sentences:  Two parallel developments in endogenous contrast set the stage of methods to map brain activity without injection of tracers or contrast agents.Contemporaneous work a decade earlier by Thulborn, and Wright at Stanford, had shown that blood oxygenation levels could be measured by NMR methods.Later groundbreaking experiments by Ogawa, et al., and by Turner had shown that oxygen depletion led to significant drops in MRI signal changes in large veins and the brain cortex itself, respectively, via a magnetic susceptibility mechanism analogous to that used by Belliveau with exogenous tracers, but in this case using deoxygenated blood itself as the contrast agent.