What does an fMRI signal measure - exactly?

Nikos K. Logothetis is at the Max Planck Institute, in an article in Nature Neuroscience explains in greater detail the relationship between the effect measured by fMRI (the Blood Oxygenation Level Dependent signal), the cerebral metabolic rate of oxygen consumption (CMRO2) and the underlying neural activity

http://www.nature.com/neuro/journal/v10/n10/full/nn1007-1230.html