physiology.
We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce high-spatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T*2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.
The role of the amygdala in enhancing declarative memory for emotional experiences has been investigated in a number of animal, patient, and brain imaging studies. Brain imaging studies, in particular, have found a correlation between amygdala activation during encoding and subsequent memory. Because of the design of these studies, it is unknown whether this correlation is based on individual differences between participants or within-subject variations in moment-to-moment amygdala activation related to individual stimuli. In this study, participants saw neutral and negative scenes and indicated how emotionally intense they found each scene. Separate functional magnetic resonance imaging responses in the amygdala for each scene were related to the participants' report of their experience at study and to performance in an unexpected memory test 3 weeks after scanning. The amygdala had the greatest response to scenes rated as most emotionally intense. The degree of activity in the left amygdala during encoding was predictive of subsequent memory only for scenes rated as most emotionally intense. These findings support the view that amygdala activation reflects moment-to-moment subjective emotional experience and that this activation enhances memory in relation to the emotional intensity of an experience.
Unsupervised models of how the brain identifies and categorizes the causes of its sensory input can be divided into two classes: those that minimize the mutual information (i.e., redundancy) among evoked responses and those that minimize the prediction error. Although these models have the same goal, the way that goal is attained, and the functional architectures required, are fundamentally different. This review describes the differences, in the functional anatomy of sensory cortical hierarchies, implied by the two models. We then consider how neuroimaging can be used to disambiguate between them. The key distinction reduces to whether backward connections are employed by the brain to generate a prediction of sensory inputs. To ascertain whether backward influences are evident empirically requires a characterization of functional integration among brain systems. This review summarizes the approaches to measuring functional integration in terms of effective connectivity and proceeds to address the question posed by the theoretical considerations. In short, it will be shown that the conjoint manipulation of bottom-up and top-down inputs to an area can be used to test for interactions between them, in elaborating cortical responses. The conclusion, from these sorts of neuroimaging studies, points to the prevalence of top-down influences and the plausibility of generative models of sensory brain function.
The brain is critically dependent on a continuous supply of blood to function. Therefore, the cerebral vasculature is endowed with neurovascular control mechanisms that assure that the blood supply of the brain is commensurate to the energy needs of its cellular constituents. The regulation of cerebral blood flow (CBF) during brain activity involves the coordinated interaction of neurons, glia, and vascular cells. Thus, whereas neurons and glia generate the signals initiating the vasodilation, endothelial cells, pericytes, and smooth muscle cells act in concert to transduce these signals into carefully orchestrated vascular changes that lead to CBF increases focused to the activated area and temporally linked to the period of activation. Neurovascular coupling is disrupted in pathological conditions, such as hypertension, Alzheimer disease, and ischemic stroke. Consequently, CBF is no longer matched to the metabolic requirements of the tissue. This cerebrovascular dysregulation is mediated in large part by the deleterious action of reactive oxygen species on cerebral blood vessels. A major source of cerebral vascular radicals in models of hypertension and Alzheimer disease is the enzyme NADPH oxidase. These findings, collectively, highlight the importance of neurovascular coupling to the health of the normal brain and suggest a therapeutic target for improving brain function in pathologies associated with cerebrovascular dysfunction.
We used two complementary methods to investigate cortical reorganization in chronic stroke patients during treatment with a defined motor rehabilitation program. BOLD ("blood oxygenation level dependent") sensitive functional magnetic resonance imaging (fMRI) and intracortical inhibition (ICI) and facilitation (ICF) measured with transcranial magnetic stimulation (TMS) via paired pulse stimulation were used to investigate cortical reorganization before and after "constraint-induced movement therapy" (CI). The motor hand function improved in all subjects after CI. BOLD signal intensity changes within affected primary sensorimotor cortex (SMC) before and after CI showed a close correlation with ICI (r = 0.93) and ICF (r = 0.76) difference before and after therapy. Difference in number of voxels and ICI difference before and after CI also showed a close correlation (r = 0.92) in the affected SMC over the time period of training. A single subject analysis revealed that patients with intact hand area of M1 ("the hand knob") and its descending motor fibers (these patients revealed normal motor evoked potentials [MEP] from the affected hand) showed decreasing ipsilesional SMC activation which was paralleled by an increase in intracortical excitability. This pattern putatively reflects increasing synaptic efficiency. When M1 or its descending pyramidal tract was lesioned (MEP from the affected hand was pathologic) ipsilesional SMC activation increased, accompanied by decreased intracortical excitability. We suggest that an increase in synaptic efficiency is not possible here, which leads to reorganization with extension, shift and recruitment of additional cortical areas of the sensorimotor network. The inverse dynamic process between both complementary methods (activation in fMRI and intracortical excitability determined by TMS) over the time period of CI illustrates the value of combining methods for understanding brain reorganization.
We present a unified statistical theory for assessing the significance of apparent signal observed in noisy difference images. The results are usable in a wide range of applications, including fMRI, but are discussed with particular reference to PET images which represent changes in cerebral blood flow elicited by a specific cognitive or sensorimotor task. Our main result is an estimate of the P-value for local maxima of Gaussian, t, χ2 and F fields over search regions of any shape or size in any number of dimensions. This unifies the P-values for large search areas in 2-D (Friston et al. [1991]: J Cereb Blood Flow Metab 11:690–699) large search regions in 3-D (Worsley et al. [1992]: J Cereb Blood Flow Metab 12:900–918) and the usual uncorrected P-value at a single pixel or voxel.
Resumen: El presente artículo analiza las bases teórico-metodológicas de la aproximación neuropsicológica de A.R. Luria, uno de los fundadores de la neuropsicología. Los éxitos de la neuropsicología de Luria se relacionan con el carácter adecuado de las ideas psicológicas que utilizó para el estudio del problema "mente-cerebro". Se presentan las diferencias principales entre la neuropsicología occidental y la neuropsicología de A.R. Luria, la cual se considera como una rama de la ciencia psicológica. Entre los principios fundamentales de esta postura se encuentran los siguientes: las estructuras cerebrales responsables para la realización de las funciones psicológicas son estructuras altamente diferenciadas; el cerebro, como sustrato de los procesos psicológicos, se organiza de acuerdo al principio sistémico. Se presentan los logros de la escuela neuropsicológica de A.R. Luria dentro del área científica y metodológica en el periodo post-Luriano. Palabras clave: Neuropsicología, Teoría neuropsicológica, Escuelas neuropsicológicas. Abstract: This article presents the theoretical and methodological bases of A.R. Luria´s neuropsychological approach, one of the founders of the new neuropsychology. The success of Luria's neuropsychology is related to the character of psychological ideas which he used for the study of the problem "brain and psycue". There are differences between occidental neuropsychology and that of Luria, which is considered to be a branch of psychological science. Among the basic principles of this approach are the following: brain structures responsible for the realization of psychological functions which are highly differentiated; the brain as the substratum of psychological processes is organized according to systemic principle. The achievements of the neuropsychological school of A.R. Luria in scientific and methodological areas in the post-Lurian period are presented.
Brain connectivity is a key concept for understanding brain function. Current methods to detect and quantify different types of connectivity with neuroimaging techniques are fundamental for understanding the pathophysiology of many neurologic and psychiatric disorders. This article aims to present a critical review of the magnetic resonance imaging techniques used to measure brain connectivity within the context of the Human Connectome Project. We review techniques used to measure: a) structural connectivity b) functional connectivity (main component analysis, independent component analysis, seed voxel, meta-analysis), and c) effective connectivity (psychophysiological interactions, causal dynamic models, multivariate autoregressive models, and structural equation models). These three approaches make it possible to combine and use different statistical techniques to elaborate mathematical models in the attempt to understand the functioning of the brain. The findings obtained with these techniques must be validated by other techniques for analyzing structural and functional connectivity. This information is integrated in the Human Connectome Project where all these approaches converge to provide a representation of all the different models of connectivity.
Copyright © 2010 SERAM. Published by Elsevier Espana. All rights reserved.
Previous research on participants with aphasia has mainly been based on standard functional neuroimaging analysis. Recent studies have shown that functional connectivity analysis can detect compensatory activity, not revealed by standard analysis. Little is known, however, about the default-mode network in aphasia. In the current study, we studied changes in the default-mode network in subjects with aphasia who underwent semantic feature analysis therapy. We studied nine participants with chronic aphasia and compared them to 10 control participants. For the first time, we identified the default-mode network using spatial independent component analysis, in participants with aphasia. Intensive therapy improved integration in the posterior areas of the default-mode network concurrent with language improvement. Correlations between integration and improvement did not reach significance, but the trend suggests that pre-therapy integration of the default-mode network may predict therapy outcomes. Functional connectivity allows a better understanding of the impact of semantic feature analysis in aphasia.
Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.
Using gradient-echo echo-planar MRI, a local signal increase of 4.3 ± 0.3% is observed in the human brain during task activation, suggesting a local decrease in blood deoxyhemoglobin concentration and an increase in blood oxygenation. Images highlighting areas of signal enhancement temporally correlated to the task are created. © 1992 Academic Press, Inc.
This article is a selective review of functional imaging investigations and brain stimulation studies addressing the neural mechanisms of recovery of stroke-associated aphasia. The imaging results show that aphasia recovery is associated with a complex pattern of brain reorganisation, involving both ipsilateral and contralateral brain regions, which is modulated by lesion size and site, time post-onset, type of training, and language task. The information provided by the imaging investigations needs to be integrated with the results of brain stimulation studies, in order to specify the most effective protocols in term of modality, locus and timing of stimulation. Further studies, using multiple imaging and neuromodulation approaches, are required to reach sound conclusions about the potential usefulness of brain stimulation approaches as an adjunct to aphasia rehabilitation.
Neural activity is intimately tied to blood flow in the brain. This coupling is specific enough in space and time that modern imaging methods use local hemodynamics as a measure of brain activity. In this review, we discuss recent evidence indicating that neuronal activity is coupled to local blood flow changes through an intermediary, the astrocyte. We highlight unresolved issues regarding the role of astrocytes and propose ways to address them using novel techniques. Our focus is on cellular level analysis in vivo, but we also relate mechanistic insights gained from ex vivo experiments to native tissue. We also review some strategies to harness advances in optical and genetic methods to study neurovascular coupling in the intact brain.
Data acquired with functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) are often interpreted in terms of the underlying neuronal activity, despite mounting evidence that these signals do not always correlate with electrophysiological recordings. Therefore, considering the increasing popularity of functional neuroimaging, it is clear that a more comprehensive theory is needed to reconcile these apparent disparities and more accurately explain the mechanisms through which various PET and fMRI signals arise. In the present article, we have turned our attention to astrocytes, which vastly outnumber neurons and are known to serve a number of functions throughout the central nervous system (CNS). For example, astrocytes are known to be critically involved in neurotransmitter uptake and recycling, and empirical data suggests that brain activation increases both oxidative and glycolytic astrocyte metabolism. Furthermore, a number of recent studies imply that astrocytes are likely to play a key role in regulating cerebral blood delivery. Therefore, we propose that, by mediating neurometabolic and neurovascular processes throughout the CNS, astrocytes could provide a common physiological basis for fMRI and PET signals. Such a theory has significant implications for the interpretation of functional neuroimaging signals, because astrocytic changes reflect subthreshold neuronal activity, simultaneous excitatory/inhibitory synaptic inputs, and other transient metabolic demands that may not elicit electrophysiological changes. It also suggests that fMRI and PET signals may have inherently less sensitivity to decreases in synaptic input (i.e. 'negative activity') and/or inhibitory (GABAergic) neurotransmission.
Attention-deficit/hyperactivity disorder (ADHD) is a prevalent condition associated with cognitive dysfunction. The Cambridge Neuropsychological Test Automated Battery is a computerized set of tests that has been widely used in ADHD and in translation/back-translation. Following a survey of translational research relevant to ADHD in experimental animals, a comprehensive literature review was conducted of studies that had used core Cambridge Neuropsychological Test Automated Battery tests 1) to evaluate cognitive dysfunction in ADHD and 2) to evaluate effects of salient drugs in patients and in volunteers. Meta-analysis was conducted where four or more independent datasets were available. Meta-analysis revealed medium-large decrements in ADHD for response inhibition (d = .790, p < .001), working memory (d = .883, p < .001), executive planning (d = .491, p < .001), and a small decrement in attentional set shifting (d = .160, p = .040). Qualitative review of the literature showed some consistent patterns. In ADHD, methylphenidate improved working memory, modafinil improved planning, and methylphenidate, modafinil, and atomoxetine improved inhibition. Meta-analysis of modafinil healthy volunteer studies showed no effects on sustained attention or set shifting. Results were paralleled by findings in experimental animals on comparable tests, enabling further analysis of drug mechanisms. Substantial cognitive deficits are present in ADHD, which can be remediated somewhat with current medications and which can readily be modeled in experimental animals using back-translational methodology. The findings suggest overlapping but also distinct early cognitive effects of ADHD medications and have important implications for understanding the pathophysiology of ADHD and for future trials.
Functional MRI (fMRI) is a non-invasive brain imaging methodology that started in 1991 and allows human brain activation to be imaged at high resolution within only a few minutes. Because it has extremely high sensitivity, is relatively easy to implement, and can be performed on most standard clinical MRI scanners. It continues to grow at an explosive rate throughout the world. Over the years, at any given time, fMRI has been defined by only a handful of major topics that have been the focus of researchers using and developing the methodology. In this review, I attempt to take a snapshot of the field of fMRI as it is in mid-2009 by discussing the seven topics that I feel are most on the minds of fMRI researchers. The topics are, in no particular order or grouping: (1) Clinical impact, (2) Utilization of individual functional maps, (3) fMRI signal interpretation, (4) Pattern effect mapping and decoding, (5) Endogenous oscillations, (6) MRI technology, and (7) Alternative functional contrast mechanisms. Most of these topics are highly interdependent, each advancing as the others advance. While most fMRI involves applications towards clinical or neuroscience questions, all applications are fundamentally dependent on advances in basic methodology as well as advances in our understanding of the relationship between neuronal activity and fMRI signal changes. This review neglects almost completely an in-depth discussion of applications. Rather the discussions are on the methods and interpretation.
Introduced is a general framework for performing group-level analyses of fMRI data using any basis set of two functions (i.e., the canonical hemodynamic response function and its first derivative) to model the hemodynamic response to neural activity. The approach allows for flexible implementation of physiologically based restrictions on the results. Information from both basis functions is used at the group level and the limitations avoid physiologically ambiguous or implausible results. This allows for investigation of specific BOLD activity such as hemodynamic responses peaking within a specified temporal range (i.e., 4-5 s). The general nature of the presented approach allows for applications using basis sets specifically designed to investigate various physiologic phenomena, i.e., age-related variability in poststimulus undershoot, hemodynamic responses measured with cerebral blood flow imaging, or subject-specific basis sets. An example using data from a group of healthy young participants demonstrates the methods and the specific steps to study poststimulus variability are discussed. The approach is completely implemented within the general linear model and requires minimal programmatic calculations.
To examine the current clinical indications for conducting a Wada test in the presurgical evaluation of epilepsy surgery candidates in the light of research on the reliability and validity of proposed, noninvasive alternatives.
There has been a significant shift in the role of the Wada test in epilepsy surgery programmes. The majority of epilepsy centres no longer conduct a Wada test on every surgical candidate. The lateralization of language via invasive methods may not be necessary for temporal lobe epilepsy patients in whom a tailored resection will spare areas associated with language function. Functional MRI is being used in some centres to lateralize and localize language function in epilepsy surgery candidates. Magnetoencephalography also shows promise in this regard. Patients at high risk of a postoperative memory decline can be identified via multivariate models that utilize noninvasive measures of cerebral function and structure, together with demographic and clinical variables.
The clinical indications for a Wada test should be determined on a case-by-case basis, with careful consideration of the available noninvasive alternatives, to ensure that the risk-benefit ratio is appropriate for every patient.
Nervous systems facing complex environments have to balance two seemingly opposing requirements. First, there is a need quickly and reliably to extract important features from sensory inputs. This is accomplished by functionally segregated (specialized) sets of neurons, e.g. those found in different cortical areas. Second, there is a need to generate coherent perceptual and cognitive states allowing an organism to respond to objects and events, which represent conjunctions of numerous individual features. This need is accomplished by functional integration of the activity of specialized neurons through their dynamic interactions. These interactions produce patterns of temporal correlations or functional connectivity involving distributed neuronal populations, both within and across cortical areas. Empirical and computational studies suggest that changes in functional connectivity may underlie specific perceptual and cognitive states and involve the integration of information across specialized areas of the brain. The interplay between functional segregation and integration can be quantitatively captured using concepts from statistical information theory, in particular by defining a measure of neural complexity. Complexity measures the extent to which a pattern of functional connectivity produced by units or areas within a neural system combines the dual requirements of functional segregation and integration. We find that specific neuroanatomical motifs are uniquely associated with high levels of complexity and that such motifs are embedded in the pattern of long-range cortico-cortical pathways linking segregated areas of the mammalian cerebral cortex. Our theoretical findings offer new insight into the intricate relationship between connectivity and complexity in the nervous system.
The success of diffusion magnetic resonance imaging (MRI) is deeply rooted in the powerful concept that during their random, diffusion-driven displacements molecules probe tissue structure at a microscopic scale well beyond the usual image resolution. As diffusion is truly a three-dimensional process, molecular mobility in tissues may be anisotropic, as in brain white matter. With diffusion tensor imaging (DTI), diffusion anisotropy effects can be fully extracted, characterized, and exploited, providing even more exquisite details on tissue microstructure. The most advanced application is certainly that of fiber tracking in the brain, which, in combination with functional MRI, might open a window on the important issue of connectivity. DTI has also been used to demonstrate subtle abnormalities in a variety of diseases (including stroke, multiple sclerosis, dyslexia, and schizophrenia) and is currently becoming part of many routine clinical protocols. The aim of this article is to review the concepts behind DTI and to present potential applications.
Finding objective and effective thresholds for voxelwise statistics derived from neuroimaging data has been a long-standing problem. With at least one test performed for every voxel in an image, some correction of the thresholds is needed to control the error rates, but standard procedures for multiple hypothesis testing (e.g., Bonferroni) tend to not be sensitive enough to be useful in this context. This paper introduces to the neuroscience literature statistical procedures for controlling the false discovery rate (FDR). Recent theoretical work in statistics suggests that FDR-controlling procedures will be effective for the analysis of neuroimaging data. These procedures operate simultaneously on all voxelwise test statistics to determine which tests should be considered statistically significant. The innovation of the procedures is that they control the expected proportion of the rejected hypotheses that are falsely rejected. We demonstrate this approach using both simulations and functional magnetic resonance imaging data from two simple experiments.
This study examined whether differential neural responses are evoked by emotional stimuli with and without conscious perception, in a patient with visual neglect and extinction. Stimuli were briefly shown in either right, left, or both fields during event-related fMRI. On bilateral trials, either a fearful or neutral left face appeared with a right house, and it could either be extinguished from awareness or perceived. Seen faces in left visual field (LVF) activated primary visual cortex in the damaged right-hemisphere and bilateral fusiform gyri. Extinguished left faces increased activity in striate and extrastriate cortex, compared with right houses only. Critically, fearful faces activated the left amygdala and extrastriate cortex both when seen and when extinguished; as well as bilateral orbitofrontal and intact right superior parietal areas. Comparison of perceived versus extinguished faces revealed no difference in amygdala for fearful faces. Conscious perception increased activity in fusiform, parietal and prefrontal areas of the left-hemisphere, irrespective of emotional expression; while a differential emotional response to fearful faces occurring specifically with awareness was found in bilateral parietal, temporal, and frontal areas. These results demonstrate that amygdala and orbitofrontal cortex can be activated by emotional stimuli even without awareness after parietal damage; and that substantial unconscious residual processing can occur within spared brain areas well beyond visual cortex, despite neglect and extinction.
Moral psychology has long focused on reasoning, but recent evidence suggests that moral judgment is more a matter of emotion and affective intuition than deliberate reasoning. Here we discuss recent findings in psychology and cognitive neuroscience, including several studies that specifically investigate moral judgment. These findings indicate the importance of affect, although they allow that reasoning can play a restricted but significant role in moral judgment. They also point towards a preliminary account of the functional neuroanatomy of moral judgment, according to which many brain areas make important contributions to moral judgment although none is devoted specifically to it.
The value of functional MR Imaging (fMRI) in assessing language lateralization in epileptic patients candidate for surgical treatment is increasingly recognized. However few data are available for left-handed patients. Moreover determining factors for atypical dominance in patients investigated with contemporary imaging have not been reported. We studied 20 patients (14 males, 6 females; 9 right handed, 11 left handed) aged from 9 to 48 years, investigated for intractable partial epilepsy. Epileptic focus location was temporal in 14 cases, extratemporal in 6, and lateralized in the left hemisphere in 11/20. Hemispheric dominance for language was evaluated by both Wada test and fMRI using a silent word generation paradigm in all patients. Furthermore, a postictal speech test was performed in 15 patients. An fMRI language lateralization index was calculated from the number of activated pixels (Student's t test, P < 0.0001) in the right and left hemispheres. The Wada test showed a right hemispheric dominance in 8 patients (6 were left handed and 2 right handed) and a left hemispheric dominance in 12 patients (5 were left handed and 7 right handed). These results were concordant with clinical postictal examination in 11/15 patients (73%). Clinical status did not allow a conclusion about hemispheric dominance for the remaining 4 patients. FMRI was concordant with the Wada test in 19/20 cases. For one left-handed patient, fMRI showed bilateral activation, whereas the Wada test demonstrated a right hemispheric dominance. Right language lateralization was significantly correlated with left lateralized epilepsy (P < 0.05) but was not correlated with age at epilepsy onset, early brain injury (before 6 years), and lobar localization of epileptogenic focus. However the lack of a significant relationship between these factors and atypical language lateralization may be related to the small sample size.
There is strong evidence that face processing in the brain is localized. The double dissociation between prosopagnosia, a face recognition deficit occurring after brain damage, and visual object agnosia, difficulty recognizing other kinds of complex objects, indicates that face and non-face object recognition may be served by partially independent neural mechanisms. In this paper, we use computational models to show how the face processing specialization apparently underlying prosopagnosia and visual object agnosia could be attributed to (1) a relatively simple competitive selection mechanism that, during development, devotes neural resources to the tasks they are best at performing, (2) the developing infant's need to perform subordinate classification (identification) of faces early on, and (3) the infant's low visual acuity at birth. Inspired by de Schonen, Mancini and Liegeois' arguments (1998) [de Schonen, S., Mancini, J., Liegeois, F. (1998). About functional cortical specializat...
Experts of abacus operation demonstrate extraordinary ability in mental calculation. There is psychological evidence that abacus experts utilize a mental image of an abacus to remember and manipulate large numbers in solving problems; however, the neural correlates underlying this expertise are unknown. Using functional magnetic resonance imaging, we compared the neural correlates associated with three mental-operation tasks (numeral, spatial, verbal) among six experts in abacus operations and eight nonexperts. In general, there was more involvement of neural correlates for visuospatial processing (e.g., right premotor and parietal areas) for abacus experts during the numeral mental-operation task. Activity of these areas and the fusiform cortex was correlated with the size of numerals used in the numeral mental-operation task. Particularly, the posterior superior parietal cortex revealed significantly enhanced activity for experts compared with controls during the numeral mental-operation task. Comparison with the other mental-operation tasks indicated that activity in the posterior superior parietal cortex was relatively specific to computation in 2-dimensional space. In conclusion, mental calculation of abacus experts is likely associated with enhanced involvement of the neural resources for visuospatial information processing in 2-dimensional space.
Blood oxygenation level dependent contrast functional MRI (BOLD-fMRI) has been used to define the functional cortices of the brain in preoperative planning for tumor removal. However, some studies have demonstrated false-negative activations in such patients. We compared the evoked-cerebral blood oxygenation (CBO) changes measured by near-infrared spectroscopy (NIRS) and activation mapping of BOLD-fMRI in 12 patients with brain tumors who had no paresis of the upper extremities. On the nonlesion side, NIRS demonstrated a decrease in deoxyhemoglobin (Deoxy-Hb) with increases in oxyhemoglobin (Oxy-Hb) and total hemoglobin (Total-Hb) during a contralateral hand grasping task in the primary sensorimotor cortex (PSMC) of all patients. On the lesion side, NIRS revealed a decrease in Deoxy-Hb in five patients (Deoxy-decrease group), and an increase in Deoxy-Hb in seven patients (Deoxy-increase group); the Oxy-Hb and Total-Hb were increased during activation in both groups, indicating the occurrence of rCBF increases in response to neuronal activation. BOLD-fMRI demonstrated clear activation areas in the PSMC on the nonlesion side of all patients and on the lesion side of the Deoxy-decrease group. However, in the Deoxy-increase group, BOLD-fMRI revealed only a small activation area or no activation on the lesion side. Intraoperative brain mapping identified the PSMC on the lesion side that was not demonstrated by BOLD-fMRI. The false-negative activations might have been caused by the atypical evoked-CBO changes (i.e. increases in Deoxy-Hb) and the software employed to calculate the activation maps, which does not regard an increase of Deoxy-Hb (i.e., a decrease in BOLD-fMRI signal) as neuronal activation.
Emotional events are remembered better than neutral events possibly because the amygdala enhances the function of medial temporal lobe (MTL) memory system (modulation hypothesis). Although this hypothesis has been supported by much animal research, evidence from humans has been scarce and indirect. We investigated this issue using event-related fMRI during encoding of emotional and neutral pictures. Memory performance after scanning showed a retention advantage for emotional pictures. Successful encoding activity in the amygdala and MTL memory structures was greater and more strongly correlated for emotional than for neutral pictures. Moreover, a double dissociation was found along the longitudinal axis of the MTL memory system: activity in anterior regions predicted memory for emotional items, whereas activity in posterior regions predicted memory for neutral items. These results provide direct evidence for the modulation hypothesis in humans and reveal a functional specialization within the MTL regarding the effects of emotion on memory formation.
The brain appears to adhere to two fundamental principles of functional organisation, functional integration and functional specialisation, where the integration within and among specialised areas is mediated by effective connectivity. In this paper, we review two different approaches to modelling effective connectivity from fMRI data, structural equation models (SEMs) and dynamic causal models (DCMs). In common to both approaches are model comparison frameworks in which inferences can be made about effective connectivity per se and about how that connectivity can be changed by perceptual or cognitive set. Underlying the two approaches, however, are two very different generative models. In DCM, a distinction is made between the 'neuronal level' and the 'hemodynamic level'. Experimental inputs cause changes in effective connectivity expressed at the level of neurodynamics, which in turn cause changes in the observed hemodynamics. In SEM, changes in effective connectivity lead directly to changes in the covariance structure of the observed hemodynamics. Because changes in effective connectivity in the brain occur at a neuronal level DCM is the preferred model for fMRI data. This review focuses on the underlying assumptions and limitations of each model and demonstrates their application to data from a study of attention to visual motion.
Emotion can exert a modulatory role on episodic memory. Several studies have shown that negative stimuli (e.g., words, pictures) are better remembered than neutral ones. Although facial expressions are powerful emotional stimuli and have been shown to influence perception and attention processes, little is known about their effect on memory. We used functional magnetic resonance imaging (fMRI) in humans to investigate the effects of expression (happy, neutral, and fearful) on prefrontal cortex (PFC) activity during the encoding of faces, using a subsequent memory effect paradigm. Our results show that activity in right PFC predicted memory for faces, regardless of expression, while a homotopic region in the left hemisphere was associated with successful encoding only for faces with an emotional expression. These findings are consistent with the proposed role of right dorsolateral PFC in successful encoding of nonverbal material, but also suggest that left DLPFC may be a site where integration of memory and emotional processes occurs. This study sheds new light on the current controversy regarding the hemispheric lateralization of PFC in memory encoding.
This chapter gives an overview of the various analysis steps that are required after a functional magnetic resonance imaging (fMRI) experiment has been designed and carried out. The resulting data must be passed through various analysis steps before the experimenter can get answers to questions about experimentally related activations at the individual or multi-subject level. The aim of fMRI analysis is to identify in which voxels' time-series the signal of interest is significantly greater than the noise level. The chapter also provides a brief overview of different approaches to obtaining activation maps, followed by a detailed introduction to analysis via the general linear model - currently the most popular statistical approach - and also various methods of thresholding the resulting statistics maps. It describes briefly the practical and numerical details involved in the analysis of a particular fMRI experiment.
To investigate the inter- and intrahemispheric reorganization of the language cortex in left temporal lobe epilepsy (TLE) with left-sided hippocampal sclerosis. A functional magnetic resonance imaging was performed on 13 right-handed patients suffering from medically intractable left TLE, and in 13 sex- and age-matched healthy controls. The activation paradigm used was a silent word generation task. A language laterality index (LI) was calculated from the number of activated voxels in the right and left anterior two-thirds of the hemispheres. Significant differences between the patients and the controls were observed in the activation of the left-sided inferior frontal gyrus. Less consistent findings in this region, as well as the relative protection of Broca's area from the activation, were revealed in the patients. In addition, different patterns of activation were proven in the cerebellum and other cortical as well as subcortical brain structures within both hemispheres. Significant differences were also found in the values of the language LIs between the investigated groups: these values suggested a more bihemispheric language representation in the patients. As anticipated, lateralization of the language functions in the epileptics significantly decreased in connection with an earlier age of initial insult. Our results support the hypothesis of a significant intra- and interhemispheric functional reorganization of language-related neuronal networks in left TLE.
The goals of the present study were twofold. First, we wished to investigate the neural correlates of aware and unaware emotional face perception after characterizing each subject's behavioral performance via signal detection theory methods. Second, we wished to investigate the extent to which amygdala responses to fearful faces depend on the physical characteristics of the stimulus independently of the percept. We show that amygdala responses depend on visual awareness. Under conditions in which subjects were not aware of fearful faces flashed for 33 ms, no differential activation was observed in the amygdala. On the other hand, differential activation was observed for 67 ms fearful targets that the subjects could reliably detect. When trials were divided into hits, misses, correct rejects, and false alarms, we show that target visibility is an important factor in determining amygdala responses to fearful faces. Taken together, our results further challenge the view that amygdala responses occur automatically.
We investigated the human face specificity by comparing the effects of inversion and contrast reversal, two manipulations known to disrupt configural face processing, on human and ape faces, isolated eyes and objects, using event-related potentials. The face sensitive marker, N170, was shortest to human faces and delayed by inversion and contrast reversal for all categories and not only for human faces. Most importantly, N170 to inverted or contrast-reversed faces was not different from N170 to eyes that did not differ across manipulations. This suggests the disruption of facial configuration by these manipulations isolates the eye region from the face context, to which eye neurons respond. Our data suggest that (i) the inversion and contrast reversal effects on N170 latency are not specific to human faces and (ii) the similar increase of N170 amplitude by inversion and contrast reversal is unique to human faces and is driven by the eye region. Thus, while inversion and contrast reversal effects on N170 latency are not category-specific, their effects on amplitude are face-specific and reflect mainly the contribution of the eye region.
This study used functional magnetic resonance imaging to investigate amygdala response in patients with acute posttraumatic stress disorder (PTSD) to emotional expressions.
Thirteen medication-free individuals with acute PTSD and no axis I psychiatric comorbidity were scanned while viewing pictures of fearful or happy faces, presented above or below consciousness, with backward masking.
There was a significant positive correlation between the severity of PTSD and the difference in amygdala responses between masked fearful and happy faces and a corresponding negative correlation for the difference between unmasked fearful and happy faces.
These findings suggest that functional abnormalities in brain responses to emotional stimuli observed in chronic PTSD are already apparent in its acute phase.
Local increase in blood flow during neural activity forms the basis for functional brain imaging, but its mechanism remains poorly defined. Here we show that cortical astrocytes in vivo possess a powerful mechanism for rapid vasodilation. We imaged the activity of astrocytes labeled with the calcium (Ca(2+))-sensitive indicator rhod-2 in somatosensory cortex of adult mice. Photolysis of caged Ca(2+) in astrocytic endfeet ensheathing the vessel wall was associated with an 18% increase in arterial cross-section area that corresponded to a 37% increase in blood flow. Vasodilation occurred with a latency of only 1-2 s, and both indomethacin and the cyclooxygenase-1 inhibitor SC-560 blocked the photolysis-induced hyperemia. These observations implicate astrocytes in the control of local microcirculation and suggest that one of their physiological roles is to mediate vasodilation in response to increased neural activity.
There is much interest currently in using functional neuroimaging techniques to understand better the nature of cognition. One particular practice that has become common is 'reverse inference', by which the engagement of a particular cognitive process is inferred from the activation of a particular brain region. Such inferences are not deductively valid, but can still provide some information. Using a Bayesian analysis of the BrainMap neuroimaging database, I characterize the amount of additional evidence in favor of the engagement of a cognitive process that can be offered by a reverse inference. Its usefulness is particularly limited by the selectivity of activation in the region of interest. I argue that cognitive neuroscientists should be circumspect in the use of reverse inference, particularly when selectivity of the region in question cannot be established or is known to be weak.