The Latest Weird Thing to Play Doom Is the Mapped-Out Brain of a Fruit Fly

Doom is such a simple, beautifully designed game that it can run on all sorts of weird things, from pregnancy tests to potatoes. Weird things can play it, too. In an effort to eternally one-up itself, the community has now gotten a map of a fruit fly’s brain to play the 1993 first-person shooter.

It started with Google, which partnered with HHMI Janelia Research and other collaborators to map the brain and central nervous system of a male fruit fly. This mapping, which is weirdly beautiful, contains 11,700 types of neurons and 166,000 neurons overall, breaking the record for the most complex neuron mapping ever done. 

The research is pretty cool: It involved taking very thin slices of a fly’s brain and body, imaging them and using AI to combine the millions of 2D images into 3D neural shapes. Google says that “knowing the structure of the brain allows neuroscientists to understand the mechanisms behind brain function.”

Now, make it play Doom

It turns out that knowing the structure of the male fruit fly brain can also help modders and developers teach it how to play Doom. A few days after Google dropped its fly brain research, software engineer Alex Wormuth created a simulated fly brain using Google’s map and then set it loose to learn how to play the pioneering FPS. 

The process is quite complicated. Each frame of the game generates 3,335 brightness inputs and 811 R8 color inputs. These inputs are fed into the simulated fruit fly brain, which contains 25.6 million connections. These inputs hit specific neurons based on what the frame shows, and that information is used to determine which controls the fly uses. This process repeats each frame the game runs. 

It even works with pain. Wormuth says that nonfatal damage targets the PPL101 dopamine receptors, which control hunger and aggression in real fruit flies. 

You can watch the fly’s progress in learning how to play Doom on Wormuth’s Doomfly website, which also shows sensory input, neural activity and other metrics so you can watch the simulated fly brain work in real time. If you swap to the 3D view, you can watch the fly shoot demons.

Flying high with other games, too

Doom isn’t the only game that a simulated fly brain is learning how to play. Developer Jessica Paquette set up a similar neural network with the fly’s brain to learn how to play Super Mario 64, while mechanistic interpretability researcher Lyra Bubbles made another one learn how to play Beat Saber. 

A few days later, X user David began teaching it how to do an escape room, and another, Shashank Jain, taught it how to play soccer. Wormuth came back a couple of days later and gave the fly brain $100 worth of bitcoin to trade to see what it would do. X user Simba made the fly brain deploy its own cryptocurrency token on Robinhood. 

The fly brain experiment is going to have a lot more impact in neuroscience in the long run, but for at least the next few weeks, don’t be surprised if you see headlines about a fruit fly brain learning how to do other random stuff, like creating real fonts that you can download right now or putting it in music videos and letting it dance.