Gaze stabilisation (horizontal) in supine ("VRT gaze stabilization" Eyes fixed turning head right/left, in supine; 01)

Lie on your back with your arms relaxed by your sides.
Focus your eyes on a mark on the ceiling.
Alternatively, use your thumb by holding it out in front of you.
Keep your eyes focused on the mark whilst you slowly turn your head from side to side.
It is important that the mark remains in focus throughout the exercise.
If it is not, slow down the speed of your head movement.