Expose Problem - "All Windows"

My top left corner is set up for a quick "All Windows," which I use a lot. When I put my mouse there it does not work as usual. Instead a clear screen remains in the middle of the screen, but all the peripheries around the clear center circle (the edges of the screen) dim. The dimming effect makes it look like I'm peering down a tube (sort of). The only thing I can think of is that I connected to a TV recently, but I checked my display settings...1280 by 800, right? I cannot figure out what is going on, and cannot find an answer elsewhere. Any help?? (all other corners work, and changing the corner does not alleviate the problem) Thanks!