It's an ATI X-series driver issue I'm afraid.
http://www.frictionalgames.com/forum/thr...age-2.html
The ATI drivers have trouble with the OpenGL features used in Amnesia. If you disable ImageTrail in the Post Processing Effects options you'll be able to play the game with all but a few caveats: you won't be able to bring up the main menu, inventory, mementos or journal screen as it will cause the game to pretty much lock up and the logos and loading graphics will still tessellate. It's a pain because it so nearly works but Thomas will hopefully be looking into the problem next week.