ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:The oculomotor system, which optimizes visual interaction with the environment, provides a valuable model system for probing the building blocks of higherorder cognition. Attention shifting, working memory, and inhibition of prepotent responses can be investigated in healthy individuals and patients with brain disorders. Although the neurophysiology of the oculomotor system has been well characterized at the single-cell level in nonhuman primates, its functional architecture in humans determined by evoked response procedures and studies of patients with focal lesions has been limited. Available evidence points to a widely distributed set of neocortical and subcortical brain regions involved in the control of eye movements, including brain stem, cerebellum, thalamus. striatum, and parietal and frontal cortices. The advent of functional magnetic resonance imaging provides a oninvasive manner of localizing, at high spatial resolution. the brain systems that subserve different aspects of sensory and cognitive processes in humans. Functional magnetic resonance imaging studies have already delineated the brain systems subserving sensorimotor and cognitive control of eye movements in adult and pediatric populations. Hence, the combination of functional magnetic resonance imaging and eye movement procedures can be used to probe the integrity of the brain in neurological and psychiatric disorders as well as provide a window into the changes in brain function subserving cognitive development

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:This article describes how fMRI can be used to examine the large-scale networks of cortical areas that subserve high-level cognition, such as sentence comprehension and visual thinking. The findings from a number of studies show that the qualitative and quantitative nature of the cognitive processes determines which cortical areas are activated (the network constituency) and the degree to which each network member is activated. For example, during sentence comprehension, activation in the left posterior temporal region and the inferior frontal gyms, as well as their right hemisphere homologs, increases as a function of the linguistic complexity of the sentence. Such findings indicate that cognition emerges from the collaboration among the multiple cortical areas that compose the large-scale networks, rather than from the aggregate of autonomously functioning modules. The patterns of activation also show systematic shifts in the activity of a network during the spontaneous recovery of function by stroke patients, demonstrating cortical plasticity in adults. Finally, the article describes some simulation models that relate the information processing activity of a computational system to its resource consumption. This construct enables a mapping from the functional properties of the cognitive systems to the biological substrate that is reflected in fMRI

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:The recent development of integrating very-high-field magnets (3.0 T) into clinical scanners was driven by the demonstration that both functional and anatomic information can be derived reliably at high spatial and temporal resolution. As veryhigh-field magnetic resonance imaging now approaches its entry into the clinical arena with a product by one major scanner manufacturer that is to be considered for Food and Drug Administration clearance, it is timely to consider the clinical applications that are likely to make such new technology significant in medical imaging. The rationale for very high field is based on the mechanism of blood oxygenation level-dependent contrast and the need for reliable functional studies on individual patients. The use of functional magnetic resonance imaging in the setting of presurgical planning is demonstrated in a number of different clinical scenarios. Such cases covering both pediatric and adult patients indicate that 3.0-T functional magnetic resonance imaging has an important role in neuroradiology

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:Functional magnetic resonance imaging (fMRI) is a novel neuroimaging technique that has enjoyed explosive growth during the past 7 years. It can be implemented relatively easily on many already existing MRI systems, it is noninvasive, and functional images may be obtained within tens of seconds. However, it measures a secondary effect of neuronal activity, the blood oxygen concentration (the blood oxygen level-dependent, or BOLD, effect), which is a somewhat sluggish and blurred measure of the actual time course of neuronal activity. Here we discuss the present limitations to temporal resolution and the degree to which they can be overcome by using specific assumptions about the coupling between neuronal activity, blood flow, and oxygen metabolism, or by specific experimental designs, in particular time-resolved fMRI

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:Although neuroimaging methods have been used successfully to map largescale neurocognitive networks distributed across the human cortex, functional mapping and differentiation of localized brain organization within a small structure has been limited by inadequate sensitivity for high spatial resolution imaging. Functional magnetic resonance imaging (fMRI) technique based on blood oxygenation level-dependent (BOLD) contrast has become one of the most useful neuroimaging techniques. It has been used extensively to study human brain function from sensory perception to cognitive performance. However, the majority of these studies used a relatively low spatial resolution (typically with a voxel size of 3.1 × 3.1 × 5.0 mm3), which is incapable of mapping on the millimeter and submillimeter spatial scale. In this article, we review the technical aspects of the high-resolution fMRI technique and the sensitivity and spatial specificity of BOLD-based fMRI. We demonstrate applications of high-resolution fMRI in studying the human visual pathway from the lateral geniculate nucleus in the thalamus to the ocular dominance columns in the primary visual cortex. Most results were obtained at very-high-magnetic fields (3.0 and 4.0 Tesla). They reveal that high-resolution fMRI at very-high-magnetic field is promising for functional mapping of brain organization from large cortical networks, small nuclei, and even to cellular layer structures

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID

摘要:

Summary:Because of their advantage in terms of signal-to-noise ratio, high-field magnetic resonance imaging systems have become favored in the last few years for functional magnetic resonance imaging (fMRI) applications. In many ways the conceptual development of these high-field scanners has involved more-or-less straightforward extensions of practices at lower field strengths. However, in other ways specific engineering challenges have been encountered and largely overcome in the quest for scanners capable of realizing the advantages of high-field systems. An understanding of the technical trade-offs that can be made in terms of hardware performance is useful in deciding on the optimum system for a given fMRI application. In this article the technical issues surrounding high-field scanning are reviewed in the context of a typical brain mapping protocol. In addition there is a discussion of the safety issues related to the use of these systems

ISSN:0899-3459    

出版商:OVID    

年代:1999

数据来源: OVID