They are heavily rewarded since rats are very food-oriented and are foragers, and the lab setup uses this trait to reinforce behaviors by feeding them treats.
They find 'enjoyment' in the video game in the same way they find 'enjoyment' finding food in the wild. It's not like a human's enjoyment of Doom, but they're not not (double-negative) enjoying it.
Having watched the video last time it was submitted, I doubt the rat has any concept of the game's existence at all, or that its actions are doing anything to influence the mass of flashing colours on the screen in front of it. If you watch it, it doesn't even once look at the screen. I'm sure it's just learned that running on the ball in certain directions produces food. As a headline, "Scientist teaches rat to play Doom" is taking dumbing down to new depths.
if rats can play doom they should be able to pilot some type of semi autonomous drone; we should probably start out with paintball guns or laser pointers.
