1. A method comprising: modifying a target neural cell population in a first region of a brain to express a light-responsive opsin polypeptide using an apparatus for delivering a nucleic acid comprising a nucleotide sequence encoding the light-responsive opsin polypeptide; stimulating, using a light source for delivering a light pulse, a second region of the brain not modified to express the light-responsive opsin polypeptide; scanning multiple regions of the brain using a functional magnetic resonance imaging device that is configured to observe a neural reaction in response to the stimulation in the second region of the brain; and determining 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.