Gee, how did I know it'd be the guy who made a virus to cure his severe lactose intolerance, infected himself with it and then gorged on dairy products to test it.
In this case, "mad scientist" is absolutely on point.
You want a toe? I can get you a toe, believe me. There are ways, Dude. You don't wanna know about it, believe me. I'll get you a toe by this afternoon--with nail polish. These fucking amateurs!
The experiment uses cortical rat neurons, because of their affordability and because they can learn well enough for these purposes (you too can order a million frozen rat neurons online, what a world). Yes, you could use human neurons to the same end, and no doubt someone watching this video will be off to order some imminently, but they're expensive and almost seem overkill because playing Doom can be stripped-down to relatively simple yes / no patterns.