Response of the primary auditory and non-auditory cortices to acoustic stimulation: A manganese-enhanced MRI study

Structural and functional features of various cerebral cortices have been extensively explored in neuroscience research. We used manganese-enhanced MRI, a non-invasive method for examining stimulus-dependent activity in the whole brain, to investigate the activity in the layers of primary cortices and sensory, such as auditory and olfactory, pathways under acoustic stimulation. Male Sprague-Dawley rats, either with or without exposure to auditory stimulation, were scanned before and 24-29 hour after systemic MnCl2 injection. Cortex linearization and layer-dependent signal extraction were subsequently performed for detecting layer-specific cortical activity. We found stimulus-dependent activity in the deep layers of the primary auditory cortex and the auditory pathways. The primary sensory and visual cortices also showed the enhanced activity, whereas the olfactory pathways did not. Further, we performed correlation analysis of the signal intensity ratios among different layers of each cortex, and compared the strength of correlations between with and without the auditory stimulation. In the primary auditory cortex, the correlation strength between left and right hemisphere showed a slight but not significant increase with the acoustic simulation, whereas, in the primary sensory and visual cortex, the correlation coefficients were significantly smaller. These results suggest the possibility that even though the primary auditory, sensory, and visual cortices showed enhanced activity to the auditory stimulation, these cortices had different associations for auditory processing in the brain network.open0

Multiple Advisors with Reputation

This paper examines reputation, the belief of a decision maker about types of advisors, in a two period cheap talk model where the decision maker obtains messages from two advisors. The decision maker believes that an advisor can be one of two types - an advisor who is biased towards suggesting any particular advice (bad advisor) or an advisor who has the same preferences as the decision maker (good advisor). I assume that each advisor perfectly knows the type of the other advisor, but his signal about the state of the world is imperfect. Strong reputational concern makes the good advisor sometimes tell a lie in the first period regardless of the type of the other advisor. It is shown that the presence of the other advisor does affect the message sent by an advisor. The good advisor has a greater incentive to tell a lie when he knows that the other advisor is bad rather than good. If each type of advisor considers his second period sufficiently important, it is better for the decision maker to have a single advisor.Reputation; Cheap talk

BIOPHYSICS MODEL IMPROVEMENT IN MAGNETIC RESONANCE IMAGING: CEREBRAL METABOLIC RATE OF OXYGEN

105 pagesCerebral metabolic rate of oxygen (CMRO2) and oxygen extraction fraction (OEF) are valuable to investigate tissue viability and function. Numerous magnetic resonance imainge (MRI) methods have been proposed to estimate them quantitatively. While those methods are investigated in healthy subjects, a robust framework is elusive from literature for reliable CMRO2 and OEF estimation in pathological scenarios. This thesis developed algorithms that improve the accuracy, robustness and applicability of CMRO2 and OEF for both healthy and pathological subjects. First, quantitative susceptibility mapping (QSM)-based and quantitative blood oxygen-level dependent magnitude (qBOLD)-based CMRO2 method were combined to resolve the issues of the individual methods. Second, the cluster analysis of time evolution (CAT) was proposed to improve robustness of the combined model against noise. With the technical advances in this thesis, CMRO2 and OEF that are robust against noise and sensitive to pathological senarios, e.g. ischemic stroke lesion, can be estimated

Sequential Cheap Talk from Advisors with Reputation

I examine two-period sequential cheap talk in situations where the decision maker seeks advice from two advisors, each of whom knows the type of the other advisor. By considering the current payoff (which is determined by the message of each advisor) and the future payoff (which is connected with the reputation of each advisor), I examine conditions which guarantee the existence of both good and bad reputation effects. Compared to situations of simultaneous cheap talk, the decision maker loses information more easily if he seeks advice sequentially.Cheap talk, reputation.

Cerebral oxygen metabolism from MRI susceptibility

This article provides an overview of MRI methods exploiting magnetic susceptibility properties of blood to assess cerebral oxygen metabolism, including the tissue oxygen extraction fraction (OEF) and the cerebral metabolic rate of oxygen (CMRO2). The first section is devoted to describing blood magnetic susceptibility and its effect on the MRI signal. Blood circulating in the vasculature can have diamagnetic (oxyhemoglobin) or paramagnetic properties (deoxyhemoglobin). The overall balance between oxygenated and deoxygenated hemoglobin determines the induced magnetic field which, in turn, modulates the transverse relaxation decay of the MRI signal via additional phase accumulation. The following sections of this review then illustrate the principles underpinning susceptibility-based techniques for quantifying OEF and CMRO2. Here, it is detailed whether these techniques provide global (OxFlow) or local (Quantitative Susceptibility Mapping - QSM, calibrated BOLD - cBOLD, quantitative BOLD - qBOLD, QSM+qBOLD) measurements of OEF or CMRO2, and what signal components (magnitude or phase) and tissue pools they consider (intravascular or extravascular). Validations studies and potential limitations of each method are also described. The latter include (but are not limited to) challenges in the experimental setup, the accuracy of signal modeling, and assumptions on the measured signal. The last section outlines the clinical uses of these techniques in healthy aging and neurodegenerative diseases and contextualizes these reports relative to results from gold-standard PET

Quantitative susceptibility mapping and R1 measurement: Determination of the myelin volume fraction in the aging ex vivo rat corpus callosum

In studies of the white matter (WM) in aging brains, both quantitative susceptibility mapping (QSM) and direct R-1 measurement offer potentially useful ex vivo MRI tools that allow volumetric characterization of myelin content changes. Despite the technical importance of such MRI methods in numerous age-related diseases, the supposed linear relationship between the estimates of either the QSM or R-1 method and age-affected myelin contents has not been validated. In this study, the absolute myelin volume fraction (MVF) was determined by transmission electron microscopy (TEM) as a gold standard measure for comparison with the values obtained by the aforementioned MR methods. To theoretically evaluate and understand the MR signal characteristics, QSM simulations were performed using the finite perturber method (FPM). Specifically, the simulation geometry modeling was based on TEM-derived structures aligned orthogonally to the main magnetic field, the construct of which was used to estimate the magnetic field shift (Delta B) changes arising from the conjectured myelin structures. Experimentally, ex vivo corpus callosum (CC) samples from rat brains obtained at 6 weeks (n = 3), 4 months (n = 3), and 20 months (n = 3) after birth were used to establish the relationship between changes quantified by either QSM or R-1 with the absolute MVF by TEM. From the ex vivo brain samples, the scatterplot of mean MVF versus R-1 was fitted to a linear equation, where R-1mean = 0.7948 x MVFmean + 0.8118 (Pearson's correlation coefficient r = 0.9138; p < 0.01), while the scatterplot of mean MVF versus MRI-derived magnetic susceptibility (chi) was also fitted to a line where chi(measured,mean) = -0.1218 x MVFmean - 0.006345 (r = -0.8435; p < 0.01). As a result of the FPM-based QSM simulations, a linearly proportional relationship between the simulated magnetic susceptibility, chi(simulated,mean), and MVF (r = -0.9648; p < 0.01) was established. Such a statistically significant linear correlation between MRI-derived values by the QSM (or R-1) method and MVF demonstrated that variable myelin contents in the WM (i.e., CC) can be quantified across multiple stages of aging. These findings further support that both techniques based on QSM and R-1 provide an efficient means of studying the brain-aging process with accurate volumetric quantification of the myelin content in WM