The first seeds of my interest in the brain were planted between junior high and high school, when my cousin went into labor. While under anesthesia during a C-section, she went into a coma that she’s been in ever since. The parts of her brain that are involved in producing consciousness and wakefulness were probably atrophied because they didn’t get enough oxygen for just a short period of time. It instantly hit me: all it takes are these little lumps of tissue in your brain to atrophy, and now everything that makes you you is evaporated.

Because the seemingly ephemeral stuff of cognition is based on the physical stuff of the brain, we can go in and manipulate it and see how something as complicated as memory works. When you are thinking of a memory, only a subset of brain cells are active, and those cells are specifically representing that memory. We can genetically modify neurons to produce a sensor that detects when brain cells are active and then installs an on-off switch in them. The switch is a protein that allows us to control the activity of a cell with light.