The Doom-playing rats are back, and now they've learned how to shoot

In 2021 PCG reported on a singular scientific experiment: rats named Carmack and Romero had been taught how to 'play' Doom. I use the inverted commas because, cute and cool as it was, the question of whether the rats were really playing Doom in any meaningful way, or just running on a ball for some delicious sugar water, was not conclusively answered.

The project leader Viktor Tóth said at the time he wasn't happy with how they'd implemented the shooting response, and four years later the team is back with a new setup that significantly expands what the rats can do (first spotted by Tom's Hardware). And this time, the rodents are rocking a trigger mechanism.

The first version had the rats suspended in a harness over a ball that rotated when they moved, with that movement mapped to a simplified version of a Doom corridor with an imp at the end. When the rat did the right things, it would get a reward in the form of sweetened water.

The limitations meant that it was a bit of a stretch to say the rats were playing Doom, but as Tóth said you could make a decent argument they were "kinda" playing Doom.

The new setup still maps the rat's real-world movement into a Doom environment, but now allows for more nuanced navigation alongside new inputs and feedback. The rats are now treated to a wraparound AMOLED display that covers more of their field of view (which "maximizes immersion without obstructing whisker space"), and any wall collisions are communicated via gentle puffs of air to the animal's snout.

Most notable of all is a new trigger mechanism that lets the rat fire a gun in Doom, meaning they now not only move around in the game environment but can shoot things.

"The shooting input is a custom-built hand-operated lever," writes Tóth. "Rats pull it with their paws to fire. The lever is held in place by small springs, encased in a 3D-printed housing. It includes a rotary encoder to detect motion and a stepper motor to actuate it."

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Everything about the setup is external: sensors, motion tracking, visual feedback, and of course the good old rewards reinforcing correct behaviours. The initial experiment was a Tóth solo project, while this v2 was built in collaboration with electrical engineer Sándor Makra. Akos Blaschek also assisted in documenting the project for open-sourcing and, should you wish to build your own rat VR hardware, here is how to do it.

The results? "The rats successfully learned to navigate the virtual environment and trigger the shooting mechanism," writes Tóth. "Habituation took approximately two weeks per rat. While advanced training wasn't completed due to time constraints, initial data showed promising engagement with the system. Full behavioral validation requires longer training periods."

So perhaps we can forget about the inverted commas, and simply say that rats can now play Doom. The rodent versions of Carmack and Romero have long since gone to the great sugar water dispenser in the sky, but the legacy is clear: they walked, so later rats could run 'n gun.