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, the light-responsive opsin polypeptide in the target neural cell population; scanning multiple regions of the brain via functional magnetic resonance imaging to observe a neural reaction in response to the stimulation in at least one of the multiple regions of the brain and, in response, determine whether neural projections in a second region of the brain are connected to at least some of the modified target cell population in the first region of the brain.