Bumps illusion

You are likely to see that the top row of circles appear as "bumps" followed by "holes" and so on in an alternating pattern. Can you try and see the opposite? Have a good hard look and try to see the first row as holes and the second as bumps. Difficult? Now hide the bottom row of circles from your field of vision (either with your hand, or scrolling your browser window appropriatly). Is it easier to see the first row as holes now?