Project Summary/Abstract Human healthy aging is associated with declines in cognition, memory, and processing efficiency. Two leading theories to explain altered function are the mitochondrial theory of aging, in which mitochondria lose the ability to provide sufficient energy to support function and altered GABA/glutamate neuromodulation. Recent developments in calibrated functional MRI (fMRI) have allowed direct imaging of alterations in neuronal activity in aging. Surprisingly, measurements of task-induced changes in oxidative metabolism (ΔCMRO2) have found increased neuronal activation despite lower baseline CMRO2 in aging. We propose to address this paradox, by using a novel combination of calibrated fMRI, functional 1H-MRS (fMRS), and 1H[13C]-MRS approach to directly test the roles of mitochondrial aging and neuromodulation in healthy aging. Our general hypotheses are: (i) The paradoxical increased functional response in healthy aging, observed primarily in sensory areas, is a compensation for lower baseline CMRO2, but it is not sufficient to achieve the same energetic support for neuronal function as in young subjects (mitochondrial aging theory); and (ii) The inverse relationship between GABA concentration and BOLD activation established in young subjects is blunted in elderly (neuromodulation theory). We will test these hypotheses with three specific aims. In Aim 1 we will use a novel gas-free calibrated fMRI method to measure ΔCMRO2 in response to visual task in healthy young and aged subjects. In Aim 2 we will perform, in parallel with calibrated fMRI, fMRS measurements of inhibitory GABA and excitatory glutamate. In Aim 3 we will use 1H[13C]-MRS to specifically measure neuronal CMRO2 (via the TCA cycle) which we showed is selectively decreased in aging visual cortex. In parallel with our testing of changes in activated brain regions like the visual cortex, we will also perform the same measurements simultaneously in the posterior cingulate cortex (PCC), which is a major hub of the default mode network (DMN) that has been shown to deactivate during tasks. Resting-state fMRI studies in aging have shown that the DMN connectivity is altered, but there have been no studies looking at its task response using quantitative fMRI or fMRS methods. Understanding the metabolic and neuromodulatory differences across activated and deactivated areas are crucial for understanding the basis of the altered functional response in healthy aging. The proposed work has high potential human health significance both for understanding basic mechanisms behind the alterations in neuronal activity in aging and in validating a novel multi-modal MRI/MRS biomarker for evaluating these mechanisms and potentially assessing the response to mechanism targeted therapeutics. Although 1H[13C]-MRS is not yet broadly available, we will evaluate whether neuronal CMRO2 at rest determined from calibrated fMRI provides similar results. Because the calibrated fMRI method does...