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Brain Function as Assessed by fMRI

Classic task-dependent fMRI paradigms use the change in BOLD signal intensity evoked by the performance of a specific task in the scanner. The increase of BOLD signal intensity is an indirect measure of activation of a distinct brain region. rsfMRI is a novel approach in which the patient does not perform a task but lies quietly in the scanner. It allows the detection of functional resting-state connectivity of brain networks. Functional connectivity has been defined as “the temporal correlation of a neurophysiologic index measured in different brain areas.” In the context of rsfMRI, the neurophysiologic index is the BOLD signal intensity, which is known to display low-frequency spontaneous fluctuations in the resting brain (0.01–0.1 Hz) that can be distinguished from respiratory (0.1–0.5 Hz) and cardiovascular (0.6–1.2 Hz) signal-intensity frequencies. These signal-intensity fluctuations are spatially independent and show temporal correlation across largely separated cortical areas. These areas form functionally plausible networks that are crucial for perceptual, such as visual or auditory, processing or sensory-motor processing. Other resting-state networks include brain regions that are known to be involved in cognitive performance such as self-referential processing or attention.

Attributing a specific function is based on inference from external evidence because by definition, the subject is not performing a specific task. This attribution is relatively obvious for some networks (eg, auditory and visual) but less straightforward for higher order cognitive networks. Such attributions often rely on patterns of task-induced activation revealed by classic task-dependent fMRI. The reliability of resting-state connectivity has been demonstrated and might be further improved by mathematic modeling such as the dual-regression approach.

The DMN is bridging the gap between task-induced and task-independent findings because it has been described as both a cluster of brain regions that are deactivated during the performance of other tasks and a network extractable from resting-state BOLD fluctuations. The localization of its components has furthermore been confirmed by PET, EEG, and magnetoencephalography. In simultaneous EEG-fMRI studies, slow EEG patterns were found to correlate negatively with functional connectivity in the DMN during the resting state. Moreover, structural data suggest that the DMN is predefined in the structural connectivity of its main components. Parietal and temporal components of the network are known to be involved in working memory and to serve as a storage for semantic knowledge. The PCC is engaged in the retrieval and modification of past experiences. The PCC seems to play a central role within the DMN and the attention network. Its activity was furthermore shown to be influenced by the process of physiologic aging. The ACC is active during affective and emotional processing and autobiographic memory retrieval to mention 2 major of many domains. In total, the DMN is a tonically active network involved in the perception, integration, and interpretation of internal and external stimuli and the application of past experiences for problem-solving and future plans.

There are 2 most commonly used approaches to study resting-state connectivity. The first approach, ICA, allows data-driven whole-brain analysis of BOLD fluctuations and does, therefore, not depend on a priori hypotheses. As a tool of blind source separation, it can extract networks from the resting-state signal intensity on the basis of their spatial independence and similar BOLD time courses. The second approach uses predefined regions of interest as seed regions and determines functional connectivity by calculating regions composed of voxels displaying a significantly correlating BOLD signal intensity.

Functional MR imaging has been widely used to discover between-group differences. The application of fMRI to detect early signs of disease in individual subjects has not yet been widely established. There are only a few examples of the clinical application on an individual patient level. The main clinical application has been in planning of epilepsy surgery. While invasive cortical mapping with subdural grids remains the criterion standard, task-dependent fMRI has been offered as a noninvasive means of functional mapping of distinct areas active during a particular task. Recent studies support the notion that resting-state functional connectivity MR imaging may provide an effective tool for presurgical mapping of eloquent cortex (eg, the motor cortex) and further brain systems as well as epileptogenic areas in a single scanning session.

The general paucity of clinical fMRI applications can be attributed to several limitations. One of the most important limitations is the poor signal intensity–to-noise ratio and the pronounced susceptibility to motion artifacts. A further important limitation, especially in neuropsychiatric disorders with prominent cognitive features such as dementia, schizophrenia, or autism, has been the dependence on language comprehension, concentration, and compliance for the performance of cognitive tasks within the scanner. Here, rsfMRI offers an intriguing novel approach that reduces the load of task fMRI on patients and their compliance.

DTI

DTI is based on the measurement of diffusion properties of water molecules. It relies on the principle that diffusion of water molecules occurs unrestricted along all directions in CSF (isotropic), whereas in the WM, it is restricted axially and increases parallel to fiber bundles, leading to anisotropic diffusion. FA can, therefore, serve as an indirect measure reflecting the underlying structural integrity of fiber tracts. Greater FA values are believed to represent greater WM integrity. Lower FA values occur due to diminished myelinization, reduced amount of axons, decreased axonal diameter, and altered axonal interspaces in fiber tracts or bundles. In this review, we focus on DTI studies by using FA as an outcome marker. This is based on the premise that FA values currently represent the most commonly reported structural connectivity scale, compared with others such as MD or relative anisotropy.

The investigation of group differences in FA values can be performed by using a whole-brain voxel-based approach such as tract-based spatial statistics. By contrast, DTI tractography analyzes distinct preselected and segmented fiber tracts, allowing simultaneous quantification of volume and microstructural integrity.

Assessment of brain anatomy includes volumetric and morphometric measurements. Brain structures such as the hippocampus can be evaluated by visual estimation and manual or automated volumetry. Regional GM changes can be assessed by cortical thickness measurement or morphometry. Morphometry is performed on the basis of local deformations 42 or voxelwise GM attenuation (VBM). VBM is an adaptation of the statistical parametric mapping technique, which permits investigators to quantitatively examine whole-brain structural changes in a variety of conditions. It relies on the assumption that the recorded MR imaging T1-weighted voxel contains a mixture of distinct tissue types. These tissue types include GM, WM, and CSF. VBM allows a voxelwise comparison of regional GM and WM concentrations of spatially normalized images. Voxelwise parametric statistics can be performed on smoothed GM segments between groups. Recently, a new study even found GM changes after hours of learning new color names, 43 indicating that VBM provides a possible basis for behavioral studies on the structural MR imaging domain. For an overview of MR imaging techniques used in neuroimaging, see Fig 1 .

A parade of Comstock's vision of a great city.

The U.S. government intended Columbia to be a showcase of American exceptionalism. Through tours across various countries and lands, America would be capable of spreading its vision of the future and the American ideal to others.

Through Rosalind Lutece's discovery of the "Lutece Particle", which held atoms at a fixed position and support by Christian Louboutin PointedToe Cutout Pumps Outlet Reliable pE6LcXE
, a charismatic American religious figure, the city of Columbia was created and presented at the Maryam Nassir Zadeh Fiorenza Metallic Booties Limit Offer Cheap awLJn
.

The city was launched to great fanfare and was later dispatched to distant shores. What began as an endeavor of achievement and hope went horribly wrong for the American Government. In 1901, during the Boxer Rebellion where Bottega Veneta Embossed Cage Sandals Outlet Wiki UqE9jN5FBA
had been taken, Columbian Forces intervened without orders from the U.S. Government and destroyed the city of Peking. The destruction of the city caused the death of untold Chinese civilians and demonstrated the danger that Columbia posed to the rest of the world.

When the floating city was revealed to be heavily armed and to have had acted on its own, a rift emerged between leadership in Columbia and Washington, resulting in the floating city being recalled to American shores. In response, on July 6th, 1902, Columbia seceded from the Union and the following year disappeared into the clouds. Comstock now had complete control over the city and its people who rallied to his beliefs and set to creating his ideal society.

This emerging utopia had its flaws, though they were not seen as problems by its leadership. Comstock and his regime, Saint Laurent Animal Print Suede Espadrilles Authentic V5yCGkA
, believed that Columbia embodied the true society envisioned by the Founding Fathers of the United States, where white Anglo-Saxons ruled over the world and that their country served a higher purpose in "civilizing" through military might and propagating their particular brand of religion. To many Columbians, America had turned away from its divine purpose, having abandoned slavery, religion, militarism, and racial supremacy. America and the rest of the world below were viewed with contempt, described as "the Sodom Below," a sinful and chaotic world which only deserved to be destroyed. Columbia in comparison was referred to as "Another Ark, for Another Time" by its citizens, meaning the city was the only source of goodness and order, and once Columbia destroyed the rest of the world, everything could restart pure and anew under the city's absolute rule. Due to Comstock's dogma, Columbians had a very narrow perspective of American history; President Abraham Lincoln was labeled "The Apostate" by Columbia for ending slavery. His killer, John Wilkes Booth, was revered as a saint. The Columbian perspective of the Civil War is that of a demonic Lincoln leading a barbarous horde against the saintly Confederate forces under a deified George Washington, underscoring Columbian beliefs about racial slavery, as well as the anarchistic nature of the "false" America.