1. A method comprising: modifying a target neural cell population in a first region of a brain to express a light-responsive opsin polypeptide; stimulating, using a light pulse, a second region of the brain not modified to express the light-responsive opsin polypeptide; scanning multiple regions of the brain via functional magnetic resonance imaging (fMRI) to observe a neural reaction in response to the stimulation in the second region of the brain; and determining whether neural projections in the second region of the brain are connected to at least some of the modified target cell population in the first region of the brain.