Unable to change brightness on a toshiba m505

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http://askubuntu.com – I got Toshiba m505 and I can't change the brightness of my screen. I can see the brightness adjusting bar move when I use fn+F6 or fn+F7 keys but nothing happens. I'm using Ubuntu 12.04. I did a lot of search on-line but I can't find anything clear for me to understand. (HowTos)