Room tilt illusion influenced by head position.

After a low brainstem stroke, a 73-year-old man experienced episodes of 90 degrees tilting of the visual environment in the sagittal plane evoked or terminated by voluntarily changing his head position. The episodes ceased 10 days after the stroke. This provocation by head position supports the idea that pathologic visual-vestibular interaction is at the basis of the room tilt illusion.