NeuroImage
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Distinct aspects of our fearful experiences appear to be mediated by separate explicit and implicit memory processes. To identify brain regions that support these separate memory processes, we measured contingency awareness, conditional fear expression, and functional magnetic resonance imaging signal during a Pavlovian fear conditioning procedure in which tones that predicted an aversive event were presented at supra and sub-threshold volumes. ⋯ In contrast, conditional fear and differential amygdala activity developed on both perceived and unperceived trials, regardless of whether contingency awareness was expressed. These findings demonstrate the distinct roles of these brain regions in explicit and implicit fear memory processes.
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Pain is associated with the activation of many brain areas involved in the multiple dimensions of the experience. Several of those brain areas may also contribute to the monitoring and regulation of autonomic activity but this aspect of pain responses has been largely overlooked in human imaging studies. This functional magnetic resonance imaging (fMRI) study relied on blood-oxygen level dependent (BOLD) signal to investigate subject-related differences in brain activity associated with the individual differences in electrodermal responses evoked by 30 s noxious (pain) and innocuous (warm) thermal stimuli. ⋯ Subjects showing larger skin conductance reactivity to the innocuous and/or noxious stimuli displayed larger stimulus-evoked brain responses in the somato-motor cortices (SI/MI, SII, and insula), the perigenual and supracallosal ACC, the orbitofrontal cortex and the medulla. Further analyses revealed brain activation more specifically associated with the pain-related skin conductance reactivity in the supracallosal ACC, amygdala, thalamus, and hypothalamus. These findings demonstrate that individual differences in electrodermal reactivity partly reflect differences in pain-evoked brain responses, consistent with a role of these structures in the monitoring/regulation of pain-related autonomic processes.
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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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Low-frequency fluctuations in fMRI signal have been used to map several consistent resting state networks in the brain. Using the posterior cingulate cortex as a seed region, functional connectivity analyses have found not only positive correlations in the default mode network but negative correlations in another resting state network related to attentional processes. The interpretation is that the human brain is intrinsically organized into dynamic, anti-correlated functional networks. ⋯ A combination breath holding and visual task demonstrates that the relative phase of global and local signals can affect connectivity measures and that, experimentally, global signal regression leads to bell-shaped correlation value distributions, centred on zero. Finally, analyses of negatively correlated networks in resting state data show that global signal regression is most likely the cause of anti-correlations. These results call into question the interpretation of negatively correlated regions in the brain when using global signal regression as an initial processing step.
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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.