Before any cutting or drilling, the surgeon pins what looks like a giant protractor to the patient’s head—a stereotactic frame. Then using CT and MRI scans, the surgeon builds a map the patient’s brain and draws a safe path from the skull to the burn zone, accurate to within fractions of a millimeter.

But because the brain is malfunctioning in invisible ways, the surgeons draw upon laboratory studies from animal and human experiments. “This whole field relies on improving our understanding of how the brain works,” Sheth says. That requires a lot of back and forth, beginning with lab work on animal brains. Once neuroscientists figure out how a certain brain circuit works in a fish or a rat, they step the research up in human studies. This involves using functional magnetic resonance imaging or EEG readings to judge what parts of the brain are most important to certain behaviors.

From there, surgeons like Sheth design their interventions, targeting a specific area of the brain as closely as possible