On a A320, there are white patterns spread regularly on the trim wheel (highlighted on the right of the picture). First I though it was to have visual feedback about THS (trimable horizontal plane) position. But looking closely, this role seems to be played by the graduations (highlighted on the left of the picture).
(photo #0393544 from airliners.net)
Why are there those white patterns on the trim wheel? What is their role?