Cerebrovascular health (and in particular arterial stiffening, or arteriosclerosis) is a significant contributor to age-related decline, and exerts a central role in a number of pathologies, including Alzheimer's disease (AD) and vascular dementia. Arteriosclerosis is progressive and considered to be irreversible. Therefore its early detection is of critical importance. As extensively demonstrated by the Framingham Heart Study and other longitudinal research, life style factors, and in particular physical inactivity, play a central role in the development of arteriosclerosis. Cerebral arteriosclerosis is currently assessed either indirectly with peripheral measures or more directly in the neck and head (with carotid or trans- cranial Doppler sonography, TCD), which provide useful clinical information but at just a few measurement points. Our lab has recently developed a new imaging approach, based on the study of the arterial pulse measured with diffuse optical tomography (pulse-DOT), which allows for the mapping of arterial status across the entire cortical mantle, with high reliability and replicability. We have shown that in normally aging older adults, low values of PReFx (pulse relaxation function, a measure of pulse shape indicative of arterial elasticity) are correlated with older age, lower cardiorespiratory fitness (CRF), and overall greater age-related brain atrophy and white matter signal abnormalities (WMSA). Crucially, we have also shown that local/regional variations in arterial stiffness from one brain region to another correlate with volumetric variations in the same regions. These regional effects provide a stronger functional link between arterial health and early structural signs of brain aging than those derived from global measures of arterial elasticity, pointing at the importance of arterial function in the chain of events that may lead, over time, to cognitive and brain volumetric losses. In the proposed research, based on a mixed cross-sectional/longitudinal (30 month) design involving 200 individuals between 50 and 70 years of age, we aim to: (1) Demonstrate that pulse-DOT will relate to cerebrovascular risk cross- sectionally and also prospectively over a 30-month period. Individuals classified by degree of presence of sub-clinical risk factors relating to low CRF are expected to show a graded relationship with the PReFx index of arterial stiffness, as well as with indices of cerebrovascular reactivity (CVR). Decreasing PReFx pulse-DOT values over follow-up are then expected to predict a worsening of risk. (2) Demonstrate that regional changes (over 30 months) in cerebral arterial function are associated with regional changes in brain structural (i.e., indicating signs of brain atrophy and WMSA) and blood flow parameters in the same regions (measured with magnetic resonance imaging, MRI). (3) Identify the relationship between cognitive function, regional vascular stiffening and CVR at baseline and over the follow-up per...