To Tim Harris, understanding the brain is like understanding a building—a really big building—from the vantage point of the sidewalk. “The brain is the Empire State Building, and it’s opaque. You’re standing there looking at the outside and wondering: is the hot water faucet on the third sink of the 65th-floor restroom on the left or the right?”

This is the situation neuroscientists find themselves in, Harris says. They can see your head, they can see you sensing your environment and doing things, but they have only the murkiest sense of your brain’s inner workings. Harris, a physicist at HHMI’s Janelia Farm Research Campus in Ashburn, Virginia, develops tools neuroscientists can use to measure the brain’s activity, to give them a quantitative view inside the elaborate structure of the brain.

Photo illustration by Fredrik Broden