In an episode of his Big Idea series, Dean Lloyd Minor welcomes Stanford neuroscientist Karl Deisseroth, MD, PhD, for a conversation about how his groundbreaking research is reshaping our understanding of the brain and the experiences that define us as human. They delve into the origins of optogenetics — a revolutionary technology Deisseroth pioneered using light to map how the brain drives emotion and behavior — and they explore his vision for Stanford Medicine’s Human Neural Circuitry research program. Deisseroth also reflects on the beauty of scientific discovery, the role of humility in research, and how advances in brain science are opening new avenues for understanding and treating mental illness.