We have an unbelievable amount to learn from using optogenetics in humans. Not just about what the brain does, but what it feels like to be a brain when it’s perturbed in precise ways. Most neuroscience tools are behavioral; we poke a circuit and see what the mouse does. But imagine being able to ask the subject directly: What changed in your experience when we activated this region? Does pulsing the retrosplenial cortex with light feel like you are dissociating as Deisseroth's lab showed?