NeuroImage
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To determine the time and location of lexico-semantic access, we measured neural activations by magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) and estimated the neural sources by fMRI-assisted MEG multidipole analysis. Since the activations for phonological processing and lexico-semantic access were reported to overlap in many brain areas, we compared the activations in lexical and phonological decision tasks. The former task required visual form processing, phonological processing, and lexico-semantic access, while the latter task required only visual form and phonological processing, with similar phonological task demands for both tasks. ⋯ Previous studies on semantic dementia and neuroimaging studies on normal subjects have shown that this area plays a key role in accessing semantic knowledge. The difference between the tasks appeared in common to all areas in the time windows of 100-150 ms and 400-450 ms, suggesting early differences in visual form processing and late differences in the decision process, respectively. The present results demonstrate that the activations for lexico-semantic access in the left anterior temporal area start in the time window of 200-250 ms, after early visual form processing.
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The blood-oxygenation-level-dependent (BOLD) signal is dependent on multiple physiological factors such as cerebral blood flow (CBF), local oxygen metabolism (CMRO(2)) and cerebral blood volume (CBV). Since caffeine affects both CBF and neural activity, its effects on BOLD remain controversial. The calibrated BOLD approach is an excellent tool to study caffeine because it combines CBF and BOLD measures to estimate changes in CMRO(2). ⋯ The results show that caffeine decreases n, the CBF:CMRO(2) coupling ratio, from 2.58 to 2.33 in motor (p=0.006) and from 2.45 to 2.23 in visual (p=0.002) areas respectively. The current study also demonstrated that caffeine does not alter cerebrovascular reactivity to CO(2). These results highlight the importance of the calibrated BOLD approach in improving interpretation of the BOLD signal in the presence of substances like caffeine.
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The changes of directional diffusivities derived from diffusion tensor imaging (DTI), i.e. decreased axial diffusivity (lambda(||)) and increased radial diffusivity (lambda( perpendicular)), have shown significant correlation with axonal and myelin damage, respectively. However, after formalin fixation, reduced sensitivity of lambda(||) in detecting axonal damage in tissue has raised the concern of applying DTI ex vivo. In order to distinguish whether death or the fixation process diminishes the sensitivity of DTI in detecting lesions, in vivo, pre-fixed postmortem, and fixed postmortem DTI were conducted on mouse optic nerves 3 and 14 days after transient retinal ischemia. ⋯ From pre-fixed postmortem to fixed postmortem, lambda(||) and lambda( perpendicular) decreased by 40 to 50% in normal and 3-day injured optic nerves, but only by 15 to 25% in 14-day injured optic nerves. Consequently, for the 14-day injured optic nerves, the differences between healthy and injured nerves were not preserved after fixation: the 40% decreased lambda(||) and 200% increased lambda( perpendicular) in injured nerves as compared to the normal nerves were measured in vivo and pre-fixed postmortem, but after the fixation process, 300% increased lambda( perpendicular) and insignificant changes in lambda(||) were found in injured nerves as compared to the normal nerves. This study clarified that fixation process, but not death, could change the sensitivity of DTI in detecting injury.
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We used the [F-18]FDG micro PET neuroimaging technique to investigate changes in brain activity induced by acute stress in rats. Animals were given immobilization stress for 1 or 2 h, or 1-h stress followed by 1-h recovery, after which their brains were scanned. Plasma corticosterone levels measured at various time points in separate groups of rats showed a rapid increase during stress and slower decrease after termination of the stress. ⋯ Additional brain areas such as the septum and prelimbic cortex now showed deactivation during recovery. Changes in glucose metabolism in the dorsal hippocampus and hypothalamus exhibited a highly significant negative correlation, supporting the view that the hippocampus is involved in regulating the stress response of the hypothalamo-pituitary-adrenal axis. The advantages and limitations of the [F-18]FDG micro PET used in this study are discussed.
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Multiple sclerosis (MS) affects both white matter and gray matter (GM). Measurement of GM volumes is a particularly useful method to estimate the total extent of GM tissue damage because it can be done with conventional magnetic resonance images (MRI). Many algorithms exist for segmentation of GM, but none were specifically designed to handle issues associated with MS, such as atrophy and the effects that MS lesions may have on the classification of GM. ⋯ The scan-rescan reproducibility test resulted in a mean coefficient of variation of 1.1% for GM fraction. Tests of the effects of varying the size of MS lesions revealed a moderate and consistent dependence of GM volumes on T2 lesion volume, which suggests that GM volumes should be corrected for T2 lesion volumes using a simple scale factor in order to eliminate this technical artifact. The new segmentation algorithm can be used for improved measurement of GM volumes in MS patients, and is particularly applicable to retrospective datasets.