Complicated........but simply.......the inner ear (fluid) and the brain have to agree what is happening and when they are in conflict, then motion sickness is inevitable.
Bazile.......your eyes are telling your brain that you are reading a book and everything is fine......BUT....the endolymphatic fluid in your ears is "slopping" around telling your brain, that all is NOT well.
Who is the brain to believe?
Now, put down your book and look out of the car window and your brain sees the moving surroundings which you inner ear fluid has been telling it for some time and ALL is in agreement.
Any problem in the pathways of the inner ears, eyes and brain, will bring on dizziness or unsteadiness, nauaea or vomiting.