Asus X401A Fn keys not controlling screen brightness

Having installed Linux Mint 14 on the Asus X401A, I found the Fn keys for controlling screen brightness Fn-F5 and Fn-F6 do not work.

In a terminal session:

gksudo pluma /etc/default/grub

and change "quiet splash" to "quiet splash acpi_osi=" thus:

GRUB_CMDLINE_LINUX_DEFAULT= "quiet splash acpi_osi="

Save the file then:

sudo update-grub

Reboot and the Fn keys will set the brightness.

Sadly, this seems to have broken Fn-F2 which switches the WiFi on/off.