>I'd say it's primarily caused by a disconnect between what the human vestibular system (your sense of balance and spatial orientation) is telling the brain your body is doing vs the acceleration forces that your body is feeling vs what your eyes are telling you about what the body is doing.

The vestibular system has a prodigious amount of control over your life. I've got hearing damage and occasionally get vertigo spells (I carry dramamine in my wallet for emergencies). Every once in a while my balance sense just begins to swing around wildly. Sounds kind of cute. It will put you on your ass for 24-48 hours with zero possibility of doing anything. Even drinking water is hard. Without drugs the nausea just becomes your entire world. You think about nothing but how much it sucks. You can't eat, move, sleep or speak.

So... yeah. Motion sickness is a very real thing. I think (based on zero evidence) that everybody has a different threshold but experiences the same effects past that. If you're lucky, your threshold is faster than you can move your head. If you aren't, chug dramamine and coffee and hope they invent something that turns off your sense of balance without making you delirious.