The brain. The brain fills in missing parts of the picture as our eyes work together, the brain adjust our vision to what we are focusing on. The brain also anticipates what we will see.
The inner ear tells the brain where gravity is and stabilizes our horizon so to speak. It the inner ear has inflammation or the little crystals shift to the wrong spot, the message is wrong and your eyes lie to you and spin your world.
Someone with the crystals in the wrong place, if you lie them down on their back and put their head back slightly off the end of the bed and turn their head 45° to the side that has the crystals out of place, and watch their eyes as you do it, their eyes will involuntarily move back and forth until the brain adjusts. The person doesn’t feel their eyes move, they experience the room moving or spinning.