Blood oxygenation level-dependent functional magnetic resonance imaging (BOLD fMRI) is a widely used technology for non-invasive whole brain imaging.  BOLD signals reflect complex changes in cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen consumption (CMRO2) following neuronal activity.  However, the neural circuits that trigger BOLD signals are not completely understood, which may confound fMRI interpretation.  Candidate circuit elements for triggering various kinds of BOLD signals include excitatory neurons, mixed neuronal populations, astroglia, and axonal tracts or fibers of passage.  Understanding the neural circuits that give rise to BOLD signals may provide a way to diagnose neurological disorders that impact specific circuits, as well as to screen for therapeutic agents to treat such disorders.